Influences on caribou foraging dynamics

Fortymile Herd caribou foraging in the alpine of Yukon-Charley Rivers National Preserve.

NPS/KYLE JOLY

A team of researchers from the University of Montana and variety of government agencies, including the National Park Service, investigated factors that affect how caribou in the Fortymile Herd forage during summer, using a combination of GPS and video camera collars in east-central Alaska. As expected, the more common a forage was, the more likely caribou were to be found eating it. However, as caribou used an area more heavily, the less likely they were to forage on lichens. This may be due to competition among the caribou and depletion of lichens due to grazing. These types of impacts are known as “density-dependent” impacts. Interestingly, foraging on shrubs increased in areas with high caribou use. This may be due to the greater quantity of forage that shrubs provide to caribou or the ability of shrubs to withstand greater foraging pressure compared to lichens. Insect harassment reduced the likelihood that caribou would be foraging. The study sheds light on the potential for (1) caribou to overgraze resources (like lichens), (2) better understanding caribou carrying capacity, and (3) how climate change may impact caribou foraging dynamics.

The CariView Experience: Migratory caribou teach researchers where critical summer foods and habitats are located – all from the animal’s perspective

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. Read more.

A taste of space: Remote animal observations and discrete-choice models provide new insights into foraging and density dynamics for a large subarctic herbivore

Abstract

1. Competition for resources and space can drive forage selection of large herbivores from the bite through the landscape scale. Animal behaviour and foraging patterns are also influenced by abiotic and biotic factors.

2. Fine-scale mechanisms of density-dependent foraging at the bite scale are likely consistent with density-dependent behavioural patterns observed at broader scales, but few studies have directly tested this assertion.

3. Here, we tested if space use intensity, a proxy of spatiotemporal density, affects foraging mechanisms at fine spatial scales similarly to density-dependent effects observed at broader scales in caribou. We specifically assessed how behavioural choices are affected by space use intensity and environmental processes using behavioural state and forage selection data from caribou (Rangifer tarandus granti)observed from GPS video-camera collars using a multivariate discrete-choice modelling framework.

4. We found that the probability of eating shrubs increased with increasing caribou space use intensity and cover of Salix spp. shrubs, whereas the probability of eating lichen decreased. Insects also affected fine-scale foraging behaviour by reducing the overall probability of eating. Strong eastward winds mitigated negative effects of insects and resulted in higher probabilities of eating lichen. At last, caribou exhibited foraging functional responses wherein their probability of selecting each food type increased as the availability (% cover) of that food increased.

5. Space use intensity signals of fine-scale foraging were consistent with density-dependent responses observed at larger scales and with recent evidence suggesting declining reproductive rates in the same caribou population. Our results highlight potential risks of overgrazing on sensitive forage species such as lichen. Remote investigation of the functional responses of foraging behaviours provides exciting future applications where spatial models can identify high-quality habitats for conservation.

Ehlers, L., E. Palm, J. Herriges, T. Bentzen, M. Suitor, K. Joly, J. Millspaugh, P. Donnelly, J. Gross, J. Wells, B. Larue, and M. Hebblewhite. 2024. A taste of space: Remote animal observations and discrete-choice models provide new insights into foraging and density dynamics for a large subarctic herbivore. Journal of Animal Ecology 93(7): 891-905.