The Erosion of Threatened Southern Mountain Caribou Migration.

Migration enables animals to access seasonally and spatially varying resources, resulting in greater abundance compared to analogous non-migratory wildlife. Both Western science and Indigenous knowledge recognize the critical role of migration in sustaining wildlife; yet these movements are increasingly disrupted by human activity worldwide. Despite their importance, long-term changes in migratory patterns for large mammals remain challenging to quantify. Therefore, to address this gap, we analyzed 35 years (1987-2022) of telemetry data for southern mountain caribou (n = 2967 animal-years), a threatened population that typically exhibits two main types of annual migration. The first type is a single horizontal migration between summer and winter ranges, while the second is a twice-per-year vertical migration between high and low elevations. We studied the extent and type of migration, changes through time, and determined if these changes correlated with landscape disturbance or shifts in weather. Our results show that caribou reduced their migratory duration (2-3 days/decade), distance (6-8 km/decade or 15%-25%) and elevation change (120-150 m/decade or 7%-23% for elevational migrants). The subpopulations of caribou adapted to extreme snow depths conducted a unique elevational migration twice a year, which is globally distinctive among ungulates. However, this elevational migration is diminishing in distance and elevation. The changes in elevational and horizontal migration correlated with increased human disturbance, especially of low-elevation winter ranges. Changes in weather did not appear to be a major driver of the migratory declines. Declines in migratory behaviour occurred concomitantly with population declines and increases in disturbance, and these processes are likely intertwined. The rapid loss of these unique migrations is a significant conservation concern that could have irreversible consequences for the social transmission of fitness-maximizing behaviors.