This blog post is provided by Libby Ehlers and tells the #StoryBehindthePaper for the paper “A taste of space: remote animal observations and discrete-choice models provide new insights into foraging dynamics and density for a large subarctic herbivore”, which was recently published in the Journal of Animal Ecology. In this blog post, Libby tells the story of how she and colleagues used video collars to study the summer diets and foraging behaviour of caribou in Alaska and Canada.

In the vast and rugged landscapes of subarctic Alaska and Canada, the Fortymile Caribou Herd roams almost continuously, foraging for nourishing foods as they navigate their extensive ranges. Migratory caribou represent iconic symbols of northern ecosystems and their importance to indigenous and native peoples spans millennia, intertwined with traditions of sustenance, clothing, and spiritual connections. Further, caribou play diverse and valuable roles in the ecosystem: caribou cycle nutrients with their movements and waste, they act as consumers and dispersers of plants, and they are a valuable food source for a diverse suite of predators, including humans. However, their resilience is being tested by the rapidly changing climate, with Arctic and subarctic regions experiencing five times faster climate change rates than anywhere else on Earth.

These climate changes are reshaping ecosystem dynamics, leading to significant shifts in vegetation communities from rising temperatures, increased precipitation (e.g., rain in winter and summer), and intensified wildfires. These transformations have been shown to favor the expansion of shrubs and trees which outcompete essential food sources like lichens, vital for caribou survival during winter. Furthermore, rising temperatures are increasing the length of time insect populations thrive, leading to increased harassment that disrupts important foraging time for caribou.

Understanding caribou behavior, especially their foraging and nutritional needs, is crucial for gauging climate change’s impact on populations. Summer nutrition is vital to support reproduction and lactation demands, but challenges like accessing high-quality foods contribute to declining caribou and reindeer populations globally. Caribou have to prioritize finding high-quality foods to replenish depleted fat reserves from winter, while nursing and raising young, avoiding intensive periods of insect harassment, and traversing challenging landscapes. It’s hard to be a caribou! Our research provides new insights into broader ecological shifts and the interconnectedness of animal behavior, ecosystem dynamics, and how populations are navigating the rapidly changing subarctic region.

Our study population of migratory caribou, the Fortymile Caribou Herd, has experienced growth from approximately 56,000 in 2009 to 84,000 in 2017, followed by a rapid decline to around 58,000 in 2020. This suggests the potential that important foods were limited and the landscape couldn’t support the higher numbers of caribou from 2017. This was concerning. Further, the Fortymile Caribou Herd is unique because their winter and summer ranges overlap extensively compared to their closest neighbors to the north, the Porcupine Caribou Herd, that spends the winter below treeline and then moves over 1000 km north to the coastal flats along the Arctic Ocean where the pregnant females give birth and raise their young. This makes the Fortymile Caribou Herd an ideal case study for studying how animal behaviours and foraging choices change as population density, or the number of animals/given amount of space (or ‘space use intensity’) changes.

Studying large herbivores like caribou in such remote and expansive territories is tough. The long-ranging and migratory life history of caribou makes observations over time and space exceedingly difficult. Despite this, recent advancements in animal-borne GPS video camera collars have revolutionized our ability to study wildlife behaviors across expansive and hard-to-access regions. We leveraged this technology to remotely observe not just what caribou were doing in summer but crucially, the locations that were valuable across their range for eating and eating high-quality foods during summer. We also examined the relationship between their food choices and how insects affected the location or duration of feeding events. Our research is part of a larger body of work using GPS video camera collars to understand caribou behaviour, summer food choices, and population changes. 

Our first results highlighted that caribou reduced their probability of eating lichen (a high-quality food in winter to sustain energy needs but lacks protein for replenishing fat reserves in summer) where more caribou were grouped together in higher densities, suggesting increased competition for this critical food. This finding supports theories on habitat use and previous evidence in other large mammals where increased densities and competition for quality foods have led to declines in reproductive success. Alternatively, caribou increased their probability of eating Salix species shrubs (a high-protein, good food source for replenishing lost fat reserves from winter) where caribou were grouped in greater densities across space. Next, we found caribou preferred Salix species shrubs over lichens in summer, indicating a shift in diet influenced by nutritional demands and the increased resilience shrubs show to grazing pressure. Finally, we found caribou were more likely to eat each food choice as the proportion of that food choice increased. 

Potential future challenges like climate change-induced shifts in vegetation will have implications for caribou foraging behavior. Our findings suggest that while some shrub expansion across the Arctic regions may benefit caribou as a high-quality summer food source, it could lead to long-term declines in lichen availability, their main food source in winter, potentially reducing reproductive success and survival.

We’ve used our findings to build seasonal maps to identify areas across the Fortymile Caribou Herd range where caribou were predicted to eat high-quality summer foods like shrubs and lichens. These maps serve as a management tool to help identify and prioritize habitats for caribou across this expansive and rapidly changing system. Further, our results provide a baseline of which to reassess in 10 years, for example, to see how food choices and the availability of these foods change over time.

The insights we’ve gained from remotely studying caribou foraging behavior using GPS video collars are valuable for conservation efforts. By leveraging advanced technology, scientific research, Indigenous and local knowledge, and collaborative conservation efforts, we can protect the habitats that sustain migratory species like the Fortymile Caribou Herd in this rapidly changing region.

Read the paper

Read the full paper here: Ehlers, L., Palm, E., Herriges, J.,Bentzen, T., Suitor, M., Joly, K., Millspaugh, J., Donnelly, P.,Gross, J., Wells, J., Larue, B., & Hebblewhite, M. (2024). Ataste of space: Remote animal observations and discrete-choice models provide new insights into foraging and densitydynamics for a large subarctic herbivore. Journal of Animal Ecology, 00, 1–15. https://doi.org/10.1111/1365-2656.14109