Predation risk imposed by an avian top predator reduces the abundance of the smaller but not the larger mammalian mesopredator species

Background/Question/Methods

Top predators may induce extensive cascading effects on lower trophic levels, e.g. implicitly through intraguild predation, where predatory species compete over shared prey but also prey on each other. The predation impacts of both mammalian and avian top predators on the species of the same guild have been widely studied, whereas the effects of raptors upon mammalian mesopredators are not well known. We examined the impacts of the predation risk of a large avian predator, the golden eagle (Aquila chrysaetos), upon the abundance of its potential mammalian mesopredator prey in Finland, northern Europe. The study combined 23 years of unique, countrywide data from nesting records of eagles and wildlife track counts of mesopredators. To assess changes in abundance of red foxes (Vulpes vulpes) and pine martens (Martes martes), the predation risk of golden eagle was modelled either through territory density, density of fledglings produced or distance to the nearest active eagle territory. High predation risk was expected to reduce the abundances of the smaller mesopredator, pine marten, in particular. We used the eagle variables to fit Generalized Additive Mixed Models, and additionally accounted for relevant landscape covariates, as well as the abundance indices of the main prey of mesopredators.

Results/Conclusions

Results showed a non-linear density-dependent response in the smaller mesopredator: pine marten abundance increased along with golden eagle territory density, but at high eagle densities the number of martens decreased. This suggests that martens and eagles prefer similar habitats but at high densities eagles are able to reduce marten abundance. Larger mesopredators, red foxes, were more numerous close to golden eagle nests and in areas with more eagles. This likely resulted from similar prey preferences of these predators and the larger size of foxes, which enables at least full-grown foxes to escape eagle predation. This study, based on long-term data essential to trophic studies, provides novel information about the population level impact of a large avian predator upon mammalian mesopredators. Our results show that the golden eagle has potential to reduce numbers of mammalian mesopredators, at least when it is clearly larger-sized than the victim species. If expanding their range, the golden eagle as well as other large raptors may also increase predation upon invasive mesopredators, such as the American mink (Neovison vison) in Europe. Lowered mesopredator abundances could in turn benefit herbivorous game species and thus improve the protection status of raptors.