All routes are not created equal: an ungulate’s choice of migration route can influence its survival

Citation

Sawyer, H., C. LeBeau, T. L. McDonald, W. Xu, and A. D. Middleton (2019). “All routes are not created equal: an ungulate’s choice of migration route can influence its survival”. In: Journal of Applied Ecology, pp. 1365-2664.13445. DOI: 10.1111/1365-2664.13445.

Abstract

1. Our knowledge of migration ecology has progressed quickly in concert with technological advances that collect fine-scale movement data through time. We now know that migration plays a critical role in the annual nutritional cycle of large herbivores and that sustaining functional migratory routes is key to long-term conservation. Yet, we lack basic information on whether one migratory route may function better than another, or more specifically, if choosing one route over another has fitness consequences - knowledge that could help inform conservation and restoration efforts.
2. Here, we examined how a suite of migratory parameters influenced the survival of mule deer Odocoileus hemionus that shared a common winter range in New Mexico, USA, but migrated to various summer ranges in Colorado, USA. We used a Cox proportional hazard model and longitudinal global positioning system data collected over a 7-year period to investigate whether the mortality risk of 66 deer was affected by choice of migratory route, summer range, migration distance, speed or the number of administrative boundaries each route crossed. We found mule deer survival was not influenced by migratory distance, speed or number of administrative boundaries, but was strongly affected by the choice of migratory route and summer range. The magnitude of these effects was surprisingly large, doubling or tripling mortality risk. Cumulative survival rates showed that regardless of summer range, individuals migrating along high-use exterior routes had cumulative survival rates approximately 30% lower than individuals migrating along high-use interior routes. To our knowledge, this is the first direct evidence that a mammal’s choice of migration route can influence its probability of survival.
3. Synthesis and applications. Our finding that large herbivores may experience up to three times higher mortality risk by using a different migratory route reveals a novel link between migration and demography. Importantly, our results also suggest that spatially explicit model parameters and predictions could help in the conservation and restoration of migratory populations by identifying specific migratory routes or seasonal ranges that reduce survival.