1 3 Oecologia DOI 10.1007/s00442-016-3680-8 POPULATION ECOLOGY – ORIGINAL RESEARCH Guardian or threat: does golden eagle predation risk have cascading effects on forest grouse? Mari S. Lyly 1 · Alexandre Villers 1,2 · Elina Koivisto 1 · Pekka Helle 3 · Tuomo Ollila 4 · Erkki Korpimäki 1 Received: 13 July 2015 / Accepted: 13 June 2016 © Springer-Verlag Berlin Heidelberg 2016 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 mesopreda- tor numbers, but instead by fear effects. Alternatively, they may be mediated by other species than fox or marten stud- ied here. In conclusion, we found support for the hypoth- esis 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. Keywords Intraguild predation · Mesopredator suppression · Top predator · Trophic cascade Introduction Top predators have been lost from many areas of the world, causing cascading effects throughout the food webs (Ter- borgh and Estes 2010; Estes et al. 2011; Ritchie et al. 2012). These cascades demonstrate how influential top predators are in many ecosystems. Typically trophic cas- cades are mediated either through herbivores or smaller predators that fall prey to the top predators. The latter case is referred to as intraguild predation, when predators utilize shared prey but also prey on each other (Polis et al. 1989; Lourenço et al. 2014). If top predators limit the numbers and the predation behaviour of mesopredators, they may even be beneficial for the shared herbivore prey (Palomares et al. 1995; Vance-Chalcraft et al. 2007). Studies of intraguild predation in terrestrial environ- ments have largely focused on mammalian carnivores and Abstract Previous studies on intraguild predation have mainly focused on within-class assemblages, even though avian top predators may also influence mammalian meso- predator 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 den- sity estimates and distance to eagle nest. Wildlife triangle counts were used as predation impact proxies of mamma- lian mesopredators and as measures of response in grouse. Our results show that eagle density correlated negatively with black grouse abundance indices while being positively Communicated by Christopher N. Johnson. Electronic supplementary material The online version of this article (doi:10.1007/s00442-016-3680-8) contains supplementary material, which is available to authorized users. * Mari S. Lyly mari.s.lyly@utu.fi 1 Section of Ecology, Department of Biology, University of Turku, 20014 Turku, Finland 2 Centre d’Etudes Biologiques de Chizé UMR 7372, CNRS and Université de La Rochelle, 79360 Villiers-en-Bois, France 3 Natural Resources Institute Finland, Oulu Game and Fisheries Research, University of Oulu, POB 413, 90014 Oulu, Finland 4 Metsähallitus, Parks and Wildlife, POB 8016, 96101 Rovaniemi, Finland