In several animal taxa such as birds, fishes, amphibians and insects, populations include individuals that migrate between two different habitats, while other individuals remain resident in only one of the habitats. For instance, populations of Atlantic salmon (Salmo salar) are composed of both migratory and non- migratory individuals. Often, the migratory type is more abundant than the non-migratory type, which causes the population to be more vulnerable to factors affecting migratory individuals. This poses the question what the effect is on the population dynamics of ecological factors that affect migratory individuals. Surprisingly, given the large body of theoretical research in the field of partial migration, this question has not been addressed in a setting where the effects of migration on the individual life history trajectories are taking into account. To address this question we studied the effect of reduced survival of migrants, high energetic cost of migration and low food availability in wintering areas (areas that only migratory individuals occupy) on the dynamics of a partially migratory population using a size-structured population model based on a Dynamic Energy Budget (DEB) model to describe the life history at the individual level.
Results/Conclusions
In a partially a migratory population, the dominance of the migratory individuals may jeopardizes the viability of the population when migratory individuals face bad ecological factors. In particular, reduced survival and high energetic cost of migration lead independently a population dominated by migratory individuals to extinction. Surprisingly, population extinction is less likely to occur when food availability in the wintering habitat is low, as high food conditions result in migrants being larger with relatively speaking less energy reserves at the onset of migration. This counterintuitive result is an emergent consequence of using an energy budget model to define the life history trajectory of individuals. To our knowledge, this persistence effect has not been documented before due to the absence of the energetic component in individual life history in previous theoretical studies. Given the increasingly documented examples of extinction and population declines of migratory species, our findings contribute to understanding the processes underling migratory behavior that are key for conserving these species.