Long-term Interactions between Migratory Caribou

Transcription

Long-term Interactions between Migratory Caribou
Report
Serge Payette, Stéphane Boudreau, Claude Morneau and Nadia Pitre
Long-term Interactions between Migratory
Caribou, Wildfires and Nunavik Hunters
Inferred from Tree Rings
Barren-ground caribou (Rangifer tarandus) herds in North
America may reach considerable size and undertake
large-scale seasonal migrations from the Arctic tundra to
the boreal forest. To test the caribou decline hypothesis
associated with native harvesting and fire, we have documented the long-term trends of caribou activity based on a
novel approach which uses tree-ring dated trampling scars
produced by caribou hooves in the extensive trails distributed over the summer and winter ranges of the Rivièreaux-Feuilles herd (RAF herd, east of Hudson Bay in northern Quebec). The age structure data of trampling scars
from lichen woodlands distributed over the entire RAF
range confirmed the overall trends of caribou activity from
the late 1700s to present time. Over the last 200 years,
the RAF herd has undergone two highs in the late 1700s
and 1900s separated by a moderate activity pattern in the
late 1800s. Native harvesting was possibly involved in the
early 1900s decline, although at a moderate level. The reduced magnitude of caribou activity during this period has
not modified the natural cycle of highs and lows, which
suggests that other demographic factors were controlling
the changing caribou abundance. Our data also show that
only exceptionally large fires may have a minor, short-lived
impact on caribou migrations but not on caribou numbers.
INTRODUCTION
Caribou (Rangifer tarandus L., including wild and semidomesticated reindeer) extends across the circumpolar and circumboreal
regions of Eurasia and North America in a variety of habitats, a
situation unequalled by other large northern mammals. Among
the 7 subspecies so far identified, 5 subspecies occur in North
America (1). The largest caribou herds totalled more than 3
million animals in the late 1900s (2). Several North American
herds undertake seasonal migratory movements across different
biomes, including the Arctic tundra and the northern boreal forest. Although the average herd size may be more than 100 000
individuals (3), the herds generally experienced recurrent, large
fluctuations in size and range. The main causes of caribou fluctuations are still a matter of debate, with predation and harvesting (2, 4–6), climate change (7, 8) and food limitation associated
with over-grazing and trampling (9–12) as the most cited.
In Nunavik (Inuit territory in northern Québec, Canada), two
major caribou herds migrate from the open boreal forest to the
Arctic tundra and return, i.e. the George River herd (GEOR)
and the Rivière aux Feuilles herd (RAF). During the 1980s,
GEOR was by far the largest herd in the world (13). According
to historical records, the herd culminated during the late 1800s
(14) before collapsing between 1905 and 1916 (15), and then
remaining at a small size until the 1960s (4, 16). Since then the
herd size increased dramatically and climaxed in the 1980s (12)
before declining again during the 1990s (17). Although less
studied than GEOR, RAF has also undergone a similar demographic trend during the late 19th century (14). The herd also
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shows a seasonal north-south migratory trajectory similar to that
of GEOR, with a summer range and calving ground located on
the rocky tundra plateau of Ungava Peninsula and a fall-winter
range in the lichen woodlands east of Hudson Bay and James
Bay (Fig. 1). In contrast to GEOR, RAF is presently at a climax
with at least 450 000 individuals (S. Couturier, pers. comm.).
Caribou was not traditionally the main food resource for Inuit
as their lifestyle has relied heavily on the marine biota. During periods of caribou abundance, however, Inuit hunters took
advantage of this resource along the Hudson Bay and Ungava
Bay coasts or at a short distance inland, especially before the
introduction of primitive firearms prior to the 20th century (18–
20). With the use of more sophisticated rifles, caribou hunting
has been facilitated and several parties of Inuit hunters spent
longer periods of time inland. The Nunamiut (i.e. Inuit living
in interior Ungava Peninsula) (20) progressively occupied the
hinterland forming a distinctive cultural group whose general
economy was based on caribou harvesting. The Nunamiut are
known to have occupied interior Ungava from the late 1800s to
1920. According to archeological investigations, the Nunamiut
were not the only group to have lived inland as paleo-Nunamiut people of Dorsetian lineage (the Dorset culture disappeared
around AD 1500) constructed long-houses in interior Ungava
(21–23). Long-houses were successively occupied by the Dorset
and modern Inuit (modern Inuit belong to the Thule culture (see
24), thus validating the hypothesis of a permanent occupation
of the hinterland by different cultural groups. The Nunamiut
abandoned interior Ungava during the 1920s because of caribou
rarity (20). Thereafter, only small parties visited occasionally
the region until the early 1940s (18). Nowadays, caribou hunting occurs mostly along the Arctic coasts of northern Québec in
spite of the increasing size of RAF during the last two decades.
The long-term trend of caribou abundance of both GEOR and
RAF, with highs in the late 1800s and 1900s and lows during
most of the 20th century, may be attributed to several causal factors. Was the trend caused by natural population fluctuations associated with biotic factors like the carrying capacity of the summer range or physical factors like climatic changes and wildfire?
What was the impact of native hunters on caribou abundance?
Most reports from traders and explorers at the end of the 19th
century as well as field naturalists in the first half of the 20th century (4, 15) identified overharvesting by Inuit and Indians as the
main cause of caribou decline. Both declines of GEOR and RAF
in the early 1900s coincided with the overall decline of the species across North America (6). Also, the importance of wildfire
as a causal factor in caribou decline has been reported repeatedly
(4, 15, 25, 26).
Because of the lack of supportive archival data on past demographic trends and site conditions during most of the 19th and
20th centuries, it is difficult to validate any hypotheses on the
causes of changing caribou abundance with time. One indirect
method to evaluate past caribou activity developed recently (27)
uses erosion marks (hereafter called trampling scars) produced
by caribou hooves on tree roots crossing caribou trails in lichen
woodlands that can be dated dendrochronologically. The dating
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Ambio Vol. 33, No. 8, December 2004
Figure 1. Distribution of the calving
ground (gray) of the Rivière aux Feuilles
caribou herd east of James Bay and
Hudson Bay and of the calving ground
of the George River herd southeast of
Ungava Bay. Black triangles indicate
winter and summer Nunamiut camps in
Arctic tundra (interior Ungava) and
northern forest tundra (modified after
Vézinet (20) and Saladin dʼAnglure (40).
of several trampling scars in sites distributed over the herd range
can provide a record of caribou activity in time and space. Caribou trails are quite abundant in lichen woodlands and produce
intricate networks corresponding to the main migratory routes
during the snowfree period.
The two main objectives of this study are i) to provide evidence for changing caribou activity during a period back in time
far longer than the present period of demographic caribou censuses; and ii) to evaluate the impact of past environmental conditions (like large wildfires) and Inuit harvesting on the inferred
changes in abundance across the entire RAF range. We have
used a retrospective analysis of caribou activity based on the
dating of trampling scars on exposed tree roots crossing caribou
trails. Our reconstruction of past caribou activity spans the last
200 years, i.e. a period long enough to identify natural fluctuations of the animal in relation to stand disturbances (wildfires)
and the alleged impact of overharvesting by native hunters.
METHODS
The study area extends from Hudson Bay to 74°W in the Rivière
aux Feuilles area (with the exception of two sites located between 72° and 73°W) and from 55°N to the Ungava Bay south
of Payne Lake at 60°N (Fig. 1). The region includes the major
biomes of northeastern Canada including from south to north
the lichen woodland zone, the forest tundra and the shrub tundra (28–30). The forest tundra and the lichen woodland zone
are dominated by black spruce (Picea mariana [Mill.] B.S.P.)
and eastern larch (Larix laricina [DuRoi] K. Koch) with white
spruce (Picea glauca [Moench] Voss) as a secondary species
with dense forests growing only along the Hudson Bay coast
(28).
Caribou activity is highlighted by trail networks easily identified in the ground cover. These networks extend from the shrub
tundra to the northern part of the lichen woodland zone, and
correspond roughly to the current annual range of RAF (Fig.
1). Presence and abundance of caribou trails were mapped from
airborne surveys and partitioned into 3 different groups: i) no
trails; ii) scattered trails corresponding to 5–100 trails 10 km1 of survey; iii) dense trails corresponding to > 100 trails 10
Ambio Vol. 33, No. 8, December 2004
km-1 of survey. Several sites along the Hudson Bay coast and
in the southern part of the forest tundra and the northern part of
the lichen woodland zone are devoid of caribou trails (group 1),
whereas scattered trails (group 2) were located in the southern
part of the forest tundra. Dense trail networks (group 3) were
distributed in the northern part of the forest tundra and the southern part of the shrub tundra.
A sampling grid over the entire RAF range (based on the trail
survey) was laid out to examine wood sections of roots damaged by caribou hooves. All the sampling sites were dominated
by black spruce trees (lichen woodland and forest tundra) and
shrubs (northern forest tundra and shrub tundra). The sites were
chosen at random and corresponded to old-growth lichen-spruce
woodlands and krummholz (stunted trees) islands used by caribou during the snow-free period for food intake and travelling
(Fig. 2). All old-growth spruce stands selected were > 500 years
old in the northern part of the forest tundra and 100–200 years
old in the southern part of the forest tundra, according to postfire stand-age surveys (31, 32). The spruce stands (10 to 30%
spruce cover) were all dominated by an extensive lichen carpet with Cladina stellaris (Opiz) Brodo (33) as the main ground
species. Most sampled sites were located on hilltops where trampling scars were recovered from large quadrats ranging from
2500 to 5000 m2. Trampling scars were sampled exhaustively
over the entire quadrats, with a focus on presently used trails
and old, abandoned trails currently colonized by a continuous
lichen (Cladina stellaris) carpet. Eleven sites were sampled in
the southern part of the summer range and the northern part of
the winter range (shrub tundra and northern forest tundra) where
caribou trail networks are well developed (group 3); the same
number of sites were sampled in the winter range (southern forest tundra) where caribou trails were scarce (group 2).
Because trampling scars can be dated accurately from tree rings
(see (27)) for details on the method used), dates of scar formation
at each site were used to reconstruct caribou activity in stands
older than 500 years in the northern forest tundra and southern
shrub tundra. Root cross-sections were sanded finely for scar dating under a dissection microscope at 40X. Trampling scars were
dated using diagnostic light rings (34). Samples were rejected
when accurate dating at ± 1 year was not possible. Age-frequency
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483
naturalists (15). These documents included general information
on the lifestyle and living conditions of Inuit and Cree groups
visiting trading posts. However, only anecdotal accounts were
retrieved from the documents about the presence and relative
abundance of caribou near the trading posts and inland. Citations of years of food scarcity and starvation among the native
groups were noted, as well as accounts of traders and explorers
on alleged slaughtering of caribou by native hunters.
RESULTS
Caribou Activity over the Last 200 Years
Figure 2. Distribution of 1950s fires (in black) from the southern
forest tundra (dark gray), northern forest tundra (medium gray)
and shrub tundra (light gray) east of Hudson Bay coast. Fires
burned extensive lichen woodlands of the winter range of the
Rivière aux Feuilles herd. White circles indicate sampling sites.
Numbers in circles correspond to sites cited in text: 1. GuillaumeDelisle Lake, 2. Eau-Claire Lake, 3. Minto Lake, 4. Chavigny Lake,
5. Payne Lake, 6. Boniface River.
distributions of trampling scars by 10-year age-classes were constructed. Underestimation of caribou trampling as one goes back
in time was taken into account, assuming a constant loss of scars
with time based on the removal of the exponential trend from the
age structures using log-linear regressions (35). The number of
scars in each study stand was log-transformed after one year was
added to each age-class to avoid value 0. Residuals of the log-linear regressions were used as an index of caribou activity.
Fire data were obtained from a survey (31) covering a large
part of RAF range, i.e. 54 000 km2, from the lichen woodland
zone (55°N) to the shrub tundra (58°N), between 74°W and 76°W.
Large wildfires occurring during the mid-1950s were also mapped
from 76°W towards Hudson Bay to supplement the latter survey.
Age of post-fire, old-growth lichen woodlands in the northern part
of the forest tundra was made available from another survey (32)
using 14C dates of charcoal beneath the lichen carpet.
The relationship between changing caribou abundance and
welfare of native communities (Inuit and Cree Indians) in the
study area, from the late 19th century to the early 1960s, was also
evaluated using archival documents from traders (reports from
managers of Hudson Bay Company trading posts), explorers,
missionaries (Anglican Mission in Kuujjuarapik) (36, 37) and
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More than 8750 trampling scars were dated from the 11 oldgrowth stands located in the northern forest tundra and shrub tundra (hereafter called northern range area), whereas a little more
than 1050 scars were recovered from the other 11 stands in the
southern forest tundra and lichen woodland zone (hereafter called
southern range area). The age structure of trampling scars (corresponding to the number of trampling scars per 10-year class)
in two stands of the northern range area, shown here as representative examples, recorded continuous caribou activity (semi-log
regressions of Boniface stand: R2 = 0.93, p < 0.001, and Chavigny
stand: R2 = 0.81, p < 0.001) from the late 18th century to the late
20th century (Fig. 3). The two stands are 50 km from each other
and showed similar and often synchronous trends in scar abundance with time. The age distribution of scars indicated an exponential decreasing trend over the 200 years of records. Variations
in number of scars around regression curves followed the same
pattern depicted in the scar age structures. Therefore, we used residuals of the regression curves to standardize the distribution of
the number of scars with age and to outline changes in number
of scars with time. Eight out of 11 stands were showing residuals
significantly correlated with each other (Spearman’s rank correlation, p < 0.10 for each comparison) and were pooled in one general chronology (Fig. 4). The three remaining stands were located
in the easternmost and northernmost parts of the northern range
area. The overall trend depicted in the chronology of residuals
included two periods of major caribou activity in the late 1700s
– early 1800s and the late 1900s, and a period of moderate caribou
activity in the late 1800s – early 1900s separating two deeper lows
during the mid-19th and mid-20th centuries. Stands distributed in
the southern range area showed discontinuous and shorter scarage structures (data not shown) with a maximum number of trampling scars recorded in the late 1900s at some sites, a trend also
similar to that identified in stands of the northern range area. The
records in stands located near the Hudson Bay coast included only
a very small number of trampling scars.
Fire Disturbance
The fire survey in a large part of RAF range showed contrasting
fire patterns associated with latitude and vegetation types (31).
The age and size of all fires determined from field data indicated
a fire cycle of 100–250 years in the southern range area and a fire
cycle of > 1000 years in the northern range area, thus explaining the advanced age of old-growth forests in the northern forest
tundra and the relatively young age of southern lichen woodlands. Most fires that occurred during the 20th century were of
small size: more than 80% of all fires in the northern range area
were < 100 ha, whereas more than 30% of the fires recorded
in the southern range area covered > 1000 ha. Very large fires
(> 100 000 ha) burned extensive tracts of lichen forests in the
southern range area during the mid-1950s when drier conditions persisted throughout the growing season from spring to
fall. More than one third of the southern range area burned in a
2-year period, in 1954 and 1955. The spatial distribution of the
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Ambio Vol. 33, No. 8, December 2004
1950s fires formed a huge west-east corridor of burned trees and
lichens except along the Hudson Bay coast where more humid
conditions prevailed (Fig. 2).
Historical Accounts of Caribou Abundance and Harvesting
According to historical reports, caribou were abundant in the
vicinity and east of Hudson Bay and James Bay during the late
1700s and early 1800s, and rare during the mid-1800s (38). Low
(14) and Elton (15) reported caribou abundance in the 1870s and
1880s and decline beginning in the early 20th century. Inuit hunters always considered the inland areas between Lac (lake) Payne
and Lac Minto to be occupied by large groups of caribou (14).
During the mid-20th century, caribou hunting was possible only
inland likely because of a reduced number of animals (39). Several years of food limitation and starvation were recorded during
the first part of the 20th century (36, 37), due in part to caribou
rarity along the Hudson Bay coast where Inuit were established
and when Nunamiut abandoned interior Ungava.
DISCUSSION
200-year Record of Caribou Activity and Native Harvesting
The chronology of residuals in the northern range area of RAF
suggests continuous but changing caribou activity during the last
200 years. Two major periods of caribou activity occurred at both
ends of the 200-year record. Also two periods of minor caribou
activity prevailed during several decades both in the 19th and the
20th centuries, separated by a somewhat low-to-moderate increase
from the late 1800s to the early 1900s. The comparison between
the RAF chronology and the GEOR chronology from eastern
Québec-Labrador (35) shows that RAF experienced an increase of
lesser magnitude during the late 1800s – early 1900s. It is possible
that this lower increase in caribou activity was due to sustained
harvesting by the Nunamiut. As reported by field anthropologists
(20, 40), winter and summer Nunamiut camps were widespread at
that time, not only in interior Ungava but also in the northern forest tundra towards Lac Minto (Fig. 5). Nunamiut hunters possibly
impacted the then increasing herd after a period of low activity
in the mid-1800s, but not to the point of compromising the survival of the animals. The harvesting pressure would have reduced
the magnitude of the rising trend of caribou abundance from the
1870s to the 1890s. When Nunamiut abandoned interior Ungava
in the 1920s the herd activity remained at a low level until the
Figure 3. Age structure (10-year class) of trampling scars of Boniface River site (solid line) and Chavigny Lake site (broken line). Note
the log scale of number of trampling scars.
Ambio Vol. 33, No. 8, December 2004
Figure 4. Standardized distribution of the number of trampling
scars (residuals of regression curves ± SD) according to time (from
the late 18th century to the late 20th century). The Nunamiut period
is indicated. The arrow corresponds to caribou crowding during
the 1950s likely associated with large wildfire conflagrations.
1950s. Because the overall trend between the late 1800s and the
late 1900s included a sequence of moderate-to-high caribou activity followed by low activity and again high activity, it is likely
that the changing RAF size inferred from tree-rings responded to
natural factors not controlled by native harvesting.
Impact of Large Fires on Caribou Migration
Of particular interest is the sudden rise of caribou activity in the
northern range area during the 1950s (Fig. 4), in a period of time
when the animals were considered rare and threatened by extinction all over Quebec-Labrador (4, 12, 16, 39). The sharp increase
of trampling scars during the 1950s testifies to anomalous caribou
activity in the northern forest tundra and southern shrub tundra.
One hypothesis that merits consideration is that the sudden rise of
caribou activity may well be associated with a short-lived density
effect of caribou caused by the repulsive influence of the large
fire conflagrations that occurred at that time in the southern range
area. Indeed, large tracks of lichen woodlands burned in 1954 and
1955 and formed, probably, a major east-west barrier against fallwinter migrations several hundred km to the south. During several
years, the animals were probably confined to the northern forest
tundra where lichen food was still available in winter (Figs 2 and
4). The general pattern of annual migrations some 500 to 800 km
from calving grounds was probably upset by several factors including natural demographic changes and random movements,
but extensive wildfires were likely forcing factors to change the
route and even maintain the animals close to the summer habitat
during several years. In a period of caribou abundance, as is the
case nowadays, the animals are moving readily to a larger winter
woodland habitat, whereas during a demographic low the reduced
herd occupies smaller wintering grounds.
Although forest fires were reported to be responsible for the
decline of caribou herds (4, 15, 25, 26), no supportive field data
are yet available. Because caribou herds are travelling across large
stretches of land, small fires have no measurable influence on the
herd’s movements. Only large fires like those reported here, showing a predominant spatial pattern perpendicular to the migration
pathways, may have an impact on the animals, and the impact
is likely short-lived. One may speculate on the ultimate causes
inducing such a retreat of the animals to the northern range area.
Food limitation may be one of the main causes, although the animals appear to return to the southern range area a few years after
the conflagrations. It is not known how the caribou are “informed”
about the large fires occurring in the southern range area.
Fire frequency and size determine to a large extent the amount
of lichen forage (41). But in the large summer and winter habitat
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485
of RAF, lichen scarcity is not a major issue. Our data set suggests
that large fires are detrimental only to caribou movements during
a short period of time. The occurrence of large fires during a short
period of time across the winter habitat reduces significantly food
availability, particularly the dominant, late-successional fruticose
lichen Cladina stellaris, which assumes dominance 60 to 80 years
after fire (42, 43). There is, however, enough food storage in the
untouched woodlands of the winter habitat to sustain the herd.
Woodland caribou is probably the only animal south of the study
area influenced by lichen limitation due to fires (2, 6).
CONCLUSION
The overall activity trends of the large Rivière-aux-Feuilles caribou herd in northern Quebec, depicted by trampling scar-age data,
indicate two highs over the last 200 years, occurring in the late
1700s and 1900s, and a low to moderate increased activity in the
late 1800s. The causal factors influencing long-term changes of
caribou activity are multiple, but harvesting from native hunters,
often considered as the main cause of the herd’s decline during the
late 1800s-early 1900s, was likely involved but not to the point
to have modified significantly the natural demographic trajectory.
Human predation possibly reduced the magnitude of caribou activity during this period, but the natural cycle of highs and lows was
maintained, thus pointing to other but still unknown demographic
factors controlling the ever changing abundance of the animals.
Because of lack of data at that time, it is not possible to evaluate
the impact of predators (wolf), which may have prolonged the low
density period of the herd (44). Large-scale disturbance factors
like fires, also considered detrimental to caribou, appear to have a
short-term impact on the migration routes of the animals. Future
investigations aimed to identify the natural factors responsible for
the long-term fluctuations of large caribou herds should focus on
the influence of climate change and ecosystem dynamics of calving grounds associated with food availability.
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45. This study has been financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the FCAR program (Québec), and the Department
of Indian and Northern Affairs of Canada. S. Boudreau received scholarships from the
FCAR program and the Association of the Canadian Universities for Northern Studies
(ACUNS). We are most grateful to Y. Poirier for laboratory assistance, M.-J. Laberge for
field assistance, and two anonymous referees for thoughtful comments.
46. First submitted 29 July 2003. Revised manuscript recieved 22 Dec. 2003. Accepted for
publication 7 Jan. 2004.
Serge Payette is professor of plant ecology and paleoecology at Université Laval, Québec City, Canada. His
main research interests are in the disturbance dynamics of boreal, subarctic and subalpine environments.
He is Chairman of the NSERC Northern Research Chair
on disturbance ecology of northern environments.
serge.payette@bio.ulaval.ca
Stéphane Boudreau has a PhD in plant ecology from
Université Laval. He is currently a postdoctoral fellow at
the School of Botany and Zoology, Forest Biodiversity
Programme, at the University of Natal, South Africa.
boudreaus@ukzn.ac.za
Claude Morneau has a PhD in plant ecology from Université Laval. He is currently a research scientist in the
Forest Biodiversity Section of the Ministère des ressources naturelles of Québec, Québec City.
claude.morneau@mrnfp.gouv.qc.ca
Nadia Pitre has a MSc in plant ecology. She is currently
associate researcher in environmental sciences at the
University of Tucuman, Tucuman, Argentina.
pitre_n@hotmail.com
Their address: NSERC Northern Research Chair, Centre
dʼétudes nordiques and Département de biologie,
Université Laval, Québec, Canada G1K 7P4.
© Royal Swedish Academy of Sciences 2004
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Ambio Vol. 33, No. 8, December 2004