Hydrobiologia 455: 87–95, 2001.
© 2001 Kluwer Academic Publishers. Printed in the Netherlands.
87
Life cycle phenology of common detritivores from a temperate
rainforest stream
John S. Richardson
Department of Forest Sciences, 3041 – 2424 Main Mall, University of British Columbia, Vancouver,
BC, V6T 1Z4 Canada. Tel: +604-822-6586. Fax: +604-822-9102. E-mail: [email protected]
Received 25 January 2000; in revised form 5 February 2001; accepted 3 April 2001
Key words: aquatic insects, Brillia, Despaxia, detritivores, Lepidostoma, life cycles, Malenka, phenology, stream,
Zapada
Abstract
The timing of life cycles, including growth rates, was determined for eight common species of detritivorous insects
in a second-order stream in southwestern British Columbia, Canada. Six of the species (Zapada cinctipes, Z.
haysi, Malenka californica, M. cornuta, Capnia sp., and Lepidostoma roafi) had simple, univoltine life cycles.
The leuctrid stonefly Despaxia augusta has a 2-year life cycle, with an apparent egg diapause of about 6 months.
The chironomid Brillia retifinis produced at least three generations per year. The major growth periods for the set
of species considered here span the entire year. Adults of several species exhibited seasonal declines in size at
emergence, but one species had larger adults as the emergence period proceeded. Closely related taxa had more
similar life cycle timing than more distantly related species suggesting a degree of phylogenetic constraint in
phenology of their life cycles. The influence of the timing of leaf drop on timing of life cycles for these animals
does not fit with proposed scenarios based on fast and slow leaf processing rates.
Introduction
The timing of life cycles has evolved to balance a number of constraints on an organism’s fitness brought on
by events in its environment. These might be temperature limitations on certain stages of an organism, timing
of abundance of food (e.g. Cummins et al., 1989), or
requirements to complete certain stages before the onset of unsuitable conditions (Rowe & Ludwig, 1991).
In streams, some of the environmental changes that
set limits to the timing of life cycles include timing of
floods (or drought), availability of food (Richardson,
1991) and seasonal temperature patterns (Sweeney et
al., 1986), including air temperatures.
Detritivorous aquatic insects (those feeding on
coarse particulate organic matter such as leaves) in
temperate streams sometimes go through periods of
resource scarcity that affect their rates of growth and
survival (e.g. Richardson, 1991; Dobson & Hildrew,
1992). Based on the seasonal availability of leaf detritus in streams, Cummins et al. (1989) have sugges-
ted that life cycles of such animals should be timed
to maximise the use of that seasonally varying resource. Testing the latter model depends upon detailed
information on food availability, temperature and food
dependent growth rates, and life cycle information
about consumers.
Details of the biology of most species of stream
invertebrates are poorly known, in part because immature stages last for upwards of 90% of the life cycle
for many species. The timing of life cycle events is
a complex response to a series of selection pressures
operating on all life stages, encompassing both aquatic
and terrestrial habitats. In addition, immature stages
are often not identifiable to species. For most of the
species I have investigated, there were no detailed reports on their natural history except for a few records
of emergence dates and geographic distribution. In
this paper, I present data on life cycle patterns and
other observations of the common detritivores collected in Mayfly Creek, British Columbia, Canada.
One goal of examining life cycle timing is to consider
88
how phenology corresponds to seasonal changes in
food abundance and the timing of life cycles of other
species.
Materials and methods
These samples all were taken from Mayfly Creek in
the University of British Columbia’s Malcolm Knapp
Research Forest near Maple Ridge, British Columbia,
Canada (49◦ 18 N, 122◦ 32 W). Mayfly Creek has
a drainage basin of approximately 3.6 km2 at the
study site (downstream of the study reach in Reece &
Richardson, in press). The channel gradient is 3.8%
over the entire reach, including a combination of steppool and riffle-pool morphologies. Channel bankfull
width is about 5 m. The region is cool, temperate
with cool, dry summers (June–September) and a wet,
warm winter period (October–April) characterised by
Pacific frontal weather systems. Average monthly air
temperature in January is 0 ◦ C and for July it is 17 ◦ C.
Mayfly Creek water temperatures extend from 0◦ C to
the maximum of 17.5◦C recorded in 10 years of work
at the site, although in most summers the temperature
does not exceed 15◦C. Annual precipitation is largely
as rain and amounts to 270 cm per year. There is no
gage on the river, but estimates of discharge (below
bankfull discharge) were 0.04–0.52 m3 /s (Reece &
Richardson, in press). The lowest flows are typically
in August or September.
The data in this paper were assembled from a number of studies, using a variety of sampling techniques,
over a 5-year period. Benthic samples included a regular sampling program, with samples collected at least
monthly using a Hess sampler equipped with staged
nets of 250 µm and 102 µm mesh size, and enclosing
a sample area of 712 cm2 . Data from benthic samples
of the Mayfly Creek experimental streams (miniature
Surber sampler with 250 µm mesh) and from leaf pack
decomposition and colonisation experiments were also
included. Sampling intensity was higher from late
spring to early autumn. Data for sizes were from different dates depending on the studies from which they
were derived. I pooled data across years and studies
into 2-week periods. If there were too few individuals for a reliable estimate for the 2-week period, then
data were combined into monthly periods. Because
data were pooled across years, variation by year was
ignored.
Data on the timing of adult emergence came from
a series of emergence cages on experimental stream
channels on the floodplain adjacent to Mayfly Creek
(description in Richardson, 1991). These adults came
from a food supplementation experiment, but since
there were no differences in emergence timing due
to experimental treatment (Richardson, 1991), the
data should be representative of the population in
the natural stream. The emergence cages were emptied at least once every 3 days during the period
May–September 1986, and at less frequent intervals
thereafter until 30 April 1987.
Individuals in samples were sorted from debris,
identified to species when possible (exceptions are
noted in the Results section), and counted. Identifications followed keys in Ricker (1943), Baumann et
al. (1977) and Merritt & Cummins (1996), and were
all verified by experts (see ‘Acknowledgements’). The
width across the head at the level of the eyes was
measured to the nearest 0.024 mm with an eyepiece
micrometer in a dissecting microscope. A subset of
larvae and adults were individually measured, dried,
and weighed to produce regressions of dry mass based
on headwidths (Table 1).
Results
The eight taxa discussed here represent five families
in three orders. The emergence periods of all taxa
combined include most of the year. Two groupings
can be discerned, those which emerge during winter
and early spring (the ‘winter stoneflies’ Zapada spp.,
and Capnia spp.) and those which emerge during
late spring, summer and early autumn (Malenka spp.,
Lepidostoma spp., Despaxia augusta (Banks), Brillia
retifinis Saether).
The most abundant non-chironomid detritivore in
Mayfly Creek was the nemourid stonefly Zapada cinctipes (Banks). The life cycle of this species was clearly
univoltine in this stream (Fig. 1). Adult emergence occurred from 8 January to 7 April, with about half of all
individuals emerging during the first half of February
(Fig. 1). Males emerged before females on average,
and mean mass of females was 76% larger than males.
The early instar larvae appeared during May and June
(Fig. 1).
Zapada haysi (Ricker) (part of the Z. oregonensis
group) is also univoltine in Mayfly Creek. The adult
emergence period occurred between 17 March and 27
May, with the peak rate of adult emergence between
7 and 27 April (Fig. 1). Early instar larvae began to
appear during the latter half of June about 6 weeks
89
Table 1. Size-mass relations for larvae measured as headwidth (mm) and mass
(mg dry weight) for each taxon for use in calculating larval biomass from head
width measurements
Species
Size-mass relation
r2
n
p
Zapada cinctipes
Zapada haysi
Malenka spp.
Despaxia augusta
Capnia spp.
Lepidostoma roafi
mass = 0.008 e3.195head
mass = 0.0133 e2.576head
mass = 0.0047 e 3.857head
mass = 0.0052 e 4.167head
mass = 0.0039 e 4.549head
mass = 0.0067 e 5.126head
0.94
0.92
0.93
0.96
0.90
0.70
264
26
55
34
48
31
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
Table 2. Adult weights by gender in mg (dry weight). Seasonal changes in adult weights over the emergence period based
on regressions of weight by date of sample collection
Species
Female - mean
mass (+1 s.e.) n
Males - mean mass
(+1 s.e.) n
Ratio of
weights
female male
Seasonal
change in adult
weights
Slope of overall
seasonal pattern
of weights
Zapada cinctipes
Zapada haysi
Malenka cormuta
Malenka californica
Despaxia augusta
Capnia nana
Lepidostoma roafi
1.551 (0.032) 108
1.702 (0.068) 46
0.591 (0.012) 94
0.892 (0.019) 142
0.722 (0.026) 47
0.688 (0.109) 15
1.698 (0.057) 66
0.882 (0.012) 185
1.107 (0.036) 37
0.451 (0.012) 47
0.557 (0.014) 108
0.518 (0.016) 62
0.365 (0.031) 13
1.159 (0.031) 81
1.76
1.54
1.31
1.60
1.39
1.88
1.47
p < 0.0001
p < 0.0001
n.s.
p < 0.0001
n.s.
n.s.
p < 0.0001
–0.0062
–0.0141
–0.00049
+0.0039
–0.0024
–0.0039
–0.0185
later than Z. cinctipes on average. Larvae were slightly
smaller than the congeneric Z. cinctipes throughout its
life cycle, although at emergence Z. haysi was actually
larger and heavier (Table 2).
Malenka californica (Claassen) and M. cornuta
(Claassen) (Plecoptera: Nemouridae) were indistinguishable as larvae, except for a slight bimodality in
the frequency distribution of head sizes during late
June and July. Early instar larvae appeared from October to late winter, and growth was rapid from April to
adult eclosion (Fig. 2). The slight decrease in the lower
part of the size range during early spring may represent the hatching of M. californica larvae, since that
species had a later emergence date. Since both species
are included (in Fig. 2), the range of larval head widths
is greater than that expected for a single species. The
emergence periods for these species also overlapped.
Malenka cornuta emerged from 29 May to 5 August (one individual was trapped between 12 and 16
September); the peak emergence was from mid-June
to mid-July. Malenka californica adults emerged from
12 June to 16 September, with most emerging in the
last 3 weeks of August.
Despaxia augusta (Plecoptera: Leuctridae) was
semivoltine at this site with the cohorts distinct and
overlapping (Fig. 2). Adult emergence occurred from
13 August to 30 September with most emerging in the
last week of August. Early instar larvae were collected
from late April through June, but none were found
during autumn or winter. Growth was rapid during
summer and early autumn but was indiscernible during
the winter.
The Capnia (Plecoptera: Capniidae) were evidently represented by two species based on two discrete
sizes of females collected in emergence cages. I was
not able to distinguish larvae, or collect males of both
species. The males that were identified were all C.
nana Claassen and presumably most of the Capnia belonged to that species. The life cycle was simple with
emergence from early January to late March (Fig. 3).
Three species of Lepidostoma (Trichoptera: Lepidostomatidae; L. roafi(Milne), L. unicolor (Banks),
and L. cascadense (Milne)) were collected from the
90
Figure 1. Seasonal development patterns of Zapada cinctipes, and
Zapada haysi in Mayfly Creek, British Columbia. Squares represent
the mean head width, and error bars indicate range (thin, longer
bars) and standard deviation (thick, shorter bars). Dashed lines
indicate periods of adult emergence. Total numbers from which figures were composed were Z. cinctipes = 5179 individuals, Z. haysi
= 889 individuals.
study streams, and of these L. roafi was the most
common. All three species were easily distinguished
as larvae except for the first instars of L. cascadense
and L. roafi which both construct cases of fine sand
in the initial instar. All three species were univoltine in Mayfly Creek and there was no suggestion of
any diapause or resting stage. The larvae of L. roafi
began to hatch during the late summer and apparently
some individuals continued to join the population until January (Fig. 4). The pattern of development was
straightforward so the frequency distribution of instars
was pooled for many dates in Fig. 4. Lepidostoma
roafi adults emerged from 24 June to 6 September, and
most left the stream between the last week of July to
mid-August.
I did not sort or identify adult chironomids, so the
description of the life cycle of Brillia retifinis (Ortho-
Figure 2. Seasonal development patterns of Malenka species and
Despaxia augusta in Mayfly Creek, British Columbia. Symbols are
as in Fig. 1. Total numbers measured were Malenka spp. = 1528
individuals, Despaxia augusta = 3099 individuals.
cladiinae) is based only on the collections of larvae
and pupae (Fig. 5). Few individuals were found in
Figure 3. Seasonal development patterns of Capnia species (mostly
C. nana based on identification of adults) in Mayfly Creek, British
Columbia. Symbols are as in Fig. 1. Total number of capniid larvae
measured was 1509 individuals.
91
Figure 4. Seasonal development patterns of Lepidostoma roafi in
Mayfly Creek, British Columbia. The width of each section of each
histogram represents the percentage of that instar in collections for
that date. The dashed line at the top of the figure indicates periods of
adult emergence; the symbols on the abscissa are roman numerals
for larval instar number, PP for prepupae, and P for pupae. Total
number of L. roafi measured was 452.
Figure 6. Seasonal patterns of growth of common detritivore species from Mayfly Creek, British Columbia. Mass on collection dates
is shown as a percentage of the largest mean mass towards the end
of the larval stage.
Figure 5. Seasonal development pattern of Brillia retifinis (Orthocladiinae) in Mayfly Creek, British Columbia. The width of each
section of each histogram represents the percentage of that instar
in collections for that date. The symbols on the abscissa are roman
numerals for larval instar number. The dashed line represents the
dates on which pupae were collected. Total numbers measured =
7295 individuals.
Figure 7. Seasonal patterns in the weights of adult Zapada cinctipes
(females shown as filled circles and males as open squares). These
data were from a food supplementation experiment and despite the
additional influence of food levels, the pattern of decline is significant. Each point represents a single individual. Regression lines
shown by gender and both were significant (p<0.0001).
winter and early spring collections, and none of these
were first instar larvae. First instar larvae were collected from May to November. Pupae were collected
from late June to November (Fig. 5). The adult emergence period was inferred from the collection dates of
pupae.
Regressions of mass based on head width were
developed for the larvae of most of the common
detritivores (Table 1). The seasonal pattern of mean individual growth is shown as a percentage of the mean
final mass (Fig. 6). The species included here had their
primary time of growth clearly divided into winter or
late spring / early summer periods. The winter stoneflies accumulated most of their growth during winter.
The pattern for all these species is typical for direct
development without diapause in that the majority of
larval growth is made up in the later instars. Malenka
spp., Lepidostoma spp., Despaxia augusta and B. retifinis had their main growth periods during late spring
and summer.
Adults of seven of the species (excluding Brillia) were dried and weighed. Females always were
on average heavier than males (Table 2). An interesting phenomenon is that some of the species
92
ficantly related to both variables (multiple regression,
F2,8 = 5.5, P<0.04, R2 =0.58; latitude p<0.023 and
altitude p<0.016). The first date of emergence was
later with increased altitude or latitude. Neither latitude nor altitude was a significant predictor of timing of
emergence by itself. There was no simple relationship
between the length of the emergence period and either
geographic variable (multiple regression, F2,8 =1.075,
P >0.3).
Discussion
Figure 8. Patterns of geographic variation in the timing of the first
date of emergence of Zapada cinctipes summarised from literature
reports. References include: 1 Cather and Gaufin, 1976, 2 Clifford,
1969, 3 Ellis, 1975, 4 Jewett, 1959, 5 Kerst and Anderson, 1975,
6 Radford and Hartland-Rowe, 1971, 7 Ricker, 1943, 8 Sheldon and
Jewett, 1967, 9 this study. ∗ - point represents two records.
showed a seasonal decline in weights, i.e. individuals of a given species and sex that emerged early in
the emergence period were larger than those emerging later (Table 2). Zapada cinctipes, Z. haysi, and
Lepidostoma roafi all declined significantly over the
emergence period (ANCOVA, sex and treatment effects removed, all p<0.0001). Despite the fact that
these animals were from a food supplement experiment (Richardson, 1991), the effect of emergence date
was highly significant and the second largest source
of variance after sex, i.e. the date effect was greater
than any treatment effects. Fig. 7 presents an example
using Z. cinctipes. Malenka californica also showed a
significant seasonal change in size, except that for this
taxon the individuals earlier in the emergence period
were smaller than those emerging later.
Zapada cinctipes have been collected in detailed
studies in a number of locations. I used data on emergence dates from the literature to examine patterns in
the timing of emergence of this species by latitude and
altitude (Fig. 8). The first date of emergence was signi-
The life cycles of most species considered in this study
were simple and in general synchronised within a species. In most species, development was direct, i.e.
no diapause stages. Presumed hatching dates reflected the variation in adult emergence dates, suggesting
no delayed egg hatching. The exception to this pattern was Despaxia augusta which had an apparent egg
diapause, as well as a semivoltine life cycle at this
site. For the majority of these species, there is little or
no detailed information with which to compare these
results, with the exceptions noted below.
Zapada cinctipes and Z. haysi are both common
detritivores found in mountain streams of western
North America (Baumann et al., 1977; Short et al.,
1980; Oberndorfer et al., 1984). All reports on the life
cycle of Z. cinctipes from other locations indicate it
is univoltine (Ellis, 1975; Kerst & Anderson, 1975;
Cather & Gaufin, 1976; Sakaguchi, 1978). The emergence period of Z. cinctipes apparently began over
two months later in an Alaskan stream and extended
into late July (Ellis, 1975), compared with a January–March emergence period in southwestern British
Columbia (this study). Differences in latitude and altitude among the locations of these various populations
account for much of the geographic variation in the
timing of emergence.
Malenka californica is found throughout the
Rocky Mountains from northern British Columbia to
New Mexico (Jewett, 1959). The recorded periods
of emergence vary among populations but are always
late summer and autumn (Ricker, 1943; Jewett, 1959;
Sheldon & Jewett, 1967; Kerst & Anderson, 1975).
The congeneric M. cornuta has a more limited range
in the Coast and Cascade Mountains from British
Columbia to Oregon (Jewett, 1959). The known emergence records for M. cornuta are late spring to the
end of summer (Ricker, 1943; Jewett, 1959; Kerst &
Anderson, 1975). As was found for Zapada spp., the
93
timing of growth and emergence by the two Malenka
congeners was almost entirely overlapping.
Adults of Despaxia augusta emerged in late summer and early autumn, and there was no evidence that
adults overwintered before laying eggs, thus indicating a 2-year life cycle. Some stoneflies with longer
life cycles have shown variation within cohorts such
that some individuals mature in 1 year and others in
2 years, a pattern known as cohort splitting (Harper,
1973; Dieterich & Anderson, 1995). There was no
evidence of cohort splitting in my data for D. augusta
in that there was a clear separation in the size ranges
of the two cohorts. There are no records of collections
of adults in the winter or spring. This species has an
egg diapause that lasts through the winter, with hatching taking place with the beginning of spring. Other
species that are associated with streams that cease to
flow in summer are known to have a summer diapause
to avoid the low-flow period (Dieterich & Anderson,
1995), which was not the case for this species. Early
instars were never collected during the late autumn
or winter, so it is unlikely that larvae hatch and remain inactive during this period. Collections of D.
augusta adults from a stream in Alaska were made
from the second week of August until the first week
of September (Ellis, 1975), very similar to the observations of emergence from Mayfly Creek. Other
unpublished data from our study area indicates that
the species has a more extended emergence period,
lasting almost 4 months, than is indicated in the data
presented here. This species is also commonly found
in seasonal streams at our research forest, which is intriguing given it is the only species of those considered
here that has a 2-year life cycle.
The case-building larvae of Lepidostoma roafi
were the most abundant of the detritivorous caddisflies. There are no detailed studies of the biology of
L. roafi, aside from work on biosystematics. In one
study in Oregon, seven males of the species were
collected in emergence traps during late August and
September (Anderson & Wold, 1972) somewhat later
than at Mayfly Creek. In Mayfly Creek, there were
three species of Lepidostoma, i.e. L. roafi, L. unicolor and L. cascadense. The latter two species had
a similar growth period and emerged about a month
before L. roafi, similar to the pattern found for the latter two species in Oregon (Grafius & Anderson, 1980).
Lepidostoma roafi was quite dense in local accumulations of coarse detritus in large pools in the late spring
along with L. unicolor and L. cascadense (see also
Winterbourn, 1971).
The number of generations each year for Brillia
retifinis was impossible to discern from the data, in
part due to overlapping generations. It apparently had
several cohorts annually and this was evident in the
exponential growth of population densities over a 3
month period in stream channels where food was experimentally supplemented (Richardson, 1991). The
extended periods over which first instar larvae and
pupae were collected suggests several generations,
probably at least three per year.
Authors studying detritivorous stream invertebrates frequently make the statement that life cycles
have become synchronized to the autumnal input of
leaf material (Petersen & Cummins, 1974). This assertion seems to derive from Ross’s (1963) observation
that there is a broad correspondence between stream
insect communities and variations in riparian forest
assemblages. The interpretation of this statement is
confounded by spatial autocorrelation between vegetation characteristics and regional climate, which affects
both vegetation and timing of life cycles of aquatic
insects. Given that any two periodic phenomena with
an annual cycle will appear synchronised (with some
lag) makes it difficult to generate a non-tautological
prediction. Experimental alterations of the relative input rates of so-called fast and slow processing detrital
materials (Petersen & Cummins, 1974; Hanson et al.,
1984; Grubbs & Cummins, 1996) might be one way
to determine the causal connection between the two.
Changes in riparian vegetation composition as a consequence of forest harvesting would be a good test, but
first species would need to be screened for their relative growth rates on different kinds of detrital materials,
as well as the temperature dependence of growth.
It is not clear if life cycle timing should be related to an individual’s maximisation of growth or
survival, although survival would usually have precedence. Optimisation of an individual’s growth rate
may be determined by the timing of the later instars
with peaks in food abundance unless food is not limiting (e.g. Abrams et al., 1996). There could be a
relation between life cycle timing and the different
discharge regimes in coastal streams (rain-driven hydrology) versus streams of the continent where spring
runoff and summer storms are more critical than
autumn/winter events. In coastal, rainforest streams
flashy autumn and winter hydrographs may make it
unsafe to be in a life stage/age class that is not mobile (eggs/pupae). One further hypothesis that needs
to be addressed is whether timing of emergence may
be a function of conditions in the terrestrial environ-
94
ment and seasonal constraints (e.g. Rowe & Ludwig,
1991). The relation between the timing of Z. cinctipes
emergence and both elevation and latitude is consistent with predictions for terrestrial invertebrates (e.g.
Bradford & Roff, 1995). The pattern does not fit
Hopkins’ (1920) bioclimatic rule in which a 122 m
increase in altitude is equivalent to 1 ◦ of latitude,
and this suggests there is local adaptation of life cycle
timing. Moreover, this indicates there is at least a
broad correspondence between life cycle events and
the regional conditions, which may not be strictly
determined within the stream.
Another pattern noted here and commonly observed for other animals is that size of adults at
emergence declined through the season (with the obvious exception of Malenka californica). These patterns
are predicted outcomes of life histories where there
is variation in hatching times or individual growth
rates, and where getting through particular life stages
sooner confers a fitness advantage (e.g. Rowe & Ludwig, 1991; Abrams et al., 1996). It is not clear what
the advantage of earlier emergence might be. One
hypothesis is that in a food-limited system, as the
detritus system within streams appears to be (Richardson, 1991; Wallace et al., 1999), animals getting an
earlier start through the larval stages may be better
able to gain access to resources over other individuals.
A food limitation study showed that for Z. cinctipes,
individuals that had access to extra detritus in summer
were able to gain a size advantage that other individuals were never able to catch up to despite high food
abundance later in their life cycle (Richardson, 1991).
This is in contrast to the suggestion that some other
detritivores in more seasonal streams may be able to
compensate for small size by increasing their growth
rates (Dieterich & Anderson, 1995).
The periods of rapid growth for most species overlapped considerably with other species. The primary
growth periods of all eight common taxa together
spanned the entire year. The sets of congeners,
Malenka spp., Zapada spp. and Lepidostoma spp.
were more similar in life cycle timing to their congener than to other taxa. This suggests two things
about evolution of life cycle timing. First, there is a
degree of phylogenetic constraint such that closely related taxa have somewhat similar timing relative to the
potential range of phenology. Second, this means there
is no evidence of temporal separation of resource use
since major growth periods for congeners are strongly
overlapping.
There are many other species of detritivores in this
stream, perhaps varying in terms of strict dependence
solely on coarse particulate organic matter (Mihuc &
Mihuc, 1995), but numbering at least 25 species. Two
of the questions that remain are how so many species
coexist on what is clearly a limiting resource (Richardson, 1991; Dobson & Hildrew, 1992; Wallace et al.,
1999), and why the late spring and summer is the dominant period of individual growth when resources are
most scarce then.
Acknowledgements
I thank Drs Charlotte Gjerløv and Bill Neill for constructive comments on an earlier version of this paper.
I am grateful for the field and laboratory assistance
of the many people that aided in various research
efforts that led to these data. Species identifications
were kindly provided by Richard Baumann, Andrew
Nimmo, Donald Oliver and John Weaver III. I appreciate the financial support from NSERC (Canada).
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