Investigating broad-scale migratory choices for a highly mobile herbivore using circuit theory
Migration is a key process for numerous species, providing access to spatiotemporally varying resources and influencing a variety of important ecological processes. Numerous studies have considered the factors influencing migration decisions and choice of routes across a variety of taxa, yet often these studies are focused at the scale of individual steps. While such a scale is unarguably important for understanding migration behavior, migratory species often operate in a landscape or regional context, making the broad scale of their entire migration route of interest for both improving understanding of migration decisions and for conservation of migratory species. We consider migration at this broad scale to investigate connectivity between seasonal ranges for the Western Arctic Caribou Herd in Alaska. Using circuit theory, we reveal environmental factors influencing migration decisions at the scale of an entire migration path, rather than at the individual steps taken along the migration route. Resistance scenarios for five environmental variables were combined to yield 81 hypotheses for landscape resistance during fall and spring migration. Current flow models under each hypothesis were generated using Circuitscape and validated using caribou telemetry locations. The relative support for each resistance scenario was compared based on the rankings it received during validation.
Our approach identified key areas of importance to caribou in the migration paths between winter and spring range. Caribou responded strongly to vegetation, displayed mixed effects of terrain ruggedness and major rivers, and appeared unaffected by vegetation senescence and duration of snow cover. Landscape resistance appeared to be stronger in the fall than spring, likely reflecting the moderating influence of snow and ice accumulation in spring as well as a biological impetus to reach calving grounds rapidly. Current understanding of migration route selection is largely based on analysis of decisions made at individual steps. By considering flow across an entire migratory route, our circuit theory approach offers the opportunity to consider migration choices made at larger scales. In Alaska, such knowledge can help managers and policy makers identify key migratory habitat for caribou to enable lasting protection.