Migration is an important process for species worldwide, providing access to spatiotemporally-variable resources and relief from predators and parasites. Partial migration, in which only a subset of a population migrates between distinct seasonal ranges, is increasingly being recognized across a diverse range of species. A better understanding of the variability of behaviors shown by partially migratory species, as well as the factors that influence this variability over time, will improve knowledge of animal movement ecology. We examine migration dynamics in the Teshekpuk Caribou Herd (TCH) of northwestern Alaska. Some of the herd spends the winter on the northern coastal plain, while other individuals migrate hundreds of kilometers to overwinter in the foothills and mountains of the continental divide. We investigate a series of questions: What proportions of the TCH exhibit residency behavior, short-distance migration, and long-distance migration? Have these proportions changed over time at the population level? Do individuals consistently display the same migration behavior across years? We compare home range overlap, net squared displacement (NSD), and visual assessment to identify the movement strategy of annual trajectories for GPS-collared caribou. State sequence analyses reveal the likelihood of behavior switching across years.
Results/Conclusions
We analyzed collar data from 79 caribou between 2004-2016. Individuals had between 1-8 years of locations, resulting in 173 caribou-years of observations. Multiple years of observations were recorded for 53% of collared caribou, allowing analysis of behavioral fidelity over time. The proportion of migrants varied from 0.52-1.00, but not in a consistent direction. Overall, 80% of the caribou-years featured migratory behavior, with 34% exhibiting long-distance migration and 46% short-distance migration. Individual-level strategies across years were flexible, with the probability of remaining in the same class in subsequent years below 0.5 for all but one movement behavior. Both home range overlap and NSD tended to underestimate migratory behavior, assigning most individuals to “no-return” and “nomadic” movement classes, respectively. These patterns likely reflect the highly mobile behavior of caribou in summer and winter, diverging from stereotypical patterns described for other species. Together, these analyses reveal high levels of migratory behavior but variability in the strategies employed (i.e., short- vs long-distance migration). In addition to improving understanding of migration ecology, our findings have implications for wildlife management of a key species harvested by subsistence hunters. Better understanding the distribution of migration behaviors provides the opportunity to develop management policies based on local conditions.