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.
Intraguild predation Mesopredator suppression Top predator Trophic cascade
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We thank all the volunteers who have helped to collect the golden eagle, mesopredator and grouse abundance data. We are grateful to Robert L. Thomson for helpful comments on the language and contents of the manuscript. The study was financially supported by the Jenny and Antti Wihuri Foundation (grants to MSL), and the Academy of Finland (Grant Nos. 123379, 136717 and 250709 to EKor).
Author contribution statement
EKor, PH and TO originally formulated the idea. PH and TO coordinated data collection; MSL, AV, EKoi and EKor developed the methodology; MSL and AV analysed the data. MSL, EKoi and EKor wrote the manuscript; other authors provided editorial advice.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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