Abstract
Many migratory ungulate species are declining globally due to climate and anthropogenic driven landscape change. Growing concerns over the effects of landscape change on migration have resulted in numerous studies evaluating behavioral and demographic responses of migratory species. However, the study of endangered species’ behavioral responses is uniquely challenging, as studies on small populations that inhabit remote or large geographical ranges can be difficult and costly to conduct. Despite this challenge, an improved understanding of how migratory endangered species respond to rapid landscape change is imperative to stabilize declining populations. Southern mountain caribou (Rangifer tarandus caribou) are a listed species under the Species at Risk Act in Canada and Endangered Species Act in the United States. Southern mountain caribou historically inhabited much of northern Idaho and southeastern British Columbia. However, due to anthropogenic disturbance, 7 of the 17 southern group herds are extirpated and many of the remaining herds are below 30 individuals. Disturbance-mediated habitat loss has influenced forage availability, increased predation risk, and decreased overall survival. Extensive timber harvest in the caribou range has increased early seral forest presence on the landscape. Early seral forests facilitate the range expansion of moose into caribou habitat and subsequently increasing predator population growth. In response, caribou avoid areas with high anthropogenic disturbance. Consequently, caribou migratory behavior and habitat selection may be altered due to this avoidance behavior. Additionally, climate change climate change is rapidly altering seasonal patterns, and by extension, plant phenology in alpine ecosystems. Specifically, earlier snow melt, decreased precipitation events, and warmer summer temperatures lead to premature plant senescence in alpine environments. Changing plant phenology can have significant negative impacts on migratory caribou recruitment and survival. However, the relationship between landscape change, caribou movement and caribou habitat selection remains unclear. As such, we sought to quantify changes in landscape structure, forage availability, and the ability of caribou to respond to such changes.
Chapter 1 evaluated the spring migratory behavior of 13 caribou herds to test whether caribou exhibit migratory plasticity in response to a changing landscape. Our work is the first to demonstrate that southern mountain caribou conduct a non-mutually inclusive two step spring migration, moving across planar and elevational gradients. We also found that caribou have little plasticity in their propensity to migrate but exhibit plasticity in the spatial and temporal characteristics of their migration. These changes primarily correspond with previous winter climate, growing season conditions, and forestry practices.
Chapter 2 estimated the resource tracking of 13 caribou herds during migration and generated resource selection functions to test whether caribou have altered habitat selection in response to changing plant phenology. We found that the growing seasons experienced by migratory caribou are shortening by 0.6 days annually. Additionally, we found that caribou are departing from their summer ranges 0.6 days earlier each year. Furthermore, we found that despite shortening growing seasons, caribou are not altering the size of or habitat selection within their summer ranges.
Our work demonstrates that southern mountain caribou are altering their migratory behavior to compensate for changes in climate and landscape disturbance. However, caribou are not changing their propensity to migrate to high-elevation summer ranges, where resource availability may be limited. Thus, existing recovery efforts that emphasize only top-down approaches may be insufficient in the long-term to stabilize the population; efforts that incorporate immediate and long-term management actions are imperative for the recovery of southern mountain caribou. Our study sheds light on the complex relationship between animal migration, habitat alteration, and climate change, highlighting how these factors collectively affect recovery actions.