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The recovery, distribution, and population
dynamics of wolves on the Scandinavian
peninsula, 1978–1998
Petter Wabakken, Håkan Sand, Olof Liberg, and Anders Bjärvall
Abstract: In 1966 the gray wolf (Canis lupus) was regarded as functionally extinct in Norway and Sweden (the Scandinavian peninsula). In 1978 the first confirmed reproduction on the peninsula in 14 years was recorded. During 20
successive winters, from 1978–1979 to 1997–1998, the status, distribution, and dynamics of the wolf population were
monitored by snow-tracking as a cooperative Swedish–Norwegian project. After the 1978 reproduction in northern
Sweden, all new pairs and packs were located in south-central parts of the Scandinavian peninsula. Between 1983 and
1990 wolves reproduced each year except 1986, but in only one territory. There was no population growth during this
period and the population never exceeded 10 animals. In 1991 reproduction was recorded in two territories. After that
there were multiple reproductions each year and the population started growing. In 1998 there were 50–72 wolves and
six reproducing packs on the peninsula. Between 1991 and 1998 the annual growth rate was 1.29 ± 0.035 (mean ±
SD). A minimum of 25 litters were born during the study period. The early-winter size of packs reproducing for the
first time was 6.2 ± 1.4 wolves (n = 9), and this decreased with time during the study. The size of packs that had
reproduced more than once was 6.4 ± 1.8 wolves (n = 12), and this increased with time over the study period. All but
1 of 30 reported wolf deaths were human-caused. The annual mortality rate was 0.13 ± 0.11, and this decreased with
time during the study period. The minimum dispersal distance was 323 ± 212 km for males and 123 ± 67 km for
females. Of 10 new wolf territories where breeding occurred, only 1 bordered other, existing territories. The distance
from newly established wolf pairs to the nearest existing packs was 119 ± 73 km. Simulation of population growth
based on known reproductions and mortalities showed a close similarity to the results from population censuses up to
the mid-1990s. To what extent this population is genetically isolated is at present unclear.
Résumé : En 1966, le Loup commun (Canis lupus) était considéré comme disparu de la Norvège et de la Suède
(péninsule de Scandinavie). En 1978 a eu lieu la première reproduction dans la péninsule en 14 ans. Durant 20 hivers
successifs, 1978–1979 à 1997–1998, le statut de la population, sa répartition et sa dynamique ont été suivis par repérage
des pistes dans la neige dans le cadre d’un projet de collaboration Suède–Norvège. Après la reproduction de 1978 dans
le nord de la Suède, tous les nouveaux couples et meutes ont été repérés dans les zones centre-sud de la péninsule. De
1983 à 1990, les loups se sont reproduits chaque année sauf en 1986, mais dans un seul territoire. Il n’y a pas eu
croissance de la population au cours de cette période et la population ne comptait jamais plus de 10 animaux. En
1991, il y a eu reproduction dans deux territoires. Par la suite, les reproductions ont été multiples et la population a
amorcé sa croissance. En 1998, il y avait 50–72 loups et six meutes reproductrices dans la péninsule. Entre 1991 et
1998, la croissance annuelle a été de 1,29 ± 0,035 (moyenne ± écart type). Au moins 25 portées sont nées au cours de
la période de l’étude. Le nombre moyen de loups dans les meutes de début d’hiver se reproduisant pour la première
fois a été de 6,2 ± 1,4 loups (n = 9) et a diminué par la suite. Ce chiffre était de 6,4 ± 1,8 loups (n = 12) au sein des
meutes à reproductions multiples et il a augmenté avec le temps jusqu’à la fin de l’étude. Parmi les 30 cas de mortalité, un seul n’était pas relié à l’activité humaine. Le taux annuel moyen de mortalité était de 0,13 ± 0,11 et ce taux a
diminué avec le temps au cours de la période de l’étude. La distance minimale de dispersion était de 323 ± 212 km en
moyenne chez les mâles et de 123 ± 67 km chez les femelles. Des 10 territoires où il y a eu reproduction, un seul
s’est avéré adjacent à d’autres territoires. La distance moyenne entre un nouveau couple et les meutes les plus proches
était de 119 ± 73 km. Une simulation de la croissance de la population, basée sur des données connues de reproduction et de mortalité, a donné des résultats semblables à ceux obtenus au cours des recensements démographiques jusqu’au milieu des années 1990. L’importance de l’isolement génétique de cette population reste à préciser.
[Traduit par la Rédaction]
725
Wabakken et al.
Received April 20, 2000. Accepted January 31, 2001. Published on the NRC Research Press Web site on April 11, 2001.
P. Wabakken. Hedmark College, Department of Forestry and Wilderness Management, N-2480 Koppang, Norway.
H. Sand and O. Liberg.1 Grimsö Research Station, Swedish University of Agriculture Sciences, S-730 91 Riddarhyttan, Sweden.
A. Bjärvall. Swedish Environmental Protection Agency, S-106 48 Stockholm, Sweden.
1
Corresponding author (e-mail: [email protected]).
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Introduction
In the last two decades much attention has been given to
the special problems of extremely small populations (Soulé
and Wilcox 1980; Soulé 1986). An element of the “smallpopulation paradigm” (Caughley 1994) that has developed
recently is restorations, both natural and artificial, of locally
extinct species (Stanley-Price 1989). In this paper we describe the first phase of a natural restoration of the gray wolf
(Canis lupus) on the Scandinavian peninsula.
The historical range of the gray wolf included most of the
northern hemisphere, but severe eradications have occurred
over large parts of this region during the last two centuries.
The gray wolf survived primarily in the most remote and
least developed tracts and is now classified as an endangered
species in many countries (Mech 1995). Human attitudes
have changed in favor of wolves, and wolf populations are
presently recovering in the northern U.S.A. and several parts
of Europe (Promberger and Schröder 1993; Fritts et al. 1995;
Mech 1995; Wydeven et al. 1995; Bangs et al. 1998).
In Eurasia, most wolf populations were at their lowest between the 1930s and 1960s (Mech 1995). In Scandinavia the
decline of the wolf population started during the 19th century, and at the beginning of the 20th century there was only
a small remnant left in the northern part (Haglund 1968).
Bounties were paid for killed wolves as late as the mid1960s. When it was finally protected in 1966 in Sweden and
1972 in Norway, the wolf was functionally extinct in Scandinavia, as the last known reproduction occurred in 1964
(Haglund 1968; Myrberget 1978; Bjärvall 1988). In the period 1965–1977 there were only a few, mostly unconfirmed,
observations of wolves. However, in winter 1977–1978 several observations of wolves were made in the northernmost
part of Sweden (these were possibly immigrants from the
large continuous Finnish–Russian population 250 km to the
east), and in 1978 a successful reproduction was recorded in
that area (Bjärvall and Nilsson 1978). In 1983, after several
years of increasing numbers of wolf sightings and tracks being reported from areas farther south (Wabakken et al. 1982;
Bjärvall 1983), a wolf pair reproduced in south-central Scandinavia, on the border between Norway and Sweden, and
since that year wolves have regularly reproduced in this part
of Scandinavia, leading to an increase in numbers and a significant range expansion (Wabakken et al. 1994; Liberg and
Glöersen 1995).
Data for this study were collected during snow-tracking
surveys in Sweden and Norway in the 20 winters from 1978–
1979 to 1997–1998. The objectives of the study were to
summarize Scandinavian wolf recovery, including (i) status
and distribution, (ii) demography, and (iii) dispersal and pair
formation, and (iv) to discuss the results in terms of the
problems commonly faced by small populations.
Study area
Sweden and Norway together constitute the 837 000-km2 Scandinavian peninsula, hereinafter also called Scandinavia (55°–72°N,
5°–31°E; Fig. 1). Boreal coniferous forest and alpine areas cover
more than 75% of the peninsula. Norway spruce (Picea abies),
Scots pine (Pinus silvestris), birch (Betula pubescens, Betula
pendula), and aspen (Populus tremula) are the dominant tree species in various mixtures. Most of the forests are managed for a mo-
711
saic of different age-class stands. The intensive forest management
has also led to the creation of an extensive network of forest roads.
Large agricultural areas are common in the southern parts of the
peninsula. Snow covers most of the Scandinavian peninsula for 5–
7 months each year.
In both countries, available important prey species for wolves
are moose (Alces alces), roe deer (Capreolus capreolus), and beaver (Castor fiber), and in Norway red deer (Cervus elaphus) and
wild reindeer (Rangifer rangifer) are also available. In the northern
half of Scandinavia, semidomesticated reindeer are widely distributed in both countries. During summer, free-ranging livestock are
also available. Of these, domestic sheep (Ovis aries) are the most
vulnerable to depredation by wolves, especially in Norway, where
approximately two million sheep graze unattended. Large carnivores other than the wolf are lynx (Lynx lynx), brown bears (Ursus
arctos), and a few wolverines (Gulo gulo). Lynx occur over most
of Scandinavia, while bears and wolverines are limited to the
northern half.
Human population density averages 16/km2, but in large areas
within the main wolf range it is less than 1/km2. A major source of
wolf–man conflicts in Scandinavia is depredation on semidomesticated reindeer (Bjärvall and Nilsson 1976; Bjärvall and
Isakson 1982) and domestic sheep (Miljoeverndepartementet 1996–
1997). Another problem is the killing of domestic dogs associated
with hunting.
The small but growing wolf population in south-central Scandinavia, which is the subject of the present paper, has developed
800–1000 km from the nearest source population, which is the
large continuous Finnish–Russian wolf population (Fig. 1). Most
of Scandinavia as well as Finland may be characterized as
semiwilderness. There are therefore few physical barriers to wolf
dispersal between these two populations. The greatest threat to genetic exchange between them, apart from distance itself, is that
presumed dispersers have to travel most of the way through reindeer country, including northern Finland, where there are hostile
reindeer herders. However, there are also one or two possible short
cuts across the narrow parts of the northern Baltic Sea (Fig. 1), but
wolf passage there requires a winter cold enough for an ice bridge
to form, which occurs relatively rarely.
Methods
Field censuses
During the 20-year study period, only three wolves were radiocollared (two dispersers and one solitary resident). Therefore, most
monitoring of wolves was based on snow-tracking. Snow was a
prerequisite for identifying species, counting individuals, recording
movements and delimiting territories, determination sex, and determining social position (i.e., distinguishing scent-marking residents
from solitary nonresidents). Wolves were snow-tracked by a number of people employed by the regional authorities and by several
volunteers through all winters from 1978–1979 to 1997–1998. Cooperative Swedish–Norwegian monitoring was established across
the international border in 1981 and has continued up to the present (Wabakken et al. 1982; Bjärvall and Isakson 1983).
Wolf tracks, sightings, and recovered prey remains were also
reported by the public. Whenever feasible, experienced field personnel checked such reports and classified them as confirmed, rejected, or uncertain. A high proportion (20–50%) of the wolf
reports were rejected because of confusion with other species, particularly lynx and domestic dogs (Wabakken et al. 1984, 1994;
Isakson 1994). Reports of wolves on bare ground were accepted
only when verified by such means as photographs, taped sounds, or
direct observations by persons known by us to be experienced and
reliable. Females could be identified from the presence of vaginal
blood in the urine on snow before and during estrus (Mech 1970;
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Can. J. Zool. Vol. 79, 2001
Fig. 1. Study area consisting of the two countries, Norway and Sweden, that together constitute the Scandinavian peninsula (lightshaded area), and the continuous area of the Finnish–Russian wolf population (dark-shaded area).
Peters and Mech 1975). Newly formed pairs and alpha pairs (reproducing adults) within breeding packs were distinguished from
other wolves by their scent-marking behavior (Peters and Mech
1975; Rothman and Mech 1979). Winter-territory boundaries of
pairs or packs were determined by the outermost locations of the
scent marks made by the alpha pair.
Estimates of population size
After each winter we determined the number of wolves in Scandinavia on the basis of all reports that were checked and confirmed
by trained personnel. A great effort was made to avoid not only
double-counting but also unnecessary merging of reports concerning different wolves. To achieve this, we used all criteria at hand,
such as time elapsed between observations/trackings in different
places, distances, number and categories of animals in a group, and
special delimitations made on snow between neighboring groups or
individuals. Single wolves were more difficult to find and distinguish
from each other than pairs and packs. Generally, only resident
wolves could be counted, and vagrant individuals were only included in the count when their separation was favoured by exceptional conditions, such as a very large distance from the nearest
location of an approved wolf occurrence.
After 1990, when the number of wolves first exceeded 10, the
total count became increasingly uncertain. For this period, therefore, we report minimum, maximum, and median population estimates for each winter. Only very well documented reports (date,
times, location, tracked distance with details of tracking, number
of animals in tracked or observed groups, and name of observer)
controlled by persons approved by us were included in the minimum estimate. Another condition was that there be absolutely no
risk of double-counting. Less well documented reports by trained
personnel or well-documented repeated observations by persons
unknown to us were included in the maximum estimate, as well as
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a few cases where there might have been some risk of doublecounting.
The management authorities in both countries monitored occurrences of large carnivores, especially in the reindeer-husbandry
area (Overskaug et al. 1984, 1985; Bergström et al. 1994), and the
Swedish Hunters’ Association conducted several large-scale wolf
and lynx surveys during the study period (Liberg and Glöersen
1995). Results from these surveys were included in our estimates
whenever they produced additional data or supported or rejected
our own evaluations.
The rate of population increase (λ) was estimated by fitting a
linear regression to the natural logarithm of annual estimates of
population size. Population-density estimates were made for the
core area of regular wolf occurrence (Fig. 2I) and within the wolfpack territories.
Wolf mortality
By law in both Sweden and Norway all dead wolves found have
to be reported to the authorities. Recovered dead wolves were
necropsied at official veterinary laboratories, and sex, age, body
mass, location, date, and cause of death were determined. Age was
determined by tooth sectioning and counting annual cementum layers (C1) at Matson’s Laboratory, Montana, U.S.A., or (in a few
cases) at the D.W.F. Game Research Division and the Norwegian
Institute for Nature Research, Trondheim (Kvam and Sorensen 1984;
Landon et al. 1998). The minimum annual mortality rate was calculated as the proportion of the number of known dead wolves in
the estimated total population size each year.
Reproduction and pack size
Wolf reproduction was confirmed by sightings, photographs, or
vocalizations of pups at rendezvous sites. When such summer observations were missing, reproduction was regarded as confirmed
within a territory if the number of individuals had increased by at
least two from one winter to the next. Consequently, for newly established pairs, reproduction was confirmed if tracks on snow revealed that the group size had increased to at least four wolves the
following winter. Within established pack territories, reproduction
was confirmed if tracks on snow in early winter (30 November ±
6 weeks) indicated play behavior and verified the addition of a
minimum of two more pack members since the previous late winter. An increase of at least two wolves was chosen because single
dispersing wolves occasionally immigrate into territorial packs
(Fritts and Mech 1981; Ballard et al. 1987, 1997; Fuller 1989;
Meier et al. 1995).
The size of litters raised by pairs reproducing for the first time
was estimated in early winter by assuming that all pack members
other than the alpha pair were pups. In consecutive years after the
first reproduction, we could not distinguish between pups and remaining older subordinate animals, and therefore did not determine
litter sizes.
In our study, a wolf pack was defined as a social unit of three or
more wolves resident within an established territory scent-marked
by an alpha pair. During snow-tracking of wolves it was uncommon to find all the pack members together in one group. Inside
their territories, packs often temporarily split into smaller groups.
Total pack size was therefore estimated as the highest minimum
count of pack members found together at any time during the winter.
Dispersal distances and pair formation
All wolves found dead outside known territories were defined as
dispersers, as well as one young wolf that was darted in 1984
(No. 6 in Table 1). The minimum dispersal distance was estimated
as a straight line from the recovery site to the closest known territory edge. For wolf pairs that established territories after 1983, the
distance from the new territory to the nearest and the most distant
713
possible parent territories was estimated as the minimum and
maximum straight-line distance, respectively. In making these estimates we assumed that no wolves were immigrants from Finland,
Russia, or the Baltic countries. If this assumption is incorrect,
some new pair formation distances will have been greatly underestimated.
Simulation of population development
If total annual reproduction and mortality rates are available for
a certain period, and the population level at the beginning of this
period is also given, it is possible to simulate population development for subsequent years in that period, independently of census
data. We simulated population growth from reproduction (pack
size) and known mortality rates in an effort to evaluate our monitoring results. Such a simulation will also produce a rate of population increase independent of that obtained using annual census
results. Because we had no direct figures on reproduction, only on
pack sizes during winter, we made the simple assumption that during the first winter after a pack was established, all animals except
the alpha pair were pups. For packs where reproduction had occurred more than once we assumed that 90% of all pack members
except the alpha pair were pups of the year. The figure of 90% was
based on data from another expanding wolf population in Minnesota, U.S.A. (Fritts and Mech 1981). This is also supported by preliminary results from radio-collared individuals in our study who
show high rates of dispersal of subadults (10–14 months of age) in
the population (H. Sand, H.-C. Pedersen, and P. Wabakken, unpublished data). For packs where the number of members had been
given as an interval, we used the mean for our calculations. For example, if the pack consisted of 4–7 animals, the mean number was
5.5 animals, and this was the second winter or later in the pack’s
existence, the calculated number of pups was 0.9 × (5.5 – 2) = 3.2
pups. Finally, the sum of all pups produced during one year was
adjusted to the nearest integer. For mortality we used the actual figures on dead wolves recovered in Scandinavia, with the additional
assumption that maximum longevity was 10 years. Our model
started with the median four wolves recorded in spring 1981 and
was then run for the entire period 1982–1998, the only inputs being our estimates of reproduction and mortality for each year.
Statistical methods
Differences in means of unpaired nonparametric measurements
were tested by Wilcoxon’s two-sample tests, one- and two-tailed,
while differences in ratios were tested by χ2. Spearman’s rank
correlation was used to determine if litter sizes and pack sizes
changed over time (years), and logistic regression analyses were
used to determine if the annual mortality rate decreased over time
(Sokal and Rohlf 1981) (SAS Institute Inc. 1992).
Results
The reappearance of breeding wolves in Scandinavia
During winter 1977–1978 and early spring 1978, after a
12-year period with very few wolf reports and no confirmed
report of reproduction in Scandinavia, eight or nine wolves
were confirmed by tracks in Sweden (Bjärvall and Nilsson
1978). Their origin is unknown, but it was assumed that
most were dispersers from the Finnish–Russian population.
The reports concerned a pair, a pack of three, and three or
four loners. Except for a loner in south-central Scandinavia,
all these reports came from the extreme north of Sweden.
There the pair successfully bred in 1978 and formed a pack
of eight the following winter (Figs. 2 and 3). In 1979 a minimum of two wolves are known to have been killed in the
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Can. J. Zool. Vol. 79, 2001
Fig. 2. Wolf-pack territories in the Scandinavian peninsula where pups were raised (Ž) and locations of newly formed pairs () between 1978 and 1997. The numbers refer to the order in which new wolf territories were established and reproduction occurred. Map I
shows the locations of resident wolf packs (Ž) and newly formed pairs () in Scandinavia by February 1998, together with the main
distribution area where wolves occurred regularly on the Scandinavian peninsula (shaded area).
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Fig. 3. Flow chart of data on wolf-pack territories, alpha-pair occurrences, and wolf reproductions on the Scandinavian peninsula between 1978 and 1997. The numbers at the left-hand side correspond to the territory numbers shown in Fig. 2. The upper line of each
pair of lines indicates the presence of an alpha male and the lower line indicates the presence of an alpha female. Numbers above the
lines indicate the size of wolf packs during early winter; a circle indicates that reproduction has been confirmed and an asterisk that
reproduction has not been confirmed but is likely to have occurred.
same area, one legally and one illegally, after which all contact with these two northern wolf packs was lost (Bjärvall
1988).
Between 1978 and 1982, a few lone wolves were tracked
along the border between Norway and Sweden in southcentral Scandinavia, 900 km farther south. It is not known
whether all or some of these were dispersers from the groups
observed in northern Sweden in 1977–1979, but this is a
possibility. That is why we regard 1978 as the starting year
for wolf recovery in Scandinavia. In this southern area a territorial pair was confirmed during the winter of 1982–1983
and a litter of pups was raised there in 1983. Since then, successful breeding of wolves has been verified on the peninsula every year except 1986, all occurrences being in southcentral Scandinavia (Figs. 2 and 3).
Numerical change, distribution, and rate of increase
Between 1983 and 1990, seven litters of wolf pups were
raised within the same territory (No. 2 in Figs. 2 and 3)
without any population increase being noted (Fig. 4). Then
in 1991, successful wolf breeding in two different places
during the same year was verified in Scandinavia for the first
time since the 1950s. After this event, the Scandinavian wolf
population started increasing and expanding (Figs. 2–4).
In winter 1997–1998, at the end of the study, the Scandinavian wolf population had increased to 50–72, including
six packs and three territorial pairs (Fig. 2I). Within the
86 000-km2 area of regular wolf occurrence the average population density was less than 1 wolf/1000 km2 (Fig. 2I).
However, within the estimated pack territories during winter, density approximated 10 wolves/1.000 km2.
For the period 1980–1981 to 1997–1998, the mean annual
rate of increase in the population was 19% (λ = 1.19 ± 0.017
(mean ± SD)), using median values of annual population estimates. Minimum and maximum estimates of population
size for the same period gave the same annual population
growth rate, 19%. Considering only the period when the
population actually increased, 1990–1991 to 1997–1998, the
mean annual rate of population increase was 29% (λ = 1.29 ±
0.035, median estimates), while minimum and maximum estimates gave mean growth rates of 26% (λ = 1.26 ± 0.033)
and 32% (λ = 1.32 ± 0.039), respectively.
Litter size and pack size
Between 1978 and 1997, 25 confirmed litters of pups were
raised (Fig. 3). For all packs and years with confirmed reproduction, pack size was 6.3 ± 1.6 (mean ± SD) (n = 21, range =
4–9). Pack size in winter after the first reproduction (6.2 ±
1.4) did not differ significantly from pack size in consecutive
years (6.4 ± 1.8; U = 58, p > 0.1, Wilcoxon’s two-sample
test).
During the study period, the size of first-litter packs decreased significantly (Figs. 3 and 5A; rs = –0.84, p = 0.018,
Spearman’s rank correlation). However, in packs where and
when breeding was verified in more than one year, pack size
increased significantly (Figs. 3 and 5B; rs = 0.81, p = 0.007,
Spearman’s rank correlation). For all years the annual proportion of scent-marking territorial alpha-pair members of
the Scandinavian wolf population was 0.29 ± 0.10).
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Fig. 4. Population trend and minimum, maximum, and average
annual numbers of wolves in Scandinavia during the winters
1980–1981 to 1997–1998.
Can. J. Zool. Vol. 79, 2001
Fig. 5. Pack size in the winter following the first reproduction
by newly formed pairs (A) (n = 9) and consecutive litters
(B) (n = 12) in Scandinavia from 1978 to 1997.
Mortality, sex, and body mass
Thirty wolves were found dead during the study period
and all deaths except one were confirmed to be humancaused (Table 1, Fig. 6). At least 9 of these 30 wolves were
killed illegally. The minimum annual mortality rate in 1983–
1997 was 0.13 ± 0.11, and it decreased significantly over
time (Fig. 7; logistic regression, χ2 = 12.62, p < 0.001).
Cause of death also changed over time. The proportion of
wolves known to have been shot or illegally killed in other
ways decreased from 73% in the period 1978–1990 to 7% in
the period 1991–1998 (χ2 = 13.08, p < 0.001; Table 1).
The overall sex ratio of dead wolves was not significantly
different from parity. Neither was there any difference in sex
ratio between the proportion of wolves killed in traffic accidents and those intentionally killed by people (75 vs. 64%
males; χ2 = 0.35, p > 0.5). However, within breeding-pack
territories, the proportion of females among the dead wolves
dominated, which was in contrast to the proportion among
wolves killed outside breeding-pack territories (71 vs. 16%;
χ2 = 7.43, p < 0.01; Table 1, Fig. 6).
Among dead adult (+1 year) wolves, males were significantly heavier than females (44.7 ± 3.0 vs. 34.2 ± 4.2 kg;
U = 82, p < 0.001, Wilcoxon’s two-sample test; Table 1).
Simulation of population growth
During the period up to 1991 the simulated estimates were
consistently higher than the census figures (Table 2, Fig. 4).
The accumulated difference between the two estimates peaked
in 1991, the simulation giving 16 wolves while the census
was 8 wolves. After this the difference between the two estimates decreased and was less than 5 wolves for several years
during the mid-1990s. Only for the last 2 years of the study
did this discrepancy increase again. The difference between
the calculated and censused population sizes at the end of
the 17-year simulation period was 13 animals (21%). The
rate of population increase based on estimates from the simulation was equal (λ = 1.19) to that based on census figures.
Dispersal and pair formation
Wolves dispersed in all directions from natal areas on the
Scandinavian peninsula (Fig. 8). Except in one case when
two wolves travelled together, all extraterritorial movements
confirmed during this 20-year study were made by single
wolves. In total for all wolves born 1983 and later, the minimum dispersal distance was 313 ± 252 km (n = 15, range =
80–880; Table 1, Figs. 6 and 8B). Eleven of 12 wolves killed
outside the area of regular distribution (Fig 2I) were males, a
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Table 1. Wolves known to have been killed in Sweden and Norway between 1978 and 1998.
No.a
Date
County and country
Sex
Social
unit
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
1979-03-14
1979-12-11
1980-01-06
1982-03-06
1984-01-11
1984-09-12
1985-07-29
1985-12-23
1986-03-21
1988-07-17
1989-04-17
1989-06-11
1989-12-12
1990-06-?
1991-06-14
1992-05-28
1992-07-?
1992-08-18
1992-08-24
1992-10-?
1993-01-28
1993-02-24
1993-08-18
1994-01-18
1996-01-04
1996-12-28
1996-12-28
1997-03-23
1997-08-?
1998-08-3
Norrbotten, Sweden
Norrbotten, Sweden
Finnmark, Norway
Finnmark, Norway
Aust-Agder, Norway
Halland, Sweden
Värmland, Sweden
Värmland, Sweden
Värmland, Sweden
Hedmark, Norway
Jämtland, Sweden
Oppland, Norway
Värmland, Sweden
Hedmark, Norway
Gävleborg, Sweden
Göteborg, Sweden
Hedmark, Norway
Aust-Agder, Norway
Norrbotten, Sweden
Möre and Romsdal, Norway
Stockholm, Sweden
Värmland, Sweden
Dalarna, Sweden
Värmland, Sweden
Älvsborg, Sweden
Jämtland, Sweden
Jämtland, Sweden
Uppsala, Sweden
Värmland, Sweden
Norrbotten, Sweden
F
—
M
M
M
M
F
M
M
M
M
M
F
F
F
M
M
M
M
M
M
F
F
F
M
—
—
M
—
M
Pack
Pack
Lone
Lone
Lone
Lone
Pack
Pack
Pack
Lone
Lone
Lone
Pack
Pack
Lone
Lone
Lone
Lone
Lone
Lone
Lone
Lone
Pair
Pack
Lone
Pack
Pack
Lone
Pack
Lone
Age
(year)
—
2
5
1
7
2
3–4c
1
1–2
—
0
—b
4
1
1
1
2
—b
0
4–5
2
1
3–4
—
—
0
1
1
Body mass
(kg)
Cause of death
30
?
49.5
40
46
48
34.4
(32)b
41.5
43
46
—
33
—b
39
48
42.5
41
44
—b
39
36
38
29
46
—
—
38
—
41
Shot, permission obtained in advance
Shot illegally
Shot
Shot
Shot, permission obtained in advance
Shot by farmer
Shot by farmer illegally
Shot illegally
Car, chased illegally
Shot by farmer
Snowmobile, illegally
Shot by farmer
Shot by hunter
Shot illegally
Car
Car
Car
Train
Train
Shot illegally
Car
Shot, permission obtained in advance
Train
Car
Car
Shot by reindeer owner illegally
Shot by reindeer owner illegally
Car
Not known, old carcass found
Train
a
See Fig. 5.
Head or part of the head missing.
Preliminary result.
b
c
significantly different sex ratio from that of the four males
and five females killed inside this area (Figs. 2I and 6,
Table 1; χ2 = 5.62, p < 0.05). One of the two juvenile (<1
year) dispersers recorded was killed by a car 300 km from
its closest possible point of origin when it was 8–9 months
old (Fig. 8, Table 1).
All monitored newly formed alpha pairs scent-marked and
thus established a territory before breeding. Nine of the 10
wolf territories were established without a common boundary with a neighboring territory (Fig. 2). For wolf packs established after 1983, the minimum distance from a possible
area of origin was 119 ± 73 km (range = 20–250 km), while
the maximum distance was 276 ± 79 km (range = 210–
430 km) (Figs. 2 and 3).
Discussion
Reliability of monitoring
Major parts of the Scandinavian peninsula consist of large
semiwilderness areas with low human density and activity. It
therefore appears that wolves may remain undetected for
long periods. However, for several reasons we believe that
all wolf packs and most pairs were detected during their first
winter after establishment. First, most of the potential wolfbreeding range is covered with snow for several months
each year, which reveals the tracks of wolves. Further, in all
forested parts of Scandinavia there is a dense network of
forestry gravel roads on which resident wolves frequently
travel and leave tracks and scats. In most areas below timberline there is high human activity in the form of forestry,
sports, and recreation. Hunters especially make frequent
use of most ground in Scandinavia. Hunting rights in Sweden and Norway are tied to land ownership, which means
that hunters use the same areas, which they either own or
rent on a long-term basis, year after year and know very
well. The probability that a resident wolf pack, leaving high
concentrations of tracks and scats and killing the same large
game that hunters also seek with tracking dogs, would go
undetected for a whole winter is extremely low.
In the alpine parts of Scandinavia, sheep and reindeer
graze over most areas. Herding activity and the concern for
livestock that are extremely vulnerable to wolf predation
make it very unlikely that resident groups of wolves would
go undetected for long.
Finally, during the years of this study the whole human
society showed great interest in the highly endangered wolf,
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Can. J. Zool. Vol. 79, 2001
Fig. 6. Locations of wolves killed or found dead on the Scandinavian peninsula from 1978 to 1998 (males outside pack territories (),
females outside pack territories (䉲), and both sexes within established pack territories (Ž)). Numbers refer to the numbers of dead
wolves given in Table 1.
and all new wolf occurrences were treated by the media as
great news, which made people very inclined to report observations. Also, there was, and still is, a large group of people in both countries with an extreme interest in wolves, and
who spend a lot of time in the field discovering and recording wolf activity.
We do not believe, therefore, that failure to detect resident
pairs and packs was a large source of error. A greater potential difficulty was to distinguish between the different packs.
However, even this problem was manageable, as most territories were separated by large tracts of empty land.
Our greatest monitoring problem concerned solitary wolves.
A few were resident, scent-marking singles, but most solitary
wolves were dispersers (Messier 1985; Fuller 1989; Geese
and Mech 1991). However, we believe that the error in esti-
mating even lone wolves was small before 1991. During this
period, a maximum of one wolf litter was raised per year,
and total wolf numbers remained low. At that time the media focus on every single wolf reported made people very
inclined to report observations. Organized monitoring
fieldwork was also intensive in both countries (Wabakken et
al. 1984, 1994; Bjärvall 1988). Moreover, in two successive
winters during these years, several hundred local volunteers
participated in independent wolf surveys within the main
wolf-distribution area, but no new individuals unknown to
personnel involved in the ongoing Scandinavian wolf monitoring were found (Overskaug et al. 1984, 1985). We therefore
conclude that for the winters of 1982–1991 the minimum
counts given here are very close to the true numbers.
After spring 1991, when the wolf population increased
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Wabakken et al.
719
Table 2. Estimated count and censused winter count of the Scandinavian wolf population from 1982 to 1998.
Annual
period
Winter
population at
beginning of
period
No. reported
dead during
period
No. assumed
dead aged
>10 years
Total dead
during
period
1981–1982
1982–1983
1983–1984
1984–1985
1985–1986
1986–1987
1987–1988
1988–1989
1989–1990
1990–1991
1991–1992
1992–1993
1993–1994
1994–1995
1995–1996
1996–1997
1997–1998
4
3
3
8
10
11
11
13
15
15
16
25
24
29
37
44
56
1
0
1
1
3
0
0
1
3
1
1
7
2
0
1
3
1
0
0
0
0
0
0
0
1
0
0
1
2
4
2
2
0
1
1
0
1
1
3
0
0
2
3
1
2
9
6
2
3
3
2
Recruitment
(non-alphas × 0.9)a
Simulated
population
size at end
of period
Census
no. at
end of
period
Simulation vs.
census
0
0
6
3
4
0
2
4
3
2
11
8
11
10
10
15
20
3
3
8
10
11
11
13
15
15
16
25
24
29
37
44
56
74
3
3
8
6
7
5
6
10
8
8
17
21
28
34
40
49
61
0
0
0
4
4
6
7
5
7
8
8
3
1
3
4
7
13
Note: Annual periods start on 1 April and end on 31 March.
a
It was assumed that 90% of the pack members except the alpha pair were pups of the year (see Fig. 2).
Fig. 7. Minimum annual mortality rate of wolves in Scandinavia
during 1984–1998.
considerably, the problem of separating reports of nonresident wolves also increased rapidly. The large lynx/wolf
censuses organized by the Swedish Hunters’ Association
were a great help during the latter period (Liberg and
Glöersen 1995; Glöersen 1996; Glöersen and Liberg 1998).
The results of our population simulation further contribute
to the credibility of our census figures. The simulations generally produced higher estimates, but for most years the discrepancy was small in absolute figures. At the end of the
period the simulated estimate was approximately 20% higher
than the corresponding median census estimate (74 vs. 61
wolves), but was close to the maximum range given for the
census (72 wolves). The rate of increase for the entire period
based on the simulation was the same as the one based on
census figures. The discrepancies between simulated and
censused population levels increased rapidly at the beginning of the period. In 1991 it was twice the size of the census figure, though both estimates were relatively low (16 vs.
8 wolves). Possible reasons for this large relative gap during
this early period are underestimation of true mortality or underestimation during censuses. For good reasons we believe
that it is primarily the mortality parameter that is underestimated. It is obvious that many natural wolf deaths will occur
unnoted and therefore not be reported. Secondly, quite a few
illegally killed wolves were reported, but it is reasonable to
assume that most poachers try to cover up their illegal kills.
This explanation fits especially well, as the “missing” deaths
are almost all from the first half of the study period, when
almost all known illegal killing of wolves took place. However, if mortality during the first half of the study period is
underestimated, then our assumption of wolves dying of old
age during the second half is exaggerated, which may explain why the discrepancy between the two estimates for this
period decreased for several years. So in the end, the discrepancies between the two sets of population estimates
could have been due to incomplete information concerning
how deaths were distributed over the period.
Population trend
Even though seven litters were confirmed in south-central
Scandinavia between 1983 and 1990, and more than 20 pups
were produced, the number of wolves monitored each winter
remained fewer than 10 during that whole period. It was after 1991, when we started recording more than one litter annually, that the population first showed a marked increase,
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Can. J. Zool. Vol. 79, 2001
Fig. 8. Assumed dispersal of wolves from territory 2 (see Fig. 2) in Scandinavia during February–June in 1984 (A) and 1984–1992 (B).
and this continued till the end of the study in 1997–1998.
There are a number of possible causes of this delay in population growth.
Prey density
Prey availability may be the ultimate factor limiting wolf
populations (Peterson et al. 1984; Ballard et al. 1987; Messier
1994). However, during the 1980s and 1990s, prey availability was unlikely to have been a limiting factor for the wolf
population in Scandinavia. Estimated from wild ungulate
biomass, Sweden alone could support 100 times more wolves
than the population size during our study (Persson 1996).2 In
south-central Scandinavia, moose (Alces alces) and roe deer
(Capreolus capreolus) are the most important prey species
for wolves (Wabakken et al. 1996; Olsson et al. 1997). During the 1980s, when no wolf population increase occurred,
the moose population was almost twice as large as during
the 1990s, when the wolf population finally increased (Lavsund
and Sandegren 1989; Hörnberg 1995). Likewise, the density
of roe deer increased during the 1980s and peaked in the
early 1990s in this part of Scandinavia (Liberg et al. 1994).
Thus, we may conclude that the delay in the wolf population
2
increase in Scandinavia was not caused by a lack of prey
during the 1980s.
Allee effects
For some populations it has been observed that below a
certain level, even if the limiting factors that brought it down
there are removed, population growth remains low. Factors
causing this reversed density dependence at low densities
have been called Allee effects (Allee et al. 1949). This is a
collective term for a number of negative processes that may
operate at low population levels, including inbreeding effects,
problems of finding breeding mates, reduced social facilitation, or stochastic factors. Thus, one possible explanation for
the lack of increase in the Scandinavian wolf population before 1990 is inbreeding depression that was later eased by
the immigration of one or a few wolves. However, this is
contradicted by recent analyses of DNA samples from killed
wolves that show a progressive decrease in genetic variation
all through this period and no sign of immigration after 1983
(Ellegren et al. 1996).
The lack of increase in the Scandinavian wolf numbers
before 1991 is more likely to have been caused by the prob-
Persson, J. 1996. Vargars populationsdynamik—ett svenskt perspektiv. Examensarbete 1996; 7, Swedish University of Agricultural Sciences,
Umeå.
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Wabakken et al.
lem of finding mates. During this period breeding took place
in only one territory, and it is known that close kinship
might prevent pair formation (Smith et al. 1997). The fact
that the rate of formation of new pairs increased dramatically once multiple reproducing units were present indeed
indicates some kind of mating avoidance between descendants from the same territory, even if they were born into
different litters.
Another important aspect of the delayed population increase is that small populations are vulnerable to stochastic
mortality (Burgman et al. 1993). When a population is as
small as the present wolf population during 1981–1991, just
a few “extra” deaths may alter the population trend. In this
period 10 wolves were found dead. According to our simple
simulation model, only eight “extra” deaths during this 10year period, i.e., less than one per year, would have been
enough to prevent population increase. Considering that most
of the mortality in this period was illegal killing, it is not unreasonable to conclude that we found only a fraction of all
dead wolves.
Change in human attitude and protection
A credible contributing factor to the changed rate of increase
during the study is the possibility that human tolerance towards wolves increased. We have no sociological evidence
that this occurred, but there is some support for this possibility in our mortality data. Almost all reported wolf mortality
during the study period was caused by human activity, but
the rate of this mortality decreased over time. Although the
number of wolves reported killed per year increased from an
average of 1.4 between 1984 and 1990 to 2.0 between 1991
and 1998, the annual mortality rate was about twice as high
during the first period as during the second, and six times
higher if it is only compared with that in the last 4 years of
the study, 1994–1998.
Comparison with other recovering wolf populations
The recovery pattern of the Scandinavian wolf population
has similarities to the recovery of several wolf populations in
the northern United States. In Wisconsin, recolonization started
in the middle 1970s by wolves dispersing from Minnesota
(Mech and Nowak 1981). As in Scandinavia, the wolf population in Wisconsin remained low during most of the 1980s
(Wydeven et al. 1995) but showed a marked increase during
the 1990s (Anonymous 1999). Between the two periods 1979–
1985 and 1986–1991, human-caused mortality of radiocollared wolves in Wisconsin decreased from 72 to 22% of
total mortality and was suggested to be the major cause of
the population increase (Wydeven et al. 1995). In two
recolonizing populations in the central Rocky Mountains,
mortality of radio-collared wolves caused by humans was 95
and 84% of total mortality (Boyd et al. 1995; Bangs et al.
1998). This suggests that human-caused mortality can be the
main mortality factor limiting the increase in newly established and recovering wolf populations.
One important difference between the recovering wolf
population in Scandinavia and those in the northern United
States is that in the latter, immigration of wolves from adjacent
populations may constitute an important additional source of
population increase (Forbes and Boyd 1996). Although dis-
721
persal may occur between the Finnish and Scandinavian
wolf populations, it is not likely to be of a magnitude that
could significantly affect the rate of increase.
Dispersal pattern
The average minimum dispersal distance found in this
study (313 km; sexes combined) was greater than reported
from North American studies (Gese and Mech 1991; Wydeven
et al. 1995; Mech et al. 1998). An important reason for the
unusually long dispersal distances among Scandinavian wolves
could be that the dispersals took place in areas of extremely
low wolf density. The longest average dispersal distance reported from North America, 213 km, was also reported from
a low-density recolonizing population in the Rocky Mountains (Boyd et al. 1995). This strongly suggests that dispersal distances in wolves are affected by population density,
and ultimately by the probability of finding a mate. The variability in dispersal distances among individuals and populations may be particularly important for increasing the
survival of dispersers in areas with scattered populations,
and long-distance movements may facilitate genetic exchange between populations (Fritts and Carbyn 1995; Forbes
and Boyd 1996).
Colonizing pattern
There are different species-specific patterns of expansion
during the colonization phase. One extreme is a “diffusion”
or “random walk” type of spread, which gives a steep and
well-defined expansion front (Skellam 1951; Caughley 1970;
Okubo 1980). The other extreme is an expansion characterized by single long leaps forward, where the expansion front
becomes “flatter” and less well defined, typical of organisms
with a high dispersal capacity and presaturation dispersal
(Usher 1986; Hartman 1995; Swenson et al. 1998). The type
of organisms expected to be extreme “long leapers” are pioneer species. Several of the most important prey species for
wolves in the northern forest biome are of this type: roe
deer, moose, and white-tailed deer (Odocoileus virginianus)
(Liberg and Wahlström 1995; Wahlström and Liberg 1995).
The wolf colonization pattern in the present study is of the
latter type. Only one of the new territories was established in
direct contact with a possible source territory. All the others
were established at unpredictable places at varying distances
from possible natal territories. At this early stage of Scandinavian wolf recovery, most wolves pass through large areas
of apparently suitable habitat before settling. Consequently,
in 1998, 20 years after the first reproduction, there were still
huge expanses of unoccupied habitat within the area encompassed by the outermost territories (Fig. 2I).
This colonization pattern is obviously not unique to our
study population. Similar patterns were found in expanding
wolf populations in Montana (Boyd et al. 1995) and Wisconsin (Wydeven et al. 1995), but Fritts and Mech (1981) described more diffusion-like colonization. This behavior in
wolves may have evolved in response to a similar dispersal
pattern of their main prey. An alternative explanation is that
settling far away from other territories may minimize competition during the early phase of colonization, when there is
a surplus of suitable habitat.
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722
Population isolation and risks of inbreeding
Small populations may be exposed to genetic deterioration
by the process of inbreeding (Frankel and Soulé 1981).
Among 15 wild gray wolves killed between 1977 and 1994
in Scandinavia, Ellegren et al. (1996) found up to five different microsatellite alleles at a single locus and concluded that
the population in south-central Scandinavia was probably
founded by as few as three individuals. This fits with our observations from the early phase of this population recovery
(Wabakken et al. 1982; Bjärvall and Isakson 1983). Ellegren
et al. (1996) also found a significant decline in genetic variability with time in the present wolf population, indicating a
high degree of inbreeding. Inbreeding had detrimental effects on several fitness traits among captive Fennoscandian
wolves (Laikre and Ryman 1991), but has never been shown
to be a problem in wild wolves (e.g., Isle Royale; Fritts and
Carbyn 1995). In this study, litter size for wolf pairs having
their first litter decreased during the study period. This could
be the first indication of inbreeding depression among wild
Scandinavian wolves, but low genetic variation without inbreeding depression is not rare among carnivores (Merola
1994).
Population viability
Recently, discussions among wildlife managers and researchers in Norway and Sweden have focused on when and
at what size the Scandinavian wolf population may be considered viable. Three major risks for small populations have
been identified: stochastic variability in demography and the
environment and genetic problems (Soulé 1986; Burgman et
al. 1993). In an analysis using the software VORTEX (Lacy
1993) it was found that a wolf population of 100–200 individuals or more would be regarded as viable (>95% probability
of survival within 100 years) according to demographic and
environmental stochasticity (Ebenhard 1999). By using data
on the minimum population size found in Scandinavia in the
winter of 1998 (50 wolves) and the growth rate (λ = 1.20), it
was found in the same analysis that the risk of extinction
within 100 years was negligible. Similarly, from modelling
disjunct wolf populations, Haight et al. (1998) found that as
few as 16 territories or 100 individuals could survive long
term if immigration was adequate and portions of the population were protected.
These analyses assume that there were no suppressing effects of inbreeding. When genetic risks are considered, the
required sizes of viable populations increase. Soulé (1980)
concluded that the minimum population size necessary to
avoid an acute risk of inbreeding depression was at least
200, assuming an effective population size (Ne) of 50. To ensure long-term (hundreds to thousands of generations) viability, no fewer than 2000 (Ne = 500) wolves would be
needed (Franklin 1980).
Ebenhard (1999) also simulated the effects of genetic deterioration of the Scandinavian wolf population and concluded that if it was totally isolated, a population of 500
wolves or more should be needed to maintain 95% of the genetic variation found in the population for 100 years. Alternatively, to achieve the same genetic goal with a population
size of 200 wolves, immigration would be necessary. If immigration occurred, one wolf per year, on average, would be
Can. J. Zool. Vol. 79, 2001
sufficient to counteract the loss of genetic variation in this
population.
Thus, the question of whether the Scandinavian wolf population is totally isolated or has connections with other populations may be vital to the future of the population, and
thus its management. The shortest distance between the edge
of the present population and the nearest possible genetic
source, the Finnish–Russian wolf population, is about 800 km,
a distance within the range known for dispersers of both
sexes (Fritts 1983; Boyd et al. 1995; this study).
Management
In a management plan for large carnivores passed by the
Norwegian Parliament, the primary goal for Norwegian management is to ensure the presence of “8–10 family groups”
in south-central Scandinavia (Miljøverndepartementet 1996–
1997). Swedish authorities have suggested a slightly higher
level, 15 “resident wolf pairs” (Naturvårdsverket 1997). Both
population levels are far below the carrying capacity of
wolves on the Scandinavian peninsula, even if only the
present wolf range is considered (Persson 19962; Persson
and Sand 1998). It is also below what is considered a viable
population size from a genetic, long-term perspective (Ebenhard 1999; Laikre and Ryman 1999). However, Fritts and
Carbyn (1995), arguing from empirical data on extant wolf
populations, stated that wolf populations in general exhibit
greater resilience than is indicated in theory, that previous
theoretical treatments of population viability exaggerated the
required population size, and therefore that viability analyses have contributed little to wolf recovery programs. They
also concluded that “the extraordinary dispersal capability
of the species—hence a large degree of metapopulation
connectedness—may be a major reason why simple theoretical models are not adequate for wolves.” In either case, the
management challenge is to maintain a viable wolf population in a small geographical area at a certain predetermined
level, given the high potential population growth rate of
wolves.
Attitude surveys have shown that both on the national
scale and locally, within the main wolf range, a great majority of Swedes and Norwegians do not want the species to be
exterminated (Dahle et al. 1987; Bjerke et al. 1998; Karlsson
et al. 1999). At present, illegal killing of wolves does not
seem to be an immediate threat to the population because it
is growing by approximately 25% annually despite some illegal killing. Still, major future challenges in Scandinavian
wolf management will be to maintain and further improve
public understanding and local acceptance of the species as
its population grows.
Another challenge is to keep the large semidomesticated
reindeer area, which covers the northern half of Scandinavia,
free of conflicts with wolves, at the same time allowing passage of at least some dispersers from the source population
in Finland/Russia to mix with the south-central Scandinavian wolves in order to avoid genetic isolation. As a consequence, more knowledge about wolf population viability in
Scandinavia is needed, and emphasis should be placed on
continued integrated research on wolf genetics and population dynamics based on radiotelemetry, including both the
Finnish and the Scandinavian wolf populations.
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Acknowledgements
We thank Diane Boyd, Steven Fritts, Rolf Peterson, and
Henrik Andrén for valuable comments that significantly
improved the manuscript. This research was supported financially by the Swedish Environmental Protection Agency,
Swedish University of Agriculture Sciences, Norwegian Ministry of Environment, Norwegian Directorate for Nature Management, Norwegian Research Council, Hedmark College,
County Governors of Hedmark, Akershus and Ostfold,
Swedish Hunters’ Association, Olle och Signhild Engkvist
Stiftelse, and World-Wide Fund for Nature (Sweden and Norway). We are also grateful to those who participated in the
fieldwork, especially P. Ahlqvist, Å. Aronsson, R. Bjornstad,
S.E. Bredvold, B.T. Baekken, J. Enerud, R. Franzén, O.R. Fremming, G. Glöersen, J. Gronbekk, E. Isakson, K. Johnsson,
E. Momb, E. Maartmann, K.A. Olander, C. Onsager, M. Rapp,
O.K. Sauge, O.K. Steinset, B. Strandberg, R. Wicklund, J. Wirtberg, I. Woxmark, and several other volunteers. Also, we are
grateful to E. Maartmann, who finished the illustrations, and
to Harry P. Andreassen for statistical advice.
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