Previous
studies on intraguild predation have mainly focused on within-class
assemblages, even though avian top predators may also influence
mammalian mesopredator prey. By using nation-wide long-term data from
Finland, northern Europe, we examined the impacts of golden eagles
(Aquila chrysaetos) together with red foxes (Vulpes vulpes) and pine
martens (Martes martes) on forest-dwelling herbivores, black grouse
(Tetrao tetrix) and hazel grouse (Tetrastes bonasia). We hypothesized
that eagles may alleviate the overall predation pressure on grouse by
imposing intraguild predation risk on mesopredators. The predation
impact of eagle was modelled using eagle density estimates and distance
to eagle nest. Wildlife triangle counts were used as predation impact
proxies of mammalian mesopredators and as measures of response in
grouse. Our results show that eagle density correlated negatively with
black grouse abundance indices while being positively associated with
the proportion of juveniles in both grouse species, irrespective of the
abundance of mesopredators. Yet, foxes and martens alone had a negative
effect on the abundance indices and the proportion of young in the two
grouse species. This suggests that the possible cascading effects of
eagles are not mediated by decreased mesopredator numbers, but instead
by fear effects. Alternatively, they may be mediated by other species
than fox or marten studied here. In conclusion, we found support for the
hypothesis that eagles provide protection for juvenile black and hazel
grouse, whereas they are a threat for adult grouse. This important
information helps us to better understand the role of avian top
predators in terrestrial ecosystems.