Year of Award

2022

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Fish and Wildlife Biology

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Mark Hebblewhite

Commitee Members

Joshua Millspaugh, Jennifer Fortin-Noreus, Jedediah Brodie, Libby Metcalf

Keywords

behavior, demography, density dependence, migratory caribou, nutrition, summer diet

Publisher

University of Montana

Abstract

The concept of density-dependence is foundational to our understanding of the ecology and management of large herbivores. As the abundance of large herbivores increases, competition for high-quality resources increases and animals select for lower-quality foods potentially resulting in reduced vital rates that cause declines in population growth. What remains unclear is how foraging at finer scales, like the ‘bite’, translates to demography. This is difficult because it’s challenging to scale bite-level foraging to population-levels. I test the fine-scale foraging mechanisms of density dependence in a large subarctic herbivore, Barrenground caribou (Rangifer tarandus granti). The nutritional landscape for arctic herbivores is changing faster than anywhere else on Earth from global climate change, so understanding population-level consequences of foraging behavior is paramount. The Fortymile Caribou Herd (FCH) is a barrenground population ranging across Alaska and Yukon, Canada that, unlike most, has recently increased in abundance and distribution. First, I used GPS-video-camera collars to understand fine-scale behavior and diet patterns. This revealed i) a pronounced tradeoff in summer foraging between lichens and shrubs and, ii) the costs of insect harassment on eating. Second, I tested how foraging choices were influenced by spatiotemporal factors in summer. I found spatial tradeoffs in behavioral and foraging choices. As spatial densities of the FCH increased, the probability of eating shrubs increased as cover (%) for Salix spp. shrubs increased. Conversely, the probability of caribou eating lichen declined with increasing spatial densities. Caribou choice for preferred summer foods also increased with the availability of such foods revealing foraging functional responses. Finally, I linked bottom-up drivers of nutritional ecology to juvenile survival to test predictions of density dependence. I found biologically important effects of bottom-up forage-related factors on maternal condition, neonate birth weight and hence, survival, and overwinter juvenile survival. However, I found opposing signals of classical density-dependence suggesting this population was not approaching food-regulated carrying capacity. Understanding the mechanisms of density-dependent foraging for an increasing population provides critical insights into potential future impacts of global change in the Arctic on caribou, and valuable information for caribou managers especially given recent declines of most populations.

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© Copyright 2022 Elizabeth (Libby) Parr Williamson Ehlers