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Ecological Monographs, 80(2), 2010, pp. 197–219
Ó 2010 by the Ecological Society of America
Climate, snow, ice, crashes, and declines in populations of reindeer
and caribou (Rangifer tarandus L.)
N. J. C. TYLER1
Centre for Saami Studies, University of Tromsø, Tromsø N-9037 Norway
Abstract. Snow is a major determinant of forage availability for reindeer and caribou
(Rangifer tarandus; hereafter Rangifer) in winter and is, consequently, a medium through
which climate variation may influence population dynamics in this species. Periodic ‘‘icing’’ of
winter ranges, where interludes of mild weather result in formation of crusted snow and basal
ice that restrict access to forage, is held to be a cause of mass starvation, catastrophic declines
in numbers, and even extirpation of local populations. It has been suggested that warming of
the Arctic may result in increased frequency of winters with unfavorable snow and ice
conditions, with serious consequences for Rangifer. This paper examines data on major
declines in populations of Rangifer to determine the mechanism(s) of these events and the role
of snow and ice conditions in them. Thirty-one declines, involving numerical decreases
between 25% and 99%, were identified in 12 populations. Declines were of two types: the
negative phase of irruptive oscillations, mainly associated with populations introduced into
new habitat, and numerical fluctuation in persistently unstable established populations. The
mechanisms of decline differed widely in both categories, ranging from wholly mortality to
almost wholly emigration. In all cases, the observed dynamics are best interpreted as a product
of interaction between internal processes (density dependence) and the external abiotic
conditions (density independence). The strength and the form of density independence,
parameterized in terms of local weather or large-scale climate, varies widely between
populations, reflecting the enormous range of climate conditions across the circumpolar
distribution of Rangifer. This complicates the search for abiotic components likely to be
consistently important determinants of population growth in the species. There are few data
demonstrating the presence of extensive hard snow or basal ice on ranges during winter(s) in
which populations declined, and none confirming ice as a ubiquitous and potent agent in the
dynamics of Rangifer. Instead, where the simultaneous effects of density-dependent and
density-independent factors are examined across the full temporal record of dynamics, climatic
conditions associated with increased amounts of snow or winter warming are generally found
to enhance the abundance of animals, at least in established populations.
Key words: ablation; Arctic; caribou; climate change; global warming; ice; mortality; population crash;
Rangifer tarandus; reindeer; snow.
INTRODUCTION
Understanding of the potency of the effects of
environmental variation in the dynamics of populations
of large herbivores has been advanced by three recent
developments. The first has been the introduction of
nonlinear statistical modeling techniques that permit
exploration of the concurrent effects of external
conditions and density on numbers (e.g., Grenfell et al.
Manuscript received 16 June 2009; revised 2 October 2009;
accepted 12 October 2009. Corresponding Editor: M. K. Oli.
1
E-mail: [email protected]
1998, Coulson et al. 2000, Ellis and Post 2004, Stenseth
et al. 2004). The second has been the incorporation of
indices of large-scale climate into such models (Forchhammer et al. 1998, Post and Stenseth 1999), which,
besides increasing their explanatory power locally (e.g.,
Hallet et al. 2004), provide a basis for interpreting largescale ecological responses in the context of global
climate change (Forchhammer and Post 2004, Post
and Forchhammer 2006). The third has been emergence
of the view that environmental stochasticity (density
independence) is best integrated into the deterministic
(density-dependent) framework rather than being con197
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198
N. J. C. TYLER
sidered largely an adjunct to it (Coulson et al. 2001,
2004, Boyce et al. 2006).
The types of abiotic conditions incorporated into
analyses of population dynamics have, by contrast,
changed little. For northern ungulates, these fall into
two broad categories. The first consists of conditions
that influence animals’ energy expenditure, and hence,
their daily requirement for food, in those months when
quality and abundance of forage are lowest. Widely used
parameters include rainfall, temperature, and wind
speed, the selection of which has a solid physiological
basis (e.g., Moen 1973, Picton 1984). Thus, increased
levels of starvation mortality in red deer (Cervus elaphus)
during wet, windy winters (Clutton-Brock and Albon
1982, 1989) reflect the fact that this species is poorly
adapted to cold, especially on a low feed intake (Mount
1979). Though the susceptibility of particular species to
chilling is often left unstated, the validity of the weather
parameters commonly used is, nevertheless, usually
evident from their high explanatory power in models
of winter survival or population growth (Putman et al.
1996, Hone and Clutton-Brock 2007). Indices of largescale climate may sometimes be biologically more
meaningful than particular weather variables, insofar
as they integrate conditions across an entire season (e.g.,
Forchhammer and Post 2004, Hallet et al. 2004,
Stenseth and Mysterud 2005), but the physiological
basis of the causal relationships between environment
and the dynamics so described remains the same.
The second category pertains particularly to reindeer
and caribou (Rangifer tarandus ssp., hereafter Rangifer).
This species has evolved a thick coat of hollow hairs
(Timisjarvi et al. 1984) of such remarkable quality as
insulation in both still (Nilssen et al. 1984) and moving
air (Moote 1955, Cuyler and Øritsland 2002, 2004) that
neither calves nor adults are likely to suffer significant
chilling in winter save under extraordinarily severe
conditions. Consequently, when considering the effects
of environmental variation on the dynamics of populations of this species, attention has turned from factors
that influence energy expenditure, and hence, food
requirements in winter, to those that influence energy
gain in terms of access to forage and, specifically, its
availability beneath crusted snow and ice.
Snow is a major determinant of the availability of
forage on Arctic ranges in winter, and there are three
principal ways in which Rangifer respond to the difficult
foraging conditions that it may sometimes cause. They
may change their diet, switching to the kinds of forage,
such as arboreal lichens, that remain exposed above the
snowpack; they may move to areas where there is little
or no snow (and walking on and through snow is itself
energetically costly (Fancy and White 1987)); and, most
commonly of all, they may dig through the snow to
reach the plants beneath (Thing 1977).
The efficacy of all three responses is influenced by the
depth, density, layer structure, and hardness of the snow
pack. These characteristics, singly or in combination,
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Vol. 80, No. 2
influence not only the spatial distribution of the animals,
the selection of feeding sites and digging behavior
(Henshaw 1968, LaPerriere and Lent 1977, Skogland
1978, Miller et al. 1982, Thomas and Edmonds 1983,
Brown and Theberge 1990, Collins and Smith 1991,
Johnson et al. 2001), but also the composition and
quality of the diet (Adamczewski et al. 1988, Rominger
and Oldemeyer 1990, Kumpula 2001), and presumably
also the animals’ daily dry matter intake. It follows from
this that snow may also influence individual life histories
and population vital rates. Snow depth has been shown
to influence the survival of reindeer in winter (Kumpula
and Colpaert 2003), the timing of parturition (Adams
and Dale 1998a) and the birth mass of caribou calves
(Adams 2005). Such responses may be direct (nonlagged) or indirect (lagged) owing to the modulation, by
snow in winter, of foraging conditions in summer (Post
and Stenseth 1999, Forchhammer et al. 2002, Post and
Forchhammer 2008, Post et al. 2009b). The epitome of
the effects of snow on this species, however, is its
potential not merely to influence the dynamics of
populations through the modulation of vital rates, but
actually to precipitate mass starvation resulting in
catastrophic declines in numbers and even the extirpation of local populations (Pinegin 1932 in Formazov
1965, Scanlon 1940, Klein and Kuzyakin 1982, Meldgaard 1986). There is broad consensus that one major
cause of catastrophic die-offs of Rangifer is the
formation, occasionally over vast areas of the winter
range, of hard, crusted snow or layers of ice in the
snowpack or on the ground (properly called ‘‘basal ice’’)
which restricts the animals’ access to forage (Formozov
1965, Vibe 1967, Krupnik 1993, Miller 1998, Klein 1999,
COSEWIC 2004, Kohler and Aanes 2004).
Ice crusts can form on Rangifer ranges in several
different ways. One is as a result of repeated cycles of
thawing and refreezing of the snowpack (e.g., Formozov
1965, Forchhammer and Boertmann 1993, Kohler and
Aanes 2004), which in some regions may occur
frequently and at any time during winter (Tyler et al.
2008). Another is as a result of freezing rains (e.g., Miller
and Barry 2009) or rain on snow (e.g., Putkonen and
Roe 2003, Grenfell and Putkonen 2008). A third is by
spring meltwater trickling through the snow pack and
freezing when it comes into contact with very cold
ground beneath (Woo and Heron 1981, Woo et al.
1982). Basal ice between 4 and 18 cm thick has been
observed on Prince of Wales Island (Miller et al. 1982)
and up to 33 cm at other sites in the Canadian high
Arctic (F. L. Miller, personal communication), while ice
up to 30 cm thick has been reported on reindeer range in
Svalbard (Kohler and Aanes 2004). The maximum
thickness of ice, however, is almost immaterial in the
present context. Prostrate vegetation that is not merely
coated but actually embedded in basal ice is rendered
completely inaccessible to hungry reindeer or caribou,
irrespective of the depth of the ice layer. Moreover,
plants encased in ice will usually remain beyond reach
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SNOW, ICE AND POPULATION CRASHES
SELECTION
OF
DATA
Analysis was restricted to numerical declines in
populations of Rangifer described by published census
data that fulfilled two criteria. First, only ‘‘major’’
declines, defined as those in which the fall in numbers
exceeded 25% of the pre-decline population, were
considered. This value represents a point above which
declines in populations of large mammals appear
exceptional in terms of magnitude and rarity (Erb and
Boyce 1999). In contrast with two recent studies (Gunn
et al. 2003, Reed et al. 2003), no limit was imposed on
the duration of declines included in the analysis, but the
majority were population crashes over the course of a
single winter. In the case of multiyear declines, data were
included only where no interval between any two
successive counts or estimates of population size
describing all or part of a drop in numbers exceeded
four years. In fact, with few exceptions, the data for the
multiyear declines examined here consisted of uninterrupted series of annual counts or estimates.
Thirty-one major declines were found that fulfilled
these criteria and which, therefore, were accepted for
analysis. These involved decreases in numbers ranging
from 25% to 99% described in 12 populations of
Rangifer (Figs. 1 and 2). The earliest occurred in the
population of reindeer on St. George Island between
1922 and 1929, while the most recent occurred in the
Adventdalen population of reindeer on Svalbard during
the winter of 2007–2008.
Of the 12, six were wild populations (Adventdalen,
Banks Island, Bathurst Island Complex, Brøgger Peninsula, Coats Island and the Reindalen Complex), four
were to a greater or lesser extent managed semidomesticated herds (mainland Alaska, St. Paul Island,
Nunivak Island and Hagemeister Island), and two were
feral, i.e., although founded from semi-domesticated
stock, the animals were never managed and so returned
to a wild state (St. Matthew Island and St. George
Island).
Seven of the 12 were introduced populations that
arose following the liberation of animals into areas
where the species did not previously occur, or had been
absent for a long time, just a relatively short time (range
6–40 years) prior to their decline(s) described here. Five
of the introductions were made onto islands (St. George,
St. Paul, Nunivak, St. Matthew, and Hagemeister
Islands), one was made onto a peninsula (Brøgger
Peninsula), and one was onto mainland (Alaska). The
remaining five (Adventdalen, Banks Island, Bathurst
Island Complex, Coats Island, and the Reindalen
Complex) were long-established, natural populations.
The data that describe the temporal pattern of
development of the different populations were originally
gathered using a variety of methods. These included (1)
total counts made (a) on the ground in summer
(Adventdalen, St. Paul Island, St. George Island) and
(b) in late winter (Brøgger Peninsula), or (c) a
combination of aerial survey and foot counts (St.
Matthew Island), and (2) estimates based on aerial
(Bathurst Island Complex, Banks Island, Coats Island)
or foot survey transects (Reindalen Complex). In three
cases, estimates seem to have been based on less rigorous
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until the ice melts in spring (May or June in the high
Arctic), which may be weeks or sometimes months
hence.
Reference to devastating effects of icing on populations of Rangifer is a prominent feature of discussion of
the potential consequences of global climate change for
this species. It has been suggested that an increase in the
frequency of the conditions that cause icing could have a
serious impact on both wild and semidomesticated
populations (Aanes et al. 2002, Miller and Gunn
2003a, b, Putkonen and Roe 2003, Callaghan et al.
2005, Weller et al. 2005, Helle and Kojola 2008, Miller
and Barry 2009; see also Ball 2003, WWF 2008, Vors
and Boyce 2009). The supporting evidence, however, is
equivocal. Remarkably few measurements have been
made of the depth and extent of basal ice on the winter
ranges of Rangifer (Miller 1998, Kohler and Aanes
2004). The causes of population crashes are sometimes
conjectural (e.g., Gunn 1995) or even unknown (Bos
1967, Stimmelmayr 1994). Two questions deserve
clarification. First, what is the evidence for the
occurrence of crashes and major declines in populations
of Rangifer and in what ecological context do they
occur? Second, what is the evidence that severe snow
and ice conditions precipitate heavy mortality in this
species?
This paper examines data on major declines in
populations of Rangifer to determine what common
features these may have had, and therefore, what
common cause(s) they potentially may have shared.
There are four aspects: (1) to compare the frequency,
duration, and temporal pattern of decline between
different populations, (2) to examine the mechanics of
decline, i.e., the respective contributions of emigration
and mortality (natural and nonnatural) to the overall
reductions in numbers, (3) to review data on causes of
death, and (4) to examine evidence on the role of snow
and ice in promoting emigration or mortality. The
analysis reveals that none of 31 major declines identified
and considered here has been unusual in the sense of
requiring explanation beyond the reigning paradigm for
the regulation of the abundance of populations of large
herbivores. In every case, the dynamics appear to have
been a product of interaction between internal processes
(density dependence) and the external conditions including winter weather (density independence). An important aspect is that the strength and even the sign of the
density-independent component of observed dynamics
seems to vary widely between populations. There
appears, moreover, to be little objective evidence that
icing of the range is a ubiquitous and potent agent in the
dynamics of Rangifer, or even that it is actually the
principal cause of drastic population declines at all.
199
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Ecological Monographs
Vol. 80, No. 2
N. J. C. TYLER
FIG. 1. Maps showing the location of populations of Rangifer and other places mentioned in the text. Populations included in
the present analysis are in roman type. Other populations or place names are in italics. The geographical range of each population is
shown as solid black. The hatched area in panel (B) is the maximum distribution of semidomesticated reindeer on the mainland of
Alaska (after Palmer [1945]).
protocols (Nunivak Island, Hagemeister Island, mainland Alaska). The data sets are not distinguished in the
present paper by method used or level of accuracy
achieved, which in most cases is not known, and are all
referred to simply as ‘‘counts.’’
None of the estimates of the magnitude of the
different declines reviewed here can be more accurate
than the counts upon which they are based. This,
however, has little bearing on the central argument. In
any case, in which counting errors in fact inflated the
magnitude of a decline, the absence of fit between the
purported event and contemporaneous records of snow
or ice would be largely unsurprising. In the reverse
situation, where a decline was in reality more severe than
reported, the prediction under the null hypothesis is for
a strong association between the purported event and
icing conditions. Once again, absence of a fit, which is
what is generally observed, actually supports the main
argument of this study.
Population data are presented throughout as numbers. Little is gained by converting these to densities
owing to the diversity of habitat types and forage
conditions represented in the sample which ranges from
alpine meadow/sub-Arctic heath to high Arctic polar
desert. Reflecting this, the pre-decline densities of
populations that suffered severe levels of decline
(.97%) span two orders of magnitude, from to 0.2
caribou/km2 in the Bathurst Island Complex (high
Arctic [Miller and Barry 2009]) to almost 20 reindeer/
km2 on St. Matthew and St. Paul islands (sub-Arctic
[Gunn et al. 2003]). The problem is exacerbated by the
fact that the sample includes populations on the
mainland and on parts of large islands, where the
determination of population density itself introduces
bias owing to the difficulty of defining and measuring
the area of the range (Schaeffer and Mahoney 2003).
RESULTS
Duration and patterns of decline
Twenty of the 31 declines consisted of marked falls in
number over the course of a single year only (Fig. 2). All
of these are described by just two points, each
representing one count made in each of two successive
years. In the remaining 11 cases the populations declined
over a number of years (range 2–18 years). Several of
these multiyear events consisted of successive one-year
crashes in each year of which losses exceeded 25% of
each precrash total (Fig. 2). Eight of the multiyear
declines are described by uninterrupted series of single
annual counts made in successive years, and in which
each year’s total was lower than the preceding one. In
three cases (Alaska, St. Paul Island I, and Banks Island),
the series of annual counts describing the declines are
interrupted by gaps of between two and four years, but
in two cases (Alaska and St. Paul Island) the trajectory
of the population decline is unambiguous owing to the
high frequency of points both preceding its onset and
throughout the aftermath. The decline of the population
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SNOW, ICE AND POPULATION CRASHES
201
on Banks Island over 16 years from 1982 to 1998 is
described by just seven counts made at intervals of 1–3
years. The largest single interruption in this time series,
however, occurred after 1994, at which point the
population had already fallen to just 7% of its initial
(1982) total, so this gap in the series has virtually no
influence on the observed pattern of decline.
Each of the seven introduced populations displayed a
pronounced irruptive oscillation (sensu Caughley 1970),
in which numbers increased rapidly from the moment of
liberation up to a single, short-lived peak before
undergoing a marked decline over one (Nunivak Island,
St. Matthew Island, Brøgger Peninsula) or several years
(range ¼ 2–18 yr; St. George Island, mainland Alaska,
St. Paul Island, Hagemeister Island). The declines
following the initial irruption were smallest in the
populations on Hagemeister Island and St. George
Island (55% and 75%, respectively) while three populations (mainland Alaska, St. Paul Island I, St. Matthew
Island) decreased by .96%. No population died out
naturally; those that ultimately went extinct, including
the populations on St. George Island, St. Matthew
Island, and possibly Hagemeister Island, did so only as a
result of human intervention (Scheffer 1951, Klein 1968,
Stimmelmayr 1994). Reindeer on the Alaska mainland
showed the slowest population rate of decline (r ¼
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!
FIG. 2. Time series depicting the development of 12
populations of reindeer or caribou accepted for inclusion in
this analysis. Data are the total number of animals in all
populations, except for Coats Island, which are from animals
aged 1 yr. Solid dots represent annual counts (or estimates) of
population size. Counts (or estimates) made in successive years
are linked with black lines. Gaps of 2 but 4 years in data
describing ‘‘major declines’’ (see Selection of data) are indicated
by gray lines. Other missing data are indicated by gray dots.
Periods of decline .1 year are indicated by thin dashed lines
above each curve. Dates of major declines are given in each
graph. Where a population is recorded as having undergone
more than one major decline, these and their respective dates
are labelled I, II, or III in chronological sequence and are
referred to as such in the text. (Owing to their high frequency of
occurrence, no dates or labels are given for declines [arrows] in
Adventdalen and the Reindalen Complex.) All graphs share a
common linear time (x) axis. All data on population size (n) are
plotted on linear axes (y), but the population size scale differs
between graphs. The upper nine graphs have been set slightly
apart to draw attention to the fact that the population
trajectories displayed in each of these resemble one or more
irruptive oscillations (sensu Riney 1964, Caughley 1970; see
Selection of data). Populations that originated through artificial
introduction of small numbers of animals into new habitat are
indicated by stars. For the graph marked with a superscript A,
estimates of numbers at the second (1964) peak in the Nunivak
population vary from 15 500 (Bos 1967) to 23 000 (Swanson
and Barker 1992). The latter value is shown here. For the graph
marked with a superscript B, numbers for the Coats Island
population have been generated by multiplying population
densities (no./km2) extracted from Ouellet et al. (1996) by the
given size of the island (5600 km2). For the site marked with a
superscript C, Gates et al. (1986) reported the second crash on
Coats Island as having occurred during the winter of 1979–
1980, but Ouellet et al. (1996) argue that most animals probably
died in the winter of 1978–1979 (shown here). Sources: St.
George Island (Scheffer 1951); Alaska, mainland (Stern et al.
1980); St. Paul Island (Scheffer 1951, Swanson and Barker
1992); Nunivak Island (Bos 1967, Swanson and Barker 1992);
St. Matthew Island (Klein 1968); Hagemeister Island (Swanson
and Barker 1992, Stimmelmayr 1994); Coats Island (Ouellet et
al. 1996); Brøgger Peninsula (Aanes et al. 2000, Kohler and
Aanes 2004); Bathurst Island Complex (Tews et al. 2007);
Banks Island (COSEWIC 2004); Adventdalen (Tyler et al.
2008); Reindalen Complex (Solberg et al. 2008).
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N. J. C. TYLER
0.18); the fastest rate was on St. Matthew Island (r ¼
4.96).
In four cases (Brøgger Peninsula, Nunivak Island,
Hagemeister Island I, St. Paul Island), the irruptive
oscillation was followed by a more moderate fluctuation
that reached a peak five (Hagemeister Island II), eight
(Brøgger Peninsula II), 20 (Nunivak Island II) and 24
(St. Paul Island II) years after the initial maximum and
after which numbers underwent a second marked
decline. In each case, with the apparent exception of
St. Paul Island II, the population rate of decrease
following the second peak was substantially lower than
the rate of decrease following the first. How the rate of
the second decline on Nunivak Island compares with the
first depends on which of the two widely differing
estimates of the size of the population in 1964 is
accepted (see legend to Fig. 2). The population of
reindeer on Hagemeister Island is unique among the
seven introduced populations in that it apparently
underwent a second irruption in the late 1980s, some
20 years after the first. This irruption apparently
terminated in a decline in 1990–1991 and a reported
die-off in the winter 1991–1992, after which most of the
remaining animals were either moved off the island or
were shot (Stimmelmayr 1994).
Two established populations each displayed a spontaneous irruptive oscillation. The population of Peary
caribou (Rangifer tarandus pearyi) on the islands known
as the Bathurst Island Complex in the Canadian high
Arctic (which include Bathurst Island, Cameron Island,
Île Vanier, Massey Island, Île Marc, Alexander Island,
and Helena Island) apparently increased approximately
10-fold over 20 years from 1974 to 1994 before declining
by 97% over the following three winters (r ¼1.23, Fig.
2 [Miller 1998, Gunn and Dragon 2002, Tews et al. 2007,
Miller and Barry 2009]). The population of barrenground caribou (Rangifer tarandus groenlandicus) on
Coats Island also irrupted, increasing approximately 16fold from 1961 to 1974 before declining almost 90% over
the next two years (r ¼ 1.00 [Gates et al. 1986, Ouellet
et al. 1996]). This oscillation was followed by a more
moderate fluctuation in which numbers increased,
reaching a peak four years after the initial maximum
before undergoing a second marked decline (Coats
Island II; Fig. 2).
The number of Svalbard reindeer (Rangifer tarandus
platyrhynchus) in Adventdalen fluctuated vigorously
from year to year, while showing a slow, but accelerating, net increase over three decades (Fig. 2). The time
series of annual counts from 1979 to 2008 included seven
major population declines, six spanning one year each
(range 25–47%) and one net decline of 31.4% over two
years (1994–1995 and 1995–1996). The rate of decrease
varied across the seven declines from r ¼ 0.19 to r ¼
0.64 (Tyler et al. 2008).
The number of Svalbard reindeer in the Reindalen
Complex also fluctuated vigorously from year to year
(Fig. 2). The time series of annual counts here from 1979
Ecological Monographs
Vol. 80, No. 2
to 2007 likewise revealed seven major declines, all of
which spanned one year only (range 28–52%) and in
which the rate of decrease varied from r ¼ 0.32 to r ¼
0.74 (Solberg et al. 2008).
The remaining two declines, both in populations of
Peary caribou, are demographically isolated incidents
insofar as no detailed quantitative records of population
size exist prior to their onset save, in each case, for a
single estimate made 9 years previously. Numbers on
the Bathurst Island Complex decreased by 67% over the
winter of 1973–1974 and the population on Banks Island
suffered a monotonic decline of 95% between 1982 and
1998. In these cases the population rates of decrease
varied from r ¼ 0.19 (Banks Island) to r ¼ 1.10
(Bathurst Island Complex) and hence, fell within the
range of rates of decrease recorded in the other ten
populations.
Mechanics of decline
Precipitous declines in numbers, which are the subject
of this paper, can result only from very high rates of
emigration, mortality, or a combination of both;
partitioning losses between these two factors is therefore
quite fundamental. Where populations are harvested in
whatever form, it is likewise of interest to distinguish
between natural and nonnatural mortality. For Rangifer, the former includes chiefly death from starvation in
winter and predation, while the latter includes commercial harvesting, sport hunting, and poaching.
Published information on the causes of losses in the 12
populations, like that on numbers, is of mixed quality.
Data on mortality range from total counts and
inspection of carcasses made annually over many years
(Adventdalen) to nothing at all (Alaska, Nunivak
Island, St. George Island). Consequently, in most
reports, the mechanism of population decline has been
either ignored or inferred from the shreds of evidence
available rather than demonstrated directly.
Thirty of the 31 population declines seem, nevertheless, to have been principally a result of heavy natural
mortality, although in only one case (St. Matthew
Island) was this apparently the sole cause. One major
decline seems to have been principally a result of
emigration. In every other case (n ¼ 29) the declines
seem to have been the result of a combination of natural
and nonnatural mortality, with or without emigration,
although only in one case (Adventdalen) is it possible to
determine the relative importance of these different
components.
Emigration.—Six of the 15 declines recorded following
irruptions occurred in populations confined on islands
surrounded by 30–500 km of permanently open water,
which obviously represents an insurmountable barrier
for even the most determined Rangifer (St. George
Island, St. Paul Island I and II, Nunivak Island I and II,
and St. Matthew Island; Fig. 1). A further three
postirruption declines occurred in the population on
Hagemeister Island, which lies 5 km off the mainland of
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SNOW, ICE AND POPULATION CRASHES
were harvested regularly prior to and during their
decline(s), although detailed information on the number
of animals killed each year is available only from St.
Paul Island. Here, the level of harvest increased from 5%
in 1938, the year the population reached its peak, to 22%
in 1940 and 28% in 1941, the second and third years of
decline, respectively. No data exist for the remainder of
the period of decline, barring a small harvest in 1945,
but ‘‘some residents . . . believe that poaching [by troops
stationed on the island from 1942 to 1944] was a major
cause’’ [of the decline in numbers] (Scheffer 1951). On
Coats Island, the mean annual harvest during the period
1968 to 1983, which included the period when both
major declines occurred (Fig. 2), was 139 caribou (Gates
et al. 1986). This is sufficient to account for between 3%
and 15% of the decline in the three annual crashes
observed. Bathurst Island was the principal caribou
hunting area for the Inuit of Resolute prior to the
decline of the population in 1973–1974, after which a
voluntary ban was imposed on hunting caribou there
(Miller 1998). Hunting resumed in 1989, but the annual
harvest in the Bathurst Island Complex during the six
years up to 1995 was ,25 caribou (Gunn et al. 2006),
and could not, therefore, account for .3% of the decline
in numbers from 1994 to 1995. Hunters killed ;85
animals in 1995–1996 (Miller 1998), equivalent to 5% of
the decline in numbers in that year. The population on
Hagemeister Island was harvested sporadically throughout its existence, but the only data available on the
number of animals killed are for the autumn of 1992,
which marked the start of the deliberate extermination
of the population (Stimmelmayr 1994). The populations
on St. George Island, St. Matthew Island, and the
Brøgger Peninsula were not harvested at the time they
declined.
The data from the nonirrupting populations are
likewise somewhat heterogeneous. Approximately 1000
caribou were shot on Banks Island during five calendar
years, 1987–1991, when numbers fell from 4251 (in 1987)
to 1469 (in 1992 [Nagy et al. 1996, COSEWIC 2004]).
The resolution of the data here, however, is not
sufficient to determine the population rate of mortality
due to hunting. In Adventdalen, where all new carcasses
within the normal annual range of the population were
counted and examined for cause of death every year
from 1979 to 1984 and from 1988 to 2008, the annual
number of deaths from nonnatural causes (range, n ¼ 7–
29 reindeer) represented between 3% and 8% of the gross
decline in numbers in crash years (where gross decline is
the difference between the total population in year t 1
and the population less calves born in year t). The
corresponding population rates of nonnatural mortality
ranged from 1% to 3% (N. J. C. Tyler, unpublished data).
Approximately 1700 reindeer were shot in the Reindalen
Complex between 1983 and 2008 (Pond et al. 1993,
Irvine et al. 2000, Côté et al. 2002; Records of the
Governor of Svalbard, unpublished data) including on
average ;70 reindeer (range ¼ 50–90 reindeer) in crash
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Alaska (Fig. 1). Even this, however, seems beyond the
aquatic range of Rangifer. The mean width of routes
across the sounds in northern Norway which herds of
semidomesticated reindeer once swam twice annually
when moving between mainland and island pastures, for
instance, is 2.3 km (maximum 4.2 km [Paine 1994]).
Finally, the spatial extent of reindeer herding operations
on mainland Alaska contracted massively during the
postirruptive decline (Stern et al. 1980). In these 10
instances, therefore, emigration can be excluded as a
factor contributing to the decline(s) in numbers.
The Canadian Arctic islands, by contrast, are linked
by sea ice in winter, enabling the caribou to move
between them on a scale sufficient to influence the
dynamics of local populations (Miller et al. 1977, 2007).
Similarly, despite its remote location, the population of
caribou on Coats Island is indigenous, and so animals
must have walked there across sea ice at some stage in
the past, probably from Southampton Island some 70
km away. There is, however, no suggestion that either of
the two declines in numbers observed in this population
in the 1970s was exacerbated by an exodus of animals
from the island (Gates et al. 1986, Ouellet et al. 1996).
There is evidence of caribou leaving Bathurst Island
across sea ice during the 1994–1995 and 1995–1996
declines, but although the numbers of animals that
moved is unknown, in the absence of evidence to the
contrary emigration is not thought to have been a major
component of the decline of the population there
between 1994 and 1997 (Miller 1998, Gunn and Dragon
2002, COSEWIC 2004).
The Brøgger Peninsula is an exception among the
irrupting populations. In this case, numbers fell from
360 reindeer prior to calving in 1993 to 78 reindeer 12
months later, yet there was no evidence of heavy
mortality. Just 20 carcasses were found following the
crash (Fuglei et al. 2003). Such a meagre tally of
carcasses, together with the appearance of reindeer in
previously unoccupied areas close (;4 km) to the
peninsula (Blomstrand Peninsula and Sarsøyra; Fig.
1E [Henriksen et al. 2003, Hansen et al. 2007]), makes it
impossible to reject the possibility that the decline was
principally a result of reindeer leaving the area by
walking across sea ice or even over glaciers.
Emigration probably also contributed to the declines
in numbers in nonirrupting island populations. Thus,
Inuit hunters believed that caribou moved off Bathurst
Island during the crash winter of 1973–1974 (Fig. 2
[M. M. R. Freeman, unpublished manuscript, as referenced in Miller et al. 1977]). There is also abundant
evidence of caribou walking over the ice to and from
Banks Island, and although no numbers are available,
emigration was suspected as having contributed to the
prolonged decline of this population during the 1980s
and 1990s (Nagy et al. 1996).
Nonnatural mortality.—At least five of the irrupting
populations (mainland Alaska, Bathurst Island Complex, Coats Island, Nunivak Island, and St. Paul Island)
203
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204
N. J. C. TYLER
years. Hunting mortality, therefore, represented between
11% and 56% of the net decline in numbers (i.e., the
difference between the total population in year t 1 and
year t) in crash years in this area. The corresponding
population rates of mortality from hunting in these
years ranged from 6% to 15%.
Natural mortality.—
1. Carcass counts.—Fifty-six carcasses counted on the
Bathurst Island Complex in June 1995 represent 2% of
the estimated postcalving populations in the preceding
summer (Gunn and Dragon 2002). The 241 carcasses
examined on St. Matthew Island represent approximately 4% of the total number of animals lost (Klein
1968). An estimated 150 carcasses seen on St. Paul
Island in the spring of 1940 represent ;8% of the
population there the previous summer (Scheffer 1951).
The magnitude of the decline of reindeer on the Brøgger
Peninsula during the winter of 1993–1994 is not known,
because no postcalving census was made in 1993. The
best estimate, based on the difference between late
winter surveys made in each of the two years, indicates a
loss of not less than 282 reindeer. Twenty carcasses
found in 1994 following the crash (Fuglei et al. 2003)
represent 7% of this value and 6% of precalving
population in 1993.
Results from more rigorous counts are qualitatively
similar. Thus, systematic aerial survey of the Bathurst
Island Complex in 1996 and 1997 yielded 143 and 82
carcasses, respectively, which represented 7% and 15%,
respectively, of the estimated postcalving populations in
each of the preceding two summers (Gunn and Dragon
2002). A systematic aerial survey of Coats Island made
following the 1974–1975 crash revealed 702 carcasses
(Gates et al. 1986), equivalent to ;11% of the
population (less calves) observed in the previous summer
(Ouellet et al. 1996). Estimates of mortality were made
in 3 of the 16 years (1982–1998) of the decline on Banks
Island. McLean and Fraser (1992) and Fraser et al.
(1992) counted 29 and 6 carcasses during aerial surveys
there in 1989 and 1991, respectively, and estimated total
mortality for 1988–1989 and 1990–1991 at 300 and 60
caribou, respectively. No estimates of population size
exist for 1988 and 1990, and the data cannot therefore be
expressed as rates of mortality. An estimate of 100
deaths in 1987–1988 (B. McLean, unpublished data, in
Nagy et al. 1996) is equivalent to 2% of the population
estimate for 1987 (COSEWIC 2004).
On average, 30 carcasses (range ¼ 1–70 carcasses)
were counted in summer following six of seven
population crashes in the Reindalen Complex, representing between 0.5% and 44% of losses in each crash;
the corresponding population rates of natural mortality
ranged from 0.2% to 12% (Solberg et al. 2008). Data
from Adventdalen are broadly similar. Here, on average
136 carcasses (range ¼ 71–178 carcasses, natural deaths
only) were counted following crashes. These represented
between 20% and 69% of recorded losses; the corresponding population rates of natural mortality ranged
Ecological Monographs
Vol. 80, No. 2
from 7% to 21% (mean ¼ 13%; N. J. C. Tyler,
unpublished data).
2. Estimates of mortality.—Using data from the
carcass counts just described, Miller (1998) and Gunn
and Dragon (2002) estimated that 1143 and 408 caribou
died on the Bathurst Island Complex during the winters
of 1995–1996 and 1996–1997, equivalent to 52% and
74%, respectively, of the estimated postcalving populations in the summers of 1995 and 1996, respectively
(Miller 1998). Likewise, the estimate by Gates et al.
(1986) of 4415 caribou having died on Coats Island
during the 1974 –1975 crash represents a rate of
mortality of 71%. Stimmelmayr (1994) estimated 276
carcasses on Hagemeister Island in June 1992, equivalent to 29% of the estimated postcalving population in
1991.
There appear to be no published data on rates of
mortality during the declines of the populations in
Alaska, on St. George Island, St. Paul Island II,
Nunivak Island (I and II), Hagemeister Island (I and
II), Coats Island (II) or on the Brøgger Peninsula (II).
Causes of death.—
1. Direct evidence.—Cause of death was determined
for cases of natural mortality following 12 of the 31
population declines considered here. In Adventdalen,
where all carcasses (n ¼ 931 carcasses) of reindeer that
died during six of seven crash years were inspected, 79%
of deaths (including 91% of natural deaths [n ¼ 812
deaths]), were attributed to starvation in winter (Tyler et
al. 2008). The second most important cause of winter
mortality, physical trauma resulting from animals falling
down cliffs, accounted on average for 2% (range ¼ 0–
3%) of annual mortality in crash years (Tyler 1987;
N. J. C. Tyler, unpublished data). Gates et al. (1986)
concluded that ‘‘winter starvation is common on Coats
Island,’’ based on examination of 34 carcasses altogether
(0.5% of estimated losses) found following the two
crashes there. Klein (1968) concluded that starvation
was ‘‘the cause of death’’ during the 1963–1964 crash of
the population on St. Matthew Island, based on the
absence of fat in the medullae of the long bones of
carcasses (sample size not given) that he examined there
in 1966. Stimmelmayr (1994) examined 54 carcasses on
Hagemeister Island in 1992, the second year of the third
decline there, and concluded that most of these and, by
inference, the remaining 222 reindeer thought to have
died that year, had probably starved to death. Six
caribou carcasses (1% of estimated losses) were inspected on Bathurst Island following the 1973–1974 crash; all
six were thought to have died ‘‘after the winter’’; three
had been killed by wolves, and three were judged to have
died from ‘‘malnutrition’’ (Parker et al. 1975). A further
six carcasses (0.2% of estimated losses) were inspected in
the Bathurst Island Complex following the 1995–1996
crash; the ultimate cause of death in this case was
considered to be ‘‘prolonged extreme malnutrition’’
(Miller 1998). It is thus apparent that not only has
cause of death been determined in rather less than half
May 2010
SNOW, ICE AND POPULATION CRASHES
of all declines, but also that even in these, postmortem
examination was generally carried out on only a tiny
fraction of the animals believed to have been lost.
2. Inference.—There are few quantitative or even
qualitative data on causes of death in any of the
remaining cases. Starvation was probably prevalent in
those populations that suffered negligible predation and
only modest (St. Paul Island, Nunivak Island, Reindalen
Complex) or no harvest (St. George Island and the
Brøgger Peninsula). Data from the Reindalen Complex,
in particular, bore a striking resemblance to results from
Adventdalen, where starvation was shown to be the
principal cause of death. In both areas the ages at death
of animals whose carcasses were inspected following
crashes were bimodally distributed, being largely restricted to very young (,1 yr) or old (7 yr) individuals
(Tyler 1987, Solberg et al. 2001). Notwithstanding the
three estimates of winter mortality on Banks Island,
Nagy et al. (1996) concluded that there was no evidence
of ‘‘substantial die-offs’’ having occurred, but that
several mortality factors including hunting, ‘‘severe
winters’’ (presumably meaning starvation), and predation contributed jointly to the decline of the population.
Snow and ice
annual sum of precipitation during days with temperature .08C from October to May, inclusive, in an
analysis of the dynamics of reindeer in the Reindalen
Complex. In neither case was the proxy estimate
accepted for inclusion in the resulting autoregressive
models of variation in population growth, though
Solberg et al.’s (2001) icing index was able to explain
part of the variation in demographic variables. Tyler et
al. (2008) used an ablation index as a proxy in their
analysis of the temporal dynamics of the population of
reindeer in Adventdalen. This index quantified the heat
input for the melting of snow in terms of the sum of
sensible heat [ambient temperature 3 mean wind speed]
during periods of mild weather (.08C) in winter (see
also Tyler and Forchhammer 2009). Contrary to
expectation, there were positive associations between
ablation and both the survival of reindeer in winter and
the subsequent abundance of reindeer in summer. The
effects were apparent in years when the population
increased (Rt . 0) but ablation had no effect on survival
or numbers in years when the population declined (Rt 0).
No direct assessment of snow hardness and ice
conditions was made during first 10 years (1982 to
1992) of the decline of caribou on Banks Island, when
the population fell from approximately 11 000 to 1400
animals. Nagy et al. (1996) referred in the legend of their
Fig. 3 to three ‘‘winters which had freezing rains’’ during
this period. These were the three years in which dead
caribou were reported (see Carcass counts) but no data
on prevailing weather conditions were provided either in
the text of their paper or in the original fieldwork reports
(Fraser et al. 1992, McLean and Fraser 1992) on which
this single mention of the ‘‘freezing rains’’ was based.
Four studies used snow depth as a proxy of range
conditions in crash years, although with varying degrees
of rigor. Klein (1968) used the greatest accumulation of
snow on the ground during the latter half of the winter
of 1963–1964, which is when the die-off of reindeer on
St. Matthew Island is believed to have occurred.
Lacking local measurements, he used data from
Nunivak and St. Paul Islands, each of which are ;350
km distant. The values for February to April 1964,
inclusive, were the highest recorded at both stations in
20 years, while the average temperature for February
was the lowest and second lowest, respectively, at each,
as well. Reviewing the status of Peary caribou on the
western Queen Elizabeth Islands in the early 1970s,
including the decline of caribou in the Bathurst Island
Complex during the winter of 1973–1974, Miller et al.
(1977:14) pointed out that no information on ‘‘. . . the
type of snow cover nor the incidence of ground-fast ice
or ice layering is available for the western Queen
Elizabeth Islands.’’ They emphasized this, commenting
that ‘‘. . . we have no quantitative measures of range
condition’’ (Miller et al. 1977:46), and it is presumably
for this reason that they did not elaborate their
observation that ‘‘. . . fragmentary weather data and
REVIEWS
With two exceptions, none of the publications in
which any of the 31 population declines considered here
were originally reported include any observations, either
objective measurements or subjective assessments, of
hard or crusted snow or ice on the range in the years of
decline. Scheffer’s (1951) report of the rise and fall of the
herd of reindeer on St. Paul Island is one exception. His
account states that in 1940, the third of the 12 years of
the decline, ‘‘According to the island records . . . a crust
of glare ice remained on the snow for several weeks . . . .’’
That is all. The other exception is Larter and Nagy’s
(2000) study on Banks Island, in which systematic
measurements of the depth and hardness of snow were
made through five successive winters. Their results,
however, throw little light on the role of snow and ice
conditions in the decline of the population of Peary
caribou on that island, because numbers had already
fallen by 87% before data collection started in October
1993. Moreover, despite freezing rain reported early in
winter that year and a mean value of snow hardness that
was 25 times greater than in the following four winters,
there was no die-off and the rate of survival of calves
over the winter was the highest they recorded.
In the absence of any direct assessment of snow
hardness and ice conditions, three studies developed
proxies, based on data on the meteorological conditions
thought likely to cause icing, which were incorporated
into retrospective analyses of patterns of population
growth. Aanes et al. (2000) examined the effect on the
rate of increase of the population of reindeer on the
Brøgger Peninsula of the number of days in winter
(October to April inclusive) each year in which
temperature was 08C. Solberg et al. (2001) used the
205
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206
N. J. C. TYLER
Ecological Monographs
Vol. 80, No. 2
FIG. 3. Magnitude of population declines (%) in relation to the duration of the pre-decline periods of growth (time to peak, in
years) in different populations of reindeer and caribou (data from Fig. 2). Time to peak is the interval from the year of introduction
or the first datum following the last recorded decline, as appropriate, to the year of peak numbers at the onset of each current
decline. Observations are in three classes (type of dynamics): declines following irruptions (solid circles); in subsequent dampened
oscillations or following a second irruption (gray circles); or in a fluctuating, persistently unstable established population
(Adventdalen, open circles).
empirical evidence, however, suggests a series of
consecutive years with early, deep snow cover; above
average snowfalls; late winter, deep snow cover;
lingering snow melts and in some years ground-fast
ice, ice layering in the snow and heavy crusting of the
snow all made forage unavailable and restricted’’ (Miller
et al. 1977:46–47). Twenty years later, following the
decline in numbers there from 1994 to 1997, Miller
(1998:49) observed that ‘‘. . . all years known to be
associated with severe mortality . . . are linked to winters
with significantly greater . . . than average total snowfall.’’ Miller and Gunn (2003a, b) subsequently showed
that the four largest known annual declines in numbers
of the caribou in Bathurst Island Complex (1973–1974,
1994–1995, 1995–1996, 1996–1997) occurred during
four of the five snowiest winters (in terms of ‘‘total
snowfall’’ in winter) recorded at Resolute (1947 to 2002)
some 160 km distant (see also Miller 1998:5). Gates et al.
(1986) and Ouellet et al. (1996) were also obliged to use
meteorological data from a remote station (Coral
Harbour on Southampton Island, some 140 km distant)
for their analyses of the dynamics of the population of
caribou on Coats Island. The former showed that
throughout the winter of 1974–1975, when the population suffered its greatest single crash (Coats Island I;
Fig. 2), snow depth at Coral Harbour was substantially
above the mean for the period 1970–1980. No data are
given for the second winter of that decline. By contrast,
in the winter of 1979–1980, which, according to their
report, was when the second crash occurred (Coats
Island II [Fig. 2]), the depth of snow at Coral Harbour
was substantially below the same 10-year mean,
although they stated that ‘‘based on subjective observations snow appeared sufficiently deep [on Coats Island]
in April to limit access to forage’’ although ‘‘[t]here was
no evidence of ice layers or ground fast ice in the snow
column’’ (Gates et al. 1986). In apparent contradiction,
Ouellet et al. (1996) found that total snowfall was
substantially below a 20-year (1971–1990) mean in both
1974–1975 and 1978–1979 (the winter in which they
believed the Coats Island II crash in fact occurred) and,
similarly, their ‘‘Relative winter severity index’’ was very
low (i.e., favorable) in both winters. Both values for the
winter of 1975–1976 (the second year of the Coats Island
I decline) were close to the 20-year mean. Finally,
commenting on the death of reindeer on Hagemeister
Island in the winter of 1991–1992, Stimmelmayr (1994)
reported a local observation that ‘‘snowfall was greater
than in other years.’’
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SNOW, ICE AND POPULATION CRASHES
DISCUSSION
Major declines of populations of Rangifer fall into
two categories: irruptive oscillation, which is mainly
associated with populations introduced into new habitat, and numerical fluctuation in persistently unstable
established populations. The mechanics of decline
appear to differ widely within both categories, ranging
from wholly mortality to almost wholly emigration,
albeit that data on this, like those on the extent and
causes of mortality, exist in only a few cases and are
largely fragmentary even in these. A conspicuous feature
of the studies examined here is the virtual absence of
data confirming the presence of hard snow or basal ice
on the range during winter(s) in which populations
declined. In the only case in which the depth and
hardness of snow was measured in the field contempo-
raneously with the collection of demographic data on
caribou, apparently exceptionally unfavorable conditions were associated with high survival over winter
(Larter and Nagy 2000). Moreover, it has been common
practice to consider declines almost as integral phenomena, in effect removing them from their ecological
context before attempting to explain them. In the few
cases where a holistic approach has been adopted, i.e.,
using time series analysis to examine the simultaneous
effects of density-dependent and density-independent
factors across the full temporal record of dynamics,
climatic conditions associated with increased amounts of
snow or with winter warming have been found to
enhance the abundance of animals, at least in established populations.
Two classes of decline
The commonest class of major decline in populations
of Rangifer is the fall in numbers during the negative
phase of an irruptive oscillation. Nine of the 12
populations considered here irrupted (sensu Caughley
1970), seven of them following the introduction of
animals into novel habitat. In all nine, numbers
described a pronounced asymmetric cycle consisting of
a steady increase over several years followed by a
marked decline. The duration of the decline phase varied
considerably, ranging from a single winter, as in the
spectacular crashes of the populations on Nunivak
Island I, St. Matthew Island, and the Brøgger Peninsula
I, to steady declines lasting as much as 18 years (Alaska;
Fig. 2). With the exception of the population on
Hagemeister Island, the irruptive oscillation was a
unique phenomenon in the development of each
introduced population, although in five cases (St. Paul
Island, Nunivak Island, Hagemeister Island I, Coats
Island, and the Brøgger Peninsula) it was followed by a
second, dampened oscillation. Taken together, falls in
numbers following irruptions and the associated secondary oscillations account for 15 of the 31 population
declines recorded here.
The second common type of decline is the negative
phase of numerical fluctuations in persistently unstable
established populations. Declines of this type represent
14 of the 31 reported here, and in contrast with irruptive
oscillations, they were far from unique in the populations in which they occurred. Each of two fluctuating
populations, both in Svalbard (Adventdalen and the
Reindalen Complex), displayed seven declines in 30
years’ continuous observation, representing a mean
frequency of one crash approximately every 4 years in
each population (Fig. 2). The two populations might be
considered special cases because, unusually among wild
populations of Rangifer, they suffer little hunting and
negligible predation. Such circumstances, however,
would normally be expected to exacerbate rather than
ameliorate the effects of weather on numbers, besides
which a similar pattern of growth has been observed in
populations of caribou in west Greenland, albeit
REVIEWS
I am not aware of any published information on snow
or ice conditions in relation to the declines in numbers of
reindeer on mainland Alaska, on Hagemeister Island (I
and II), on Nunivak Island (I and II), on St. George
Island, or on St. Paul Island (II).
There are three cases in which icing, though not
initially presented as an explanatory variable, has
subsequently been advanced as a major cause of
observed declines. Thus, the original report of the
irruption of reindeer on Brøgger Peninsula from 1978
to 1998, including the huge fall in numbers in 1993–
1994, made no reference to icing (Aanes et al. 2000). Nor
was any relationship found between the annual rate of
increase of the population and the conditions believed to
be associated with the formation of basal ice. Aanes et
al. (2000) concluded, instead, that precipitation in winter
was a major determinant of variation in the annual rate
of increase. In a subsequent paper reporting the same
irruption and crash, Aanes et al. (2002) mentioned an
‘‘ice-crust,’’ which was believed to have formed over the
range during rainy weather in November 1993, and they
now attributed the crash to this, instead. Two years
later, Kohler and Aanes (2004), carefully emphasizing
the paucity of records of basal ice in Svalbard,
developed a model in which a ‘‘ground-icing parameter’’
best explained fluctuations in reindeer numbers on the
Brøgger Peninsula. The two other cases are the declines
of caribou in the Bathurst Island Complex during 1973–
1974 and 1994–1997. The original report of the 1973–
1974 decline included no data on weather conditions
(Miller et al. 1977), while the first report of the 1994–
1997 decline included only data on, and emphasized the
role of, ‘‘total snowfall,’’ which was significantly greater
than average in those years (Miller 1998:49). As in the
case of the crash on the Brøgger Peninsula, emphasis on
ice appeared later. Four subsequent reports unambiguously attributed the declines in numbers of caribou in
the Bathurst Island Complex to the effects of deep snow
and ice without, however, supporting the revised claim
with new data (Gunn Miller and Nishi 2000, Gunn and
Dragon 2002:68, Gunn et al. 2003, COSEWIC 2004).
207
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N. J. C. TYLER
reflected in a proxy of population size (Forchhammer et
al. 2002). Hence, persistent instability is not a peculiar
feature of high-Arctic populations of Rangifer, or those
that suffer little persecution.
The declines of populations of Peary caribou in the
Bathurst Island Complex in 1973–1974 and on Banks
Island between 1982 and 1998 do not readily fit either
category. In both cases there are simply too few counts
prior to and following the decline to place its trajectory
in a broader numerical context. The sustained decline on
Banks Island is unique among the five established
populations in the present sample by virtue of its
duration (16 years). Without exception, all the other
declines in established populations, whether irrupting
(Bathurst Island Complex II, n ¼ 1) or fluctuating
(Adventdalen and the Reindalen Complex, n ¼ 14) were
precipitous, lasting generally only one year, and never
more than three years.
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De-demonizing declines
Classifying declines in this way to a large extent dedemonizes their significance in the temporal pattern of
the growth of populations of Rangifer, while not,
however, diminishing it. Neither class of decline presents
any challenge to, nor requires any explanation beyond,
the reigning paradigm for the regulation of abundance
in populations of large herbivores. Though several of the
accounts are meagre and the data sparse, in every case
the observed dynamics are consistent with numbers
responding to the simultaneous effects of competitive
interactions between individuals (density dependence)
and climate perturbations (density independence) according to principles that have been tested and explored
in detail in many species of large herbivores (e.g., red
deer [Forchammer et al. 1998], Soay sheep [Ovis aries;
Grenfell et al. 1998, Coulson et al. 2000, 2001, Stenseth
et al. 2004], moose [Alces alces; Post and Forchhammer
2001, Ellis and Post 2004], and Rangifer [Post 2005,
Forsyth and Caley 2006, Tyler et al. 2008]).
Without exception, all the really spectacular declines,
ranging in size from 52% to 99% and therefore
‘‘cataclysmic’’ sensu Miller (1998), were associated with
population irruptions. A remarkable feature of these
declines is the variety of ecological circumstances under
which they occurred. The irruptions occurred in a
mainland, a peninsula, and seven island populations;
in the high Arctic and in the subArctic; in introduced
and in established populations; in semidomesticated,
feral, and wild populations, some of which were heavily
harvested while others were free of persecution. No less
remarkable is the fact that, notwithstanding their
ecological diversity, in every case numbers described a
trajectory which, from the initial steady increase to the
subsequent marked decline, closely matches the main
features of Riney’s (1964) sketch of the numerical
response of populations of large herbivores adjusting
to the carrying capacity of their environment or, as we
are more inclined to express it today, responding to an
Ecological Monographs
Vol. 80, No. 2
abundance of forage (see also Caughley 1970, 1976,
Caughley and Lawton 1981, Forsyth and Caley 2006).
In this model, which Riney (1964) considered generally
applicable, irruptive oscillation is regarded as a direct
consequence of the interaction between a population
and its food supply, and the postirruptive decline in
numbers, in particular, is viewed primarily as a result of
the depletion of resources caused by the animals
themselves. The situation is complicated at high
latitudes by the fact that the per capita supply of food
in winter is a function not only of the standing crop of
edible plants, but also of the characteristics of the
snowpack that determines the availability of forage for
the animals (Vibe 1967, LaPerriere and Lent 1977,
Skogland 1978, Miller et al. 1982, Adamczweski et al.
1988, Miller 1998). Nevertheless, in six of the irrupting
populations (mainland Alaska [Palmer 1926, 1934, Stern
et al. 1980]; St. Paul Island [Scheffer 1951]; Nunivak
Island [Bos 1967]; St. Matthew Island [Klein 1968];
Hagemeister Island [Swanson and Barker 1992, Walsh et
al. 2007]; Brøgger Peninsula [Hansen et al. 2007]), the
initial rise in numbers was shown to have been
accompanied by heavy exploitation of the natural
forage, especially lichens. In all these cases, therefore,
the fundamental, and predictable, cause of the cessation
of the irruption and of the subsequent decline in
numbers was almost certainly the depletion of resources
(see also Leader-Williams 1988). Moreover, on the
Brøgger Peninsula, which is the best documented of
those six cases, the environment has shown signs of
recovery during the immediate postirruptive phase, with
an increase in the cover of important forage species,
exactly in accordance with the prediction of Riney’s
model (Hansen et al. 2007). Little is known about the
role of plant–herbivore interaction in the two remaining
cases (St. George Island and Bathurst Island Complex
II) because range conditions were not evaluated at the
time. It is reasonable to assume both that the reindeer
enjoyed an abundance of forage at the time of their
liberation onto St. George Island, and that this was
principal cause of the ensuing irruption. However, the
impact of the animals on the vegetation was never
documented: Scheffer’s (1951) comment that ‘‘lichens
are still present in fair amounts’’ tells us little, because it
refers to the situation on the island almost three decades
after the peak of the irruption. There are no data on
changes in the biomass or cover associated with the
irruption and decline of the population of Peary caribou
in the Bathurst Island Complex.
Two decades ago Leader-Williams (1988) pointed out
that Riney’s (1964) model does not explain the dramatic
population crashes seen among some introduced populations of reindeer. He singled out the crash on St.
Matthew Island as a case in point. Today, we might also
include, for instance, the crashes on Coats Island (I), on
the Brøgger Peninsula (I), and in the Bathurst Island
Complex (II), albeit that one of these was not on an
island, and the latter occurred in an established
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SNOW, ICE AND POPULATION CRASHES
ecological systems. In both cases, the erratic pattern of
growth has been shown to be a consequence of the
simultaneous and interactive effects of weather and
population density on the annual schedule of survival
and fecundity (Solberg et al. 2001, Tyler et al. 2008). The
dynamics are complex, reflecting the fact that the
relative strengths of the density-dependent and the
density-independent processes acting on numbers are
not fixed but vary between different phases of population growth (Tyler et al. 2008). This observation,
however, only reinforces the view that the erratic pattern
of growth is in no sense ecologically aberrant and begs
no explanation outside the conventional model.
The present observations (Fig. 2) are, in fact, not
merely consistent with the view that the trajectory of
population growth is governed chiefly through the
dynamical relationship between the animals and their
resources, as originally outlined in Riney’s (1964) model,
but actually provide support for it. The test is whether
the magnitude of decline principally reflects the level of
depletion of resources, irrespective of other considerations including the form of management (semidomesticated or wild population), predation/hunting pressure,
and even abiotic forcing. A key assumption is that the
level of depletion of resources is proportional to the
duration of the population growth phase prior to each
major decline. Comparison across populations reveals a
strong influence of duration of the pre-decline period of
population increase on magnitude of decline, albeit with
type of dynamics as a major contributing factor.
Proportional declines have been largest following initial
irruptions, smallest in population fluctuations and
intermediate following secondary, dampened oscillations (Fig. 3). The consistency of this relationship across
populations implicates the action of a common factor in
determining the magnitude of major declines in populations of Rangifer. Cumulative effect of sustained
grazing is obviously a prime candidate for this.
Moreover, the fact that secondary oscillations have
been detected in some postirruptive populations of
Rangifer, a species so obviously far removed from those
Riney (1964) considered, is a wonderful confirmation of
the general applicability of his model. Their existence in
populations of Rangifer emphasizes the fundamental
importance of the dynamics of plant–herbivore interactions in determining the trajectory of population
irruptions in large herbivores in general.
Another notable feature of the examples drawn
together here is the diversity of ways in which
populations in fact decrease in size. The crash of the
population of reindeer on St. Matthew Island is the only
one among all 31 major declines that was unequivocally
wholly a result of natural mortality. In every other case
one or more factors, including emigration, hunting/
harvesting, predation, and starvation, contributed to the
drop in numbers (e.g., Scheffer 1951, Gates et al. 1986,
Nagy et al. 1996, Gunn and Dragon 2002). Even the few
cases supported by direct evidence of heavy natural
REVIEWS
population. Emigration and climate, he suggested,
probably modified the form of population declines on
some islands (Leader-Williams 1988). Gunn et al. (2003)
revived this argument, suggesting that there is a
qualitative as well as a quantitative difference between
a postirruptive decline (sensu Riney 1964) and a
population crash. Their definition of a crash (‘‘at least
a 30% decrease in the estimated population size in 1
year’’) comfortably embraces the declines on St.
Matthew Island (99%), both years of the 1974–1976
decline on Coats Island (71% and 53%, respectively), on
the Brøgger Peninsula (I: 78%) and all three years of the
1994–1997 decline on Bathurst Island Complex (33%,
75%, and 86%, respectively). The abruptness of a
‘‘crash,’’ they pointed out, contrasts sharply with the
relatively slow rate of the postirruptive decline described
in Riney’s (1964) model, the period of which is
approximately half that of the phase of population
growth it succeeds (see Forsyth and Calow 2006). The
difference, in their view, was that while the postirruptive
decline is by definition a product of the dynamical
relationship between animals and their food supply,
populations of Rangifer crash owing to shortage of food
caused principally by ‘‘extreme environmental variation,’’ in particular ‘‘exceptionally severe snow and ice
conditions.’’ In their view ‘‘the evidence for the primary
effect of . . . weather [was] overwhelming,’’ while densitydependent effects caused by intraspecific competition for
forage were ‘‘secondary,’’ though they could not be
‘‘completely exclude[d]’’ (Gunn et al. 2003). The issue
hinges on the relative importance of, and the strength
and form of associations between, intrinsic factors and
environmental variation in determining the dynamics of
the populations. A key point, now well recognized, is
that the effects of climate and population density
interact, such that the influence of weather becomes
increasingly pronounced as population density increases
(see Grenfell et al. 1998, Coulson et al. 2001, Ellis and
Post 2004, Stenseth et al. 2004). Indeed, recent analyses
of the crashes on the Brøgger Peninsula and the Bathurst
Island Complex II have tended to adopt an orthodox
position, suggesting that these crashes may be best
explained in terms of the synergistic effects of depletion
of forage associated with high population density and
unfavorable weather conditions (Hansen et al. 2007,
Tews et al. 2007, but see Miller and Barry 2009). This
emerging view is supported by results from analyses of
demographic time series in established populations (see
Heterogeneous dynamics, regional variation in climate,
and icing conditions).
The recurring crashes observed in persistently unstable populations likewise fit comfortably within a
conventional theoretical framework. The two detailed
long-term series available show how crashes are a
frequent and characteristic feature of the temporal
dynamics of established populations of Svalbard reindeer. They are in no way exceptional phenomena that
belong at the periphery of the normal working of these
209
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210
N. J. C. TYLER
mortality reveal disconcerting heterogeneity. Thus, there
is only one instance, once more the crash on St.
Matthew Island, in which death cut right across a
population such that animals of all age classes and both
sexes perished (Klein 1968). In every other case for
which there are data, the distribution of age-at-death
was strongly skewed, indicating large differences in agespecific rates of mortality, even during major declines.
Thus, carcasses recovered following each of five die-offs
in Svalbard consisted almost exclusively of calves (age
,1 yr) and old animals (age 7 yr), while intermediate
age classes (1–6 yr old) represented just 2% of the
combined sample (n ¼ 557 [Tyler 1987, Solberg et al.
2001]). Evidently subadult and prime-age Svalbard
reindeer are magnificently robust and capable of
withstanding the very conditions that precipitate great
crashes of the populations of which they are members.
In the one remaining case, the data were strongly skewed
with respect both to age and sex. Thus, 40% of the
carcasses of reindeer found following the decline of the
population on Hagemeister Island during the winter of
1991–1992 were subadults (age 1 yr [Stimmelmayr
1994]). This apparently contrasts with the situation in
Svalbard, where death of 1 yr olds from starvation in
winter is virtually unknown. However, it is evident from
the ages at death, which Stimmelmayr gives in months,
that only 20 of the 50 reindeer in her sample in fact died
in mid or late winter and, of these, nine were calves (age
,1 yr), while all the others belonged to her oldest age
class (5 yr [ Stimmelmayr 1994]). Hence, the age
distribution of the winter carcasses in her sample is
similar, after all, to that observed in Svalbard. The single
curious feature of her data is the fact that 94% of the
carcasses examined on Hagemeister Island were males
(Stimmelmayr 1994). There is unavoidable doubt over
whether her collection constituted a representative
sample of the animals that died in that year.
Another feature of the examples reviewed here is the
paucity of evidence on cause(s) of death, and hence, also
of the prevalence of starvation as a principal cause of
death during major declines of populations of Rangifer,
albeit that there are cases in which some classes of
animals starved to death in large numbers. This is not
new. One claim for starvation as the principal cause of
decline in a population of Rangifer (Young 1994), cited
as evidence a review (Davis 1978), which in fact
mentioned neither starvation nor, indeed, any cause of
mortality. What is clear is that, comparing across
populations, no two major declines have ever been quite
alike in terms of their proximal mechanism, i.e., how
declines in numbers fractionated between starvation and
other factors involved. That being the case, it is perhaps
not altogether surprising that no single environmental
factor nicely explains all or even most major declines.
The point that emerges is that no major decline of
reindeer or caribou yet observed seems to have been
exceptional in terms either of rarity or manner of
occurrence, or in the sense of its requiring explanation
Ecological Monographs
Vol. 80, No. 2
outside the current ecological paradigm for the regulation of abundance of populations of large herbivores.
Empirical basis of current claims
There appears, therefore, to be little empirical basis
for claims that icing of the range, i.e., severe crusting of
snow or the formation of basal ice over a large area of
winter pasture, is a potent and ubiquitous cause of heavy
mortality of Rangifer or, ultimately, that it is a major
driving variable in the temporal dynamics of populations of this species. How can a belief in the murderous
effects of icing have arisen? At least four factors have
contributed. The first is a blurred distinction between
snow and ice as agents of mortality. The second, closely
related to this, is the heterogeneity of the parameters
used to describe snow and ice conditions on winter
ranges in years when populations have declined. The
third is bias in what gets reported and, in particular,
repeated. The fourth is a tendency for accounts of ice
and mortality to evolve in the literature independently of
the slim factual basis from which they originally arose.
Papers reporting direct effects of winter weather on
the dynamics of populations of Rangifer make frequent
reference to ‘‘snow and ice conditions.’’ This is an
inconsistent phrase. Any measureable amount of solid
ice on the ground or in the snowpack is likely to hinder
animals in their attempts to reach forage. The effects of
snow, on the other hand, range from insignificant to
severe depending on the extent, depth, or hardness of the
snowpack. The population crashes best linked to
conditions on the ground have all unambiguously
documented deep snow but never ice (St. Matthew
Island [Klein 1968]; Bathurst Island Complex I and II
[Miller and Gunn 2003a]; Coats Island I [Gates et al.
1986]), a point made equally unambiguously by Miller
and Gunn (2003b):
. . . total snowfall is the best indicator that we have to
date of the potential for an extremely severe ‘weatheryear’ causing die-offs . . . .
The clarity of this statement contrasts with the
elusiveness of the succeeding sentence:
. . . icing compound[s] the impact of deep snow and
tends to cloud the relative importance of the role of deep
snow vs. icing in . . . die-off years.
These clouds, however, are not easily dispersed. None
of the studies mentioned in Miller and Gunn’s (2003b)
review, nor those reviewed in the present paper, contain
any objective data on the extent of the coverage or the
depth of ice on the range. Several included quantitative
empirical data on meteorological covariates considered
likely to influence the characteristics of snow cover and
the formation of ice, but the resulting proxies either fell
from the final models or were associated with improved
survival and enhanced population growth (see Snow and
ice).
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SNOW, ICE AND POPULATION CRASHES
The severity of a drought may be described by three
measurements: intensity, duration, and areal extent.
This makes for difficulties when comparing one drought
with another, and it explains why ‘‘the worst drought in
living memory’’ occurs about once every 10 years. By
unconsciously juggling the weighting ascribed to intensity, duration, and extent, almost any drought can be
remembered as worse than its numerous predecessors.
Surveying literature on the effects of hard snow and
icing on populations of Rangifer, one is also struck by
the paucity of negative results, i.e., situations in which
the presence of ice or of icing conditions did not result in
heavy mortality. One possibility is that such situations
never occur. An alternative is that they have been poorly
documented (Edwards 1956). Another alternative is that
they have rarely been reported. Bias in reporting is an
insidious problem, because no amount of study of
published results can yield information on the extent to
which contrary evidence escapes publication. The
essence of this problem was beautifully outlined by
King and Murphy (1985):
It is known from sound evidence that endotherms
sometimes die . . . of starvation or hypothermia. That
is a fact. There is an ample literature of such accounts,
most of it anecdotal and some of it dramatic . . . . The
summary of a typical publication might be: ‘‘I toured
the woods after a bad snowstorm and found many dead
birds and some deer that seemed very weak.’’ Most
reports . . . fall into one category: positive evidence of
episodic death . . . , sometimes as a result of malnutrition. The countering evidence, since it is negative, is
rarely published. One can easily imagine the reaction of
a journal editor to a manuscript whose summary might
be: ‘‘I toured the woods after a bad snowstorm and did
not find any dead birds or scrawny deer.’’ It is, of
course, the positive evidence that lingers in mind and
comes to dominate our impressions.
Some negative results in studies of Rangifer, in fact,
have been reported. For instance:
There was ice over virtually all the flat ground but food
they found . . . . The animals showed no sign of
starvation; quite the reverse, they seemed to be in good
condition (Lønø 1959);
[T]he winter of 1973–74 . . . had several periods of
temperatures above 08C, often coinciding with precipitation, with subsequent cold spells . . . . These conditions led to the formation of hard snow or ice over the
vegetation and forced the reindeer to . . . seek higher . . .
marginal areas . . . . In spite of these adverse weather
conditions, no particular high winter mortality was
suffered by animals over 3 to 4 mon [sic] of age.
(Reimers 1977);
. . . there was no relationship between . . . overwinter
survival and mean snow depth . . . (Larter and Nagy
2000);
REVIEWS
The durability of the view that icing has potent
effects on populations may be due in part to the fact
that no agreed ‘‘ice standard,’’ nor even ‘‘snow
standard,’’ is used in studies of the direct effects of
winter weather on the dynamics of populations of
Rangifer. A range of parameters, all bearing on snow
and ice conditions to a greater or lesser extent, have
been employed, and these fall into four not entirely
discrete categories. Of assessments of snow and ice,
seven are based on quantitative objective measurements: greatest accumulation of snow (Klein 1968),
total snowfall (Ouellet et al. 1996, Miller and Gunn
2003a, b), monthly snow depth (Gates et al. 1986),
mean maximum monthly snow depth (Solberg et al.
2001), and the mean depth and mean hardness of snow
(Larter and Nagy 2000). There are three subjective
assessments, none of which exceed a single sentence: ‘‘a
crust of glare ice’’ (Scheffer 1951), ‘‘snowfall’’ (Stimmelmayr 1994) and ‘‘terrestrial ice-crust’’ (Aanes et al.
2002). To these may be added many qualitative reports
of glazed crust (‘‘gololeditsa’’) and ice referred to in
Russian accounts (Formozov 1965, Krupnik 1993).
The second category includes low temperature contemporaneous with record snowfall (Klein 1968). The third
focuses on warm temperatures, and includes four
slightly different quantitative assessments based on
empirical data: ‘‘the number of days in winter . . . in
which temperature 08C’’ (Aanes et al. 2000), ‘‘the
annual sum of precipitation during days with temperature .08C’’ (Solberg et al. 2001), the sum of ‘‘sensible
heat input during periods of mild weather (.08C) in
winter’’ (Tyler et al. 2008), and a snowpack (degreeday) model (Kohler and Aanes 2004). There is also one
qualitative assessment: ‘‘freezing rains’’ (Nagy et al.
1996). The fourth category includes total precipitation
in winter (Aanes et al. 2000, Solberg et al. 2001). This
parcel of parameters includes physical opposites (warm
weather and cold weather) and assessments ranging in
quality from the sharply defined (‘‘monthly snow
depth’’) to the nebulous (‘‘snowfall’’). It is almost
inconceivable, however, that these various aspects of
snow and ice influence the survival of animals to a
similar degree. The diversity of quantitative and qualitative assessments inevitably complicates the task of
identifying components of winter weather that may have
a powerful and ubiquitous influence on the dynamics of
populations. It perhaps also accounts for the appeal of
blanket terms like ‘‘gololeditsa’’ and ‘‘icing’’ and of
intuitively plausible parameters such as ‘‘rain-on-snow’’
(Putkonen and Roe 2003, Grenfell and Putkonen 2008,
Rennert et al. 2009). The problem is not a new one. In an
analysis of the effect of drought on populations of
kangaroos, Caughley et al. (1985) pointed out how the
apparent uniformity of the concept of ‘‘drought’’ was
superficial and potentially misleading:
211
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N. J. C. TYLER
The highest overwinter survival occurred during the
winter 1993–1994 when fall icing conditions were
documented on a substantial proportion of the caribou
winter range (Larter and Nagy 2000);
REVIEWS
Not all relatively deep snow years result in major dieoffs; 1989–90 ranked fourth out of the 55 years . . . and
no greater loss of 1þ old caribou was detected (Miller
and Gunn 2003a).
Reports of this kind, however, are rare. The chief
problem is that field studies are frequently opportunistic.
When a population has suffered a severe decline, there is
usually considerable incentive to explain what happened, and usually a good way to start is to trawl
meteorological records for unusual values. Such an
approach will obviously never provide any basis for a
general statement about the effects of a given set of
weather conditions on population size, because the
opposite case is not tried. Where there has been no
decline, there is little urgency to report the conditions
that prevailed at the time. Population declines are
important phenomena and worth study in themselves
(e.g., Young 1994, 1999; Erb and Boyce 1999), especially
where they provide warning of increased risk of
extinction (e.g., Mace et al. 2008). However, most extant
natural populations of large herbivores have persisted
for a very long time, and even major declines in their
numbers, which may have occurred from time to time,
are necessarily only elements of the unbroken temporal
pattern of their development across hundreds or even
thousands of years. Hence, while it is legitimate to ask
specifically what environmental factor(s) triggered a
particular decline, it is ultimately more instructive to
determine what factors mould the dynamical behavior
of the populations of which those declines are but a part.
To achieve this it is necessary to analyze population time
series in their entirety, rather than merely focusing on
bits of them, and usually only the negative bits. Time
series analysis has not only identified important
interactions between the simultaneous effects of extrinsic
factors (weather) and intrinsic factors (population
density), which can lead to highly unstable dynamics
(e.g., Grenfell et al. 1998, Coulson et al. 2001, Ellis and
Post 2004, Berryman and Lima 2006), but crucially, it
has also revealed counterintuitive effects of environmental conditions on numbers (Forchhammer et al. 2002,
Tyler et al. 2008).
Accounts of the effects of snow and ice on populations
of Rangifer seem to evolve with repeated telling,
sometimes as new data or insights emerge, but not
invariably so. Descriptions of conditions on the ground
evolve fastest, probably because these are often more
vague at the outset than the census data that they were
recruited to explain. Thus, as noted above, the crash of
the population of reindeer on Brøgger Peninsula in 1993–
1994 was originally explained in terms of heavy
precipitation in winter, without any mention of icing
Ecological Monographs
Vol. 80, No. 2
(Aanes et al. 2000). A subsequent paper (Aanes et al.
2002) included an anecdotal report of the formation of an
extensive terrestrial ice-crust which was ‘‘in some places
. . . more than 10 cm thick.’’ Two years later, the same
observation became an ‘‘ice layer (up to 10–30 cm)
deposited over large parts of the lowland plain’’ (Kohler
and Aanes 2004), while Chan et al. (2005) subsequently
conferred on the same icing event, and the population
crash now vicariously associated with it, the status of
‘‘The ‘text-book’ example of a dramatic effect of such
locked pastures.’’ This interpretation was matched by
Callaghan et al. (2005), who incorrectly cited the original
reference (i.e., Aanes et al. 2000) in partial support of the
statement that ‘‘Dramatic reindeer population crashes
resulting from periodic ice crusting have been reported
from . . . Svalbard’’ (see also Grenfell and Putkonen
2008). Almost no new information on the crash itself has
emerged following the original report. Neither the
magnitude of the decline of the population during the
winter of 1993–1994, nor the level of mortality to which it
was in part due, were accurately documented (see
Emigration and Carcass counts). There is no doubt that
the population of reindeer there declined precipitously;
however, lacking comprehensive data it seems unlikely
that either the conditions on the ground or the mechanics
of the decline will ever be known.
Another example of an emerging emphasis on the
importance of icing can be clearly traced in accounts of
the declines of caribou on the Bathurst Island Complex.
Following the 1973–1974 decline, Miller et al. (1977)
remarked that ‘‘. . . we have no quantitative measures of
range conditions.’’ Twenty years later, when examining
the 1994–1997 decline, Miller (1998:49) emphasized the
role of ‘‘significantly greater . . . than average total
snowfall,’’ and went on to point out that ‘‘. . . detailed
range-wide information on type of snow cover and the
incidence of ground-fast ice or ice layering on an annual
basis is generally unavailable for the [Queen Elizabeth
Islands]’’ (Miller 1998:50). Subsequent papers, however,
made clear reference to extensive icing of the range and
attributed the falls in number to it. Thus, Gunn et al.
(2000) commented cautiously that ‘‘Widespread deep
snow and icing . . . that restricted forage availability
appears to be the sole cause of the decline.’’ Gunn and
Dragon (2002:68) suggested that decline during the
winter of 1996–1997 was a result of an unusually severe
winter and that ‘‘. . . deep snow, freezing rain (causing
icing) and the formation of ground fast ice . . . lead[s] to
. . . high levels of death among . . . Peary caribou . . . .’’
The position hardened when Gunn et al. (2003) stated
unambiguously that
The most complete data set is for Bathurst Island.
Hundreds of caribou died . . . during four winter and
spring periods which were four of the five most severe on
record (1973–74, 1994–95, 1995–96 and 1996–97) . . .
extensive ice formed over the vegetation as a result of
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SNOW, ICE AND POPULATION CRASHES
warm temperatures and high winds in both autumn and
spring.
fact, the inclusion of sources of this kind merely
highlights the paucity in primary scientific literature of
concrete evidence for the effects of hard snow and basal
ice on the dynamics of populations of Rangifer.
Heterogeneous dynamics, regional variation in climate,
and icing conditions
There is good evidence that the annual rate of increase
of established populations of Rangifer can be strongly
influenced by the weather in winter, and that snow
conditions of one form or another can be an important
density-independent factor influencing the dynamics of
populations of this species. It is also clear, however, that
neither the strength nor even the form of density
independence is invariable. Instead, associations between aspects of winter weather and dynamics vary
across time within and across space between populations, and evidently not at random (Post 2005, Tyler et
al. 2008). Pronounced gradients have been detected in
the dynamics of populations of Rangifer across the
Northern Hemisphere with respect, in particular, to the
degree of density independence (Post 2005, Post et al.
2009a). The effects of climate on numbers vary in terms
both of temporal relationships (direct or lagged) and
sign (enhancing or suppressing growth). In Post’s (2005)
analysis, the effect of climate variation (in terms of the
North Atlantic Oscillation winter index) on numbers
was direct (nonlagged) in 10 of 27 populations, while
lags of one or two years were detected in the dynamics of
the remainder. Direct effects of winter weather on
numbers normally reflect variation in overwinter survival and in calving success among surviving females
(calving success being generally a composite measurement embracing both fecundity and the survival of
newborn calves [e.g., Adams and Dale 1998b, Solberg et
al. 2001, Tyler et al. 2008]); lags, on the other hand,
reflect more complex ecological relationships (see Forchhammer et al. 2002, Post and Forchhammer 2008, Post
et al. 2009a, b). Given this degree of heterogeneity, the
results of analyses made specifically to determine how
the dynamics of established populations have been
affected by variation in local weather conditions
associated with snow and icing are surprisingly consistent. Eight time series in as many populations have been
examined. Climatic conditions associated with increased
amounts of snow during winter had a positive effect on
the abundance of five populations of caribou on
Greenland, in each case with a lag of two years (no
significant effect of climate was detected in a sixth
[Forchhammer et al. 2002]), while indices of winter
warming were associated with increased abundance of
one established population of Svalbard reindeer (Tyler
et al. 2008) and had no significant effect on annual rate
of increase in a second (Solberg et al. 2001).
The diversity and sometimes contradictory nature of
the ways in which populations of Rangifer respond to
particular kinds of climate and weather conditions is
almost certainly related to the diversity of the habitats
REVIEWS
Their view was subsequently included in a review of
the status of Peary caribou which concluded that
‘‘Severe winter weather characterized by deep snow with
icing events caused the crashes’’ (COSEWIC 2004).
None of these publications, however, provided information to supplement the absence of data on the depth or
extent of ice in the original reports. The caribou, of
course, almost certainly faced difficult foraging conditions in those winters, and Miller et al. (1977:47) did
actually mention the existence of empirical evidence of
ice, but they presented none of it. (A few measurements
made then and in the springs of 1994–1995 and 1996–
1997 at fuel caches throughout the range included
observations of ground-fast ice with thickness in the
range 5–20 cm, maximum 33 cm; F. L. Miller, personal
communication 2008.) Tracing the reports of the declines
on the Brøgger Peninsula and in the Bathurst Island
Complex, therefore, demonstrates how enthusiasm for
the purported effects of icing on populations of Rangifer
can, in just a few years, lift slim sets of unpublished field
observations from obscurity to prominence at the heart
of the interpretation of major ecological events. It is, of
course, legitimate to shine the spotlight of new
understanding on old observations; it is best, however,
that the light be bright but not dazzling.
Similar enthusiasm may also contribute to overly
generous interpretation of published material. Among
publications repeatedly cited in support of the view that
difficult snow conditions and formation of basal ice have
led to major declines of populations of Rangifer are
several which, in fact, contain no information on either
population mortality rates, or snow and ice conditions
or both. These include papers by Lønø (1959), Miller et
al. (1975), Parker et al. (1975), Alendal and Byrkjedal
(1976), Larsen (1976), Reimers (1977, 1982, 1983),
Øritsland (1998), Aanes et al. (2000), Griffith et al.
(2002) and Kohler and Aanes (2004), cited variously, for
instance, by Aanes et al. (2000), Solberg et al. (2001),
Kumpula and Colpaert (2003), Putkonen and Roe
(2003), Kohler and Aanes (2004), Callaghan et al.
(2004, 2005), Grenfell and Putkonen (2008). Curiously,
the chief evidence in two of these papers (Lønø 1959,
Reimers 1977) actually describes the reverse situation,
i.e., instances where the presence of basal ice or
presumed icing conditions did not result in starvation
or heavy mortality. Similarly, the citing of reports that
contain no data on the purported detrimental effects of
icing on populations of reindeer, but which, in turn,
refer to sources that, likewise, contain no first-hand data
(e.g., Syroechkovski and Kuprianov 1995 cited by
Callaghan et al. 2004, 2005), or citing press reports
and newspaper articles the basis of which cannot readily
be examined (e.g., Beltsov 2002, McFarling 2002, both
cited by Putkonen and Roe 2003) indicates an eagerness
for a cause that surpasses normal scientific caution. In
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214
N. J. C. TYLER
Rangifer occupy and the climatic conditions to which
they are exposed. Temperature profiles and snow
conditions in winter and the physical structure of the
terrain all vary hugely across the geographical range of
this species. Interludes of mild weather (.08C), for
instance, are a common feature of winter weather on the
west coast of Svalbard (median ¼ 11 periods per winter
[October–April], range ¼ 5–23 periods [Tyler et al. 2008])
when they may occur in any month (mean frequency ¼
1.5 interludesmonth1year1, range ¼ 1.0 interludes/yr
[February] to 2.5 interludes/yr [October]; data from
1975 to 2009 from the Norwegian Meteorological
Institute; see also Putkonen and Roe [2003]). High
frequency of warming in Svalbard, especially early in
winter, evidently creates favorable conditions for the
reindeer there (Tyler et al. 2008). The situation in the
Canadian Arctic Archipelago, by contrast, is quite
different. Here, interludes of mild weather in winter
are exceedingly rare. At Resolute (748 N, 948 W), for
instance, the frequency of days per month with
maximum temperature .08C ranges from 0.07 to 0.03
in October and November, respectively, and is zero from
December to April, inclusive (data 1971–2000 from
Environment Canada’s Web site [available online]).2 In
high-Arctic Canada, therefore, the occurrence of mild
weather and the concomitant formation of basal ice is a
phenomenon associated almost exclusively with the
spring melt in May–June (Miller et al. 1982, Woo et
al. 1982, but see Rennert et al. 2009). On a larger scale,
the stable, cold conditions typical of Rangifer winter
habitat throughout western and north-central North
America become progressively replaced eastward across
the continent by an unstable, more oceanic winter
climate. Most significantly, there is a concomitant
increase along this gradient in the frequency of rainon-snow events in winter, with values at the east coast
up to two orders of magnitude greater than in central
and western regions (Rennert et al. 2009). Under such
conditions the normal logic, whereby midwinter thaws
and rain-on-snow events are precursors of icing, is
sometimes reversed. In coastal habitats interludes of
mild weather can induce levels of local warming that
benefit rather than disadvantage the animals. Thus, in
parts of Newfoundland, periodic thaws and winter rains
may melt snow completely away, increasing the availability of forage for caribou over substantial areas as a
result (Damman 1983, Mahoney and Schaefer 2002).
Likewise, on the west coast of Greenland, foehn winds
can convert snowfields to snow-free ground in just a day
or two. Commenting on this, Vibe (1967) wrote that
‘‘This type of storm makes the snow evaporate or melt,
and gives the Reindeer access to the vegetation.’’
Finally, in parts of central Norway the normal pattern
of Saami transhumance is reversed, such that, instead of
moving inland in winter to find the good grazing
conditions afforded by a colder, drier climate, reindeer
2
hwww.climate.weatheroffice.ec.gc.cai
Ecological Monographs
Vol. 80, No. 2
herders deliberately take their animals to the coast,
where they benefit from the mild climate that keeps the
pastures largely free of snow throughout winter (Skjenneberg and Slagsvold 1968).
Even beyond the reach of maritime climate conditions, the formation of basal ice is influenced by a range
of factors that modulate the influence of rain or melting
of the snowpack on grazing conditions. For basal ice to
form it is necessary that (a) sufficient water be delivered
(b) onto very cold substrate with which (c) it remains in
contact for long enough to freeze. Where the source of
water is melted snow, its amount will depend not only
on the rate of melting, but also on the amount (cover
and depth) of snow on the ground. Warming can create
little water where there is little snow, and the depth of
snow on winter pastures is regionally highly variable.
Mean snow depths in forest habitat, for instance, fall in
the range of 70–120 cm (Brown and Theberge 1990,
Rominger and Oldemeyer 1990, Kumpula and Colpaert
2007), while on high-Arctic tundra they are generally
,25 cm even in the snowiest months (e.g., Larter and
Nagy 2001, Tyler et al. 2008). Where the source of water
is rainfall, it must occur in ‘‘substantial’’ amounts if it is
to percolate through the snowpack (Grenfell and
Putkonen 2008). The subsequent fate of the water, from
whichever source, depends on a variety of factors.
Where the cold content of the substrate and the
overlying snowpack is sufficiently high, it will freeze
(Woo et al. 1982). But where the ground is sufficiently
insulated by deep snow, the latent heat released during
freezing may warm the soil sufficiently to allow the
formation of an ice/water bath that can potentially
survive for weeks (Putkonen and Roe 2003). Moreover,
the percolation of rain or melt water through snow from
the surface to the substrate is a complex process strongly
influenced by the layer and crystal structure of the
snowpack (Conway and Benedict 1994, Waldner et al.
2004). Large horizontal variations in flow and the
channelling of water down preferential flow paths result
in considerable spatial heterogeneity in its delivery onto
the ground (Woo et al. 1982, Marsh 1999). Once on the
ground, local relief and in particular downslope
drainage has a major influence on the buildup of ice,
resulting in it being absent in some areas and
accumulating in others (e.g., topographic depressions
[Woo and Heron 1981, Woo et al. 1982]). This was
observed by Øritsland (1998), who described the
situation on Edgeøya in Svalbard where ‘‘Due to
micro-topographic features such as small hummocks,
ridges, stones, pebble-sized rubble, slope and soil
coarseness that affected water runoff, the thickness of
the ice layers ranged from zero to l0 cm.’’ Associations
between climate, local weather, and conditions literally
on the ground, are thus inherently complex and vary not
only regionally but also locally. This almost certainly
contributes to the lack of consistency across populations
of Rangifer in the relationship between declines in
May 2010
SNOW, ICE AND POPULATION CRASHES
numbers and the meteorological parameters believed to
trigger them.
CONCLUSIONS
namics of reindeer herding in the context of global
change have shown how, in terms of influence on the
size and structure of herds and, potentially, on the
form of the entire industry, the effects of climate
variation seem to be dwarfed by the effects of a
battery of potent density-independent factors, all of
which lie outside the realm of biology, but which,
instead, are part of the sociological, political, and
economic environments that govern the fate of
animals (Tyler et al. 2007, Rees et al. 2008, see also
Vors et al. 2007).
ACKNOWLEDGMENTS
This paper was originally presented at a conference ‘‘After
the Melt: Ecological Responses to Arctic Climate Change,’’
held at the University of Aarhus, Denmark, in 2008, and I
thank the organizers for inviting me to attend that meeting. I
also thank Mike Ferguson for comments on the occurrence of
basal ice in the Canadian High Arctic, Greg Finstad for
commenting on the reliability of population data from
Hagemeister Island, Anne Gunn for observations on snow
and ice conditions in the Bathurst Island Complex in the late
1990s, Jan Petter Holm for assistance with preparing Fig. 1,
Johnny-Leo Jernsletten for assistance with sources in Russian,
David Klein for information on populations of reindeer in
Alaska, Bob van Oort for assistance with Fig. 2, Tor Punsvik
for providing data on hunting mortality in the Reindalen
Complex, and Jaakko Putkonen for kindly sending me copies of
two papers then still in press. Most particularly I thank Mads
Forchhammer, Eric Post, and especially Frank Miller, as well
as two anonymous referees, for reading and criticizing an earlier
version of the paper.
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