Migration is a key behavioral strategy to maximize fitness by tracking spatiotemporal variation in resource abundance. However, if migration is such a good strategy, why doesn’t every individual do it? In fact, many migratory populations are comprised of both migratory and resident animals. The ecological challenge is explaining variation in migratory behavior within a population, and its drivers. Migration in large ungulates is of both practical and ecological interest, because migration is declining due to anthropogenic causes, and understanding the drivers of migration, and hence reasons for declines, is a critical gap in knowledge. Despite the intellectual challenge, there are no long-term demographic studies of partially migratory ungulates. Ungulates are long-lived, with generation times ~ 3-6 years, thus, long-term for ungulates practically means >10 years. Here, we report both expected and surprising results from our long-term research on the ecology of the Ya Ha Tinda elk herd from 2001 – 2015 testing i) the ecological drivers of migration, ii) the mechanism for the maintenance of migratory variation within this population, and test iii) the potential population consequences of individual variation in behavior.
We monitored the demography, migration, and resource selection of > 250 individual adult female elk, and juvenile survival using population surveys, mark-resight and telemetry methods. Migrants benefited from higher nutritional resources especially during July and August, resulting in 10% higher pregnancy rates, and higher 6-month-old calf weights. These bottom-up benefits were complemented by large-scale reductions in exposure to wolf predation risk. However, migration itself was quite risky, with increased risk of mortality on the actual migratory route. Accordingly, we documented changes in use of migration routes over time driven by predation risk exposure. In contrast, residents experienced poorer nutrition, and higher large-scale exposure to wolf predation, but exploited fine-scale predation risk refugia surrounding human development. Surprisingly, there were no differences in vital rates or estimates of population growth rate of migrants and residents over the 15 years. This was initially puzzling, but seen through the lens of density-dependent resource selection, equal fitness is expected if individuals can switch between strategies. Indeed, our long-term monitoring provided the first evidence that elk switched migratory behavior from year to year. Switching between strategies was determined by density-dependence, predation, and age. The consequences of density-dependent switching between strategies apparently stabilized population dynamics and lead to a dynamic equilibrium in the stable ratio of migrant and resident elk within our population.