Shifting and expanding ranges of a sub-Arctic caribou herd and associated changes in vegetation

Abstract

Rapid climate warming has contributed to significant changes in Arctic and boreal vegetation over the past half century. Changes in vegetation can impact wildlife by altering habitat and forage availability, which can affect behavior and range use. However, animals can also influence vegetation through foraging and trampling and therefore play an important role in determining ecosystem responses to climate change. As wildlife populations grow, density-dependent processes can prompt range expansion or shifts. One mechanism for this is density-dependent forage reduction, which can contribute to nutritional stress and population declines, and can also alter vegetation change trajectories. We assessed the range characteristics of a migratory caribou (Rangifer tarandus) herd in east-central Alaska and west-central Yukon Territory as it grew (1992-2017) then declined (2017-2020). Furthermore, we analyzed the correlation between caribou relative spatial density and vegetation change over this period using remotely sensed models of plant functional type cover. Over this period, caribou population density increased in all seasonal ranges. This was most acute in the calving range where density increased 8-fold, from 1.5 to 12.0 animals km^-2. Concurrent with increasing density, we documented range shifts and expansion across summer, post-calving and winter ranges. In particular, summer range size doubled (12,000 km² increase) and overlap with core range (areas with repeated year-round use) was halved. Meanwhile, lichen cover, a key forage item, declined more in areas with high caribou density (2.4% absolute, 22% relative decline in cover) compared to areas where caribou were mostly absent (0.3% absolute, 1.9% relative decline). Conversely, deciduous shrub cover increased more in high caribou density areas. However, increases were dominated by less palatable shrubs whereas more palatable shrubs (i.e., willow [Salix spp.]) were stable or declined slightly. These changes in vegetation cover were small relative to uncertainty in the map products used to calculate change. Nonetheless, correlations between vegetation change and caribou range characteristics, along with concerning demographic trends reported over this same period, suggest changing forage conditions may have played a role in the herd's subsequent population decline. Our research highlights the potential of remotely sensed metrics of vegetation change for assessing the impacts of herbivory and trampling and stresses the importance of in situ data such as exclosures for validating such findings.

Keywords: Fortymile Caribou Herd; Rangifer tarandus; caribou; climate change; density dependence; forage; herbivory; range expansion; remote sensing; shrub expansion; vegetation change.

FIGURE 1

(a) Map of study area with seasonal ranges for radio‐collared adult female caribou (Rangifer tarandus) in the Fortymile Caribou Herd (FMCH), 1992–2021. Black dashed lines show the extended range of the FMCH (outer) and the Core Uplands (inner). (b) Annual FMCH population size estimates derived from photo census (circles) or population models (triangles). Population estimates within the shaded gray rectangle were derived using the Rivest method, which corrects for caribou aggregations missed during photo census (Rivest et al., 1998). Estimates derived using the Rivest method are generally higher than those derived without it. (c) A female Fortymile caribou and her calf travel and forage across core summer range. Photo credit: Jim Herriges and Libby Ehlers.

FIGURE 2

Workflow for vegetation change analysis. (a) Cover weighted area was calculated by multiplying plant functional type (PFT) cover (as a proportion) by pixel area. This was done on a pixel‐wise basis. Weighted area was calculated separately for each PFT and for the start and end year, producing ten 30‐m resolution maps of weighted area in total. (b) Continuous utilization distributions are binned into four caribou spatial density classes. Here, density refers to relative spatial density, not absolute density. These density values are unitless and do not represent animals per unit area. (c) Land cover for the year 1992 (the start of the time series) was retrieved from Wang et al. (2019). (d) Before they were used in the analysis, the weighted area maps were masked following the steps in Vegetation data . (e) For each unique combination of caribou spatial density class and land cover class, all weighted area pixels within those areas were retrieved and summed to provide the overall area covered for that density class and land cover class. Here, we show an example using the high caribou spatial density class and the Woodlands land cover class. The summed weighted area values were then used to calculate absolute and relative changes in PFT top cover.

FIGURE 3

Change in Fortymile Caribou Herd 95% isopleth range area (top), population density across range (middle), and percent overlap with the Core Uplands (bottom) from 1991 to 2021.

FIGURE 4

Annual summer 50% isopleth ranges for radio‐collared adult female caribou (Rangifer tarandus) in the Fortymile Caribou Herd, 1992–2021. Polygons denote range boundaries, points denote range centroids. The dashed black area denotes the Core Uplands, which is the area of greatest caribou use across all seasons.

FIGURE 5

Relationship between caribou spatial density and absolute percent change in plant functional type (PFT) top cover from 1992 to 2020. The x‐axis shows levels of relative caribou spatial density, from “None” (or very low density) to “High” density. Each facet represents a different PFT, and each color represents a different season. The seasons dictated the spatial extent of each caribou density bin and the probability of selection mask used. The “overall” season represents an aggregate of caribou use across all seasons.

FIGURE 6

Relationship between caribou spatial density and absolute percent change in shrub genus top cover from 1992 to 2020. The x‐axis shows levels of relative caribou spatial density, from “None” (or very low density) to “High” density. Each facet represents a different shrub genus, and each color represents a different season. The seasons dictated the spatial extent of each caribou density bin and the probability of selection mask used. The “overall” season represents an aggregate of caribou use across all seasons.