Red deer migration and dietary quality: testing the role of landscape characteristics for the forage maturation hypothesis

The regular movement of animals between summer and winter ranges, termed seasonal migration, is a common phenomenon among ungulates at northern latitudes. The most prominent hypothesis to explain seasonal migration is the forage maturation hypothesis; it predicts that large herbivores will preferentially follow phenological (or green-up) gradients in order to access the highest possible forage quality with sufficient quantity. Surprisingly few studies provide empirical testing of the forage maturation hypothesis, and a question not often addressed is which landscape variables underlie the phenological gradient that makes migration a beneficial strategy. I tested the forage maturation hypothesis using the normalized difference vegetation index (NDVI), an index of plant phenology, and feces analyses as measures of forage quality and linked it with space use data from partially migratory red deer (Cervus elaphus) in Norway. Specifically, I investigated whether migratory individuals had access to higher quality forage than stationary, and the role of landscape variables for access to high quality forage. Generalized linear mixed models were used to analyze different measures of forage quality as responses to a set of covariates, including landscape variables. I found evidence that migratory red deer gain access to more high quality forage than stationary red deer, consistent with the forage maturation hypothesis. I found a positive effect of elevation on access to high quality forage for migratory individuals, and a diverse effect of other landscape characteristics, such as aspect and home range size. There was a positive effect of distance to fjord on accessed forage quality. This study sheds light on the relationship between the landscape an ungulate inhabit and the possibilities and limitations to access high quality forage within this landscape. Considering future climate change, and consequent changes in phenological gradients, it is important to understand the mechanisms causing the phenological gradients that migratory ungulates benefit from. Given the high diversity of impact by different variables found in this study, the effect of climate change may differ for populations across ecosystems.