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.

Dynamic selection for forage quality and quantity in response to phenology and insects in an Arctic ungulate

“…At both spatial scales, caribou selected forage components most strongly during early summer when the trade-offs between forage quantity and quality were the greatest and exhibited high spatial variability. Caribou selection for high quality forage was enabled by their movements north across the Arctic Coastal Plain (Figure 1), following the receding snow and emergent plant growth (Severson et al, 2021;Skogland, 1980Skogland, , 1984. Indeed, habitat used by caribou during the calving period had an average snowmelt date of 27 May, while habitat used further north during the post-calving period had an average snowmelt date of 2 June, and habitat used even further north during the mosquito period had a snowmelt date of 5 June.…”

“…Because proximity to the Arctic Ocean is associated with cooler temperatures and delayed phenology (Gustine et al, 2017), tracking early summer plant growth north across the coastal plain enabled caribou to maximize their exposure to high DN and DE. Although ungulates in temperate systems have been observed to "surf the green wave" during spring (Aikens et al, 2017;Merkle et al, 2016), CAH caribou appear to exhibit this behavior during early summer, given delayed phenology in the Arctic (Severson et al, 2021 early summer, and their distributions much more diffuse (Figure 1), likely because biomass and DE were sufficient across most of the CAH summer range (Figure 3). During the years of our study, the CAH was at moderate abundance (ADFG, 2020), so we do not expect that caribou density strongly influenced their access to forage quality or quantity.…”

“…Unlike ungulates in temperate systems, Arctic caribou typically migrate through snow, arriving at their calving grounds prior to the onset of vegetation green-up (Gurarie et al, 2019;Laforge et al, 2021). Once on their summer ranges, Arctic caribou often exhibit dynamic patterns of habitat use, shifting their distributions and habitat selection patterns every few weeks (Wilson et al, 2012; Bureau of Land Management [BLM], Bureau of Land Mangement, 2019a; Johnson et al, 2020;Severson et al, 2021; Figure 1). Researchers have assumed that their behavior is influenced, in part, by spatiotemporal variation in the nutritional value of forage (Cameron et al, 2005;Griffith et al, 2002), but the underlying drivers remain unknown.…”

Migratory vertebrates shift migration timing and distributions in a warming Arctic

“…Reindeer have often been considered to display strong site fidelity, especially during the calving season in summer [178,179]. This idea of site fidelity is under discussion, as reindeer can shift their wintering ranges following overgrazing of pastures [180], and recently, two reindeer herds have started to adjust their calving grounds to annual variation in forage quality, moving further westwards into Alaska in earlier springs [25,181]. Similarly, moose in Alaska have shifted their summering ranges northwards following shrub encroachment in tundra habitats [182].…”

“…For example, marine mammals are largely reliant on sea ice for feeding (and reproducing in case of pinnipeds), and may find themselves without any available habitat with the predicted disappearance of sea ice in summer [21]. Changes in timing of migration and reproduction [22,23], and to a limited extent shifts in breeding distribution [24,25], have already been observed in Arctic migratory animals. Those species which have been unable to shift timing of reproduction sufficiently, often suffer from reductions in reproductive success and survival [15,16].…”