Spring phenology drives range shifts in a migratory Arctic ungulate with key implications for the future

Abstract

Annual variation in phenology can have profound effects on the behavior of animals. As climate change advances spring phenology in ecosystems around the globe, it is becoming increasingly important to understand how animals respond to variation in the timing of seasonal events and how their responses may shift in the future. We investigated the influence of spring phenology on the behavior of migratory, barren-ground caribou (Rangifer tarandus), a species that has evolved to cope with short Arctic summers. Specifically, we examined the effect of spring snow melt and vegetation growth on the current and potential future space-use patterns of the Porcupine Caribou Herd (PCH), which exhibits large, inter-annual shifts in their calving and post-calving distributions across the U.S.-Canadian border. We quantified PCH selection for snow melt and vegetation phenology using machine learning models, determined how selection resulted in annual shifts in space-use, and then projected future distributions based on climate-driven phenology models. Caribou exhibited strong, scale-dependent selection for both snow melt and vegetation growth. During the calving season, caribou selected areas at finer scales where the snow had melted and vegetation was greening, but within broader landscapes that were still brown or snow covered. During the post-calving season, they selected vegetation with intermediate biomass expected to have high forage quality. Annual variation in spring phenology predicted major shifts in PCH space-use. In years with early spring phenology, PCH predominately used habitat in Alaska, while in years with late phenology, they spent more time in Yukon. Future climate conditions were projected to advance spring phenology, shifting PCH calving and post-calving distributions further west into Alaska. Our results demonstrate that caribou selection for habitat in specific phenological stages drive dramatic shifts in annual space-use patterns, and will likely affect future distributions, underscoring the importance of maintaining sufficient suitable habitat to allow for behavioral plasticity.

Keywords: Rangifer tarandus; Arctic; Porcupine Caribou Herd; caribou; climate change; large herbivore; migration; phenology; range shift; resource selection.

FIGURE 1

Study area boundary (yellow polygon) for assessing resource selection associated with calving (blue dots) and post‐calving (red dots) locations of the Porcupine Caribou Herd along the Arctic coast of Alaska and Yukon (black lines) during 2012–2018. Black arrows indicate the typical movement path during these seasons. The boundaries of the Arctic National Wildlife Refuge (ANWR), sub‐units of ANWR (1002 Area and Wilderness Area), Ivvavik National Park (NP), and Vuntut NP are shown in orange. Locations represent a random subsample of 20% of the total data for clarity

FIGURE 2

Variable importance plots for models of caribou resource selection during the calving (top) and post‐calving (bottom) seasons by the Porcupine Caribou Herd, 2012–2018

FIGURE 3

Partial dependence plots for caribou selection of phenology variables during the calving (a, b) and post‐calving (c) seasons by the Porcupine Caribou Herd, 2012–2018. Gray polygons depict the timing of either the calving or post‐calving season, respectively. During the calving season, snow melt date (a) was included in the model at three spatial scales and the onset of greenness was included at two scales (b). During the post‐calving season, the phenology variables were included in the model at only one spatial scale (c)

FIGURE 4

Predicted probability of calving habitat use for the Porcupine Caribou Herd from 2012 (top) to 2018 (bottom) as a function of annual variation in snow melt and vegetation greening. The left column shows only the predicted probabilities and the right column includes caribou locations (blue dots). The number of individuals tracked each year is represented as “n.” The black outline depicts areas classified as suitable habitat. White lettering represents the median ordinal date of snow melt (S), onset of greenness (O), and 50% max NDVI (N) in the study area each year

FIGURE 5

Predicted probability of post‐calving habitat use for the Porcupine Caribou Herd from 2012 (top) 2018 (bottom) as a function of annual variation in snow melt and vegetation greening. The left column shows only the predicted probabilities and the right column includes caribou locations (blue dots). The number of individuals tracked each year is represented as “n.” The black outline depicts areas classified as a suitable habitat. White lettering represents the median ordinal date of snow melt (S), onset of greenness (O), and 50% max NDVI (N) in the study area each year

FIGURE 6

Examples of predicted probabilities of Porcupine Caribou Herd calving habitat use during years when phenology was early (2015; top) and late (2018; bottom). The black outline depicts areas classified as suitable habitat. The median phenology ordinal dates in 2015 and 2018, respectively, were snow melt: 135 and 155; onset of greenness: 134 and 162; 50% max NDVI: 146 and 174

FIGURE 7

Examples of predicted probabilities of Porcupine Caribou Herd post‐calving habitat use during years when phenology was early (2015; top) and late (2018; bottom). The black outline depicts areas classified as a suitable habitat. The median phenology ordinal dates in 2015 and 2018, respectively, were snow melt: 135 and 155; onset of greenness: 134 and 162; 50% max NDVI: 146 and 174

FIGURE 8

Average predicted calving resource use for the Porcupine Caribou Herd during 2012–2018 (top) and average projected use during the 2030s, 2040s, and 2050s. The black outline depicts areas considered suitable habitat. Predictions are based on projected climate data from representative concentration pathway 8.5

FIGURE 9

Average predicted post‐calving resource use for the Porcupine Caribou Herd during 2012–2018 (top) and average projected use during the 2030s, 2040s, and 2050s. The black outline depicts areas considered a suitable habitat. Predictions are based on projected climate data from representative concentration pathway 8.5