title: Guardian or threat: does golden eagle predation risk have cascading effects on forest grouse?
abstract: 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.
authors.affiliation: Section of Ecology, Department of Biology, University of Turku, 20014, Turku, Finland. mari.s.lyly@utu.fi.
authors.forename: Mari S
authors.initials: MS
authors.lastname: Lyly
authors.affiliation: Section of Ecology, Department of Biology, University of Turku, 20014, Turku, Finland.|Centre d'Etudes Biologiques de Chizé UMR 7372, CNRS and Université de La Rochelle, 79360, Villiers-en-Bois, France.
authors.forename: Alexandre
authors.initials: A
authors.lastname: Villers
authors.affiliation: Section of Ecology, Department of Biology, University of Turku, 20014, Turku, Finland.
authors.forename: Elina
authors.initials: E
authors.lastname: Koivisto
authors.affiliation: Natural Resources Institute Finland, Oulu Game and Fisheries Research, University of Oulu, POB 413, 90014, Oulu, Finland.
authors.forename: Pekka
authors.initials: P
authors.lastname: Helle
authors.affiliation: Metsähallitus, Parks and Wildlife, POB 8016, 96101, Rovaniemi, Finland.
authors.forename: Tuomo
authors.initials: T
authors.lastname: Ollila
authors.affiliation: Section of Ecology, Department of Biology, University of Turku, 20014, Turku, Finland.
authors.forename: Erkki
authors.initials: E
authors.lastname: Korpimäki
mesh_terms: D000818:Animals; D020307:Eagles; D065928:Forests; D046094:Galliformes; D011157:Population Dynamics; D011235:Predatory Behavior
keywords: Intraguild predation; Mesopredator suppression; Top predator; Trophic cascade