Annabel Josien Slettenhaar , Jan Eivind Østnes, Børje Cato Moen, Rolf Terje Kroglund, Erlend Birkeland Nilsen , Torgeir Nygård

Species interactions can be altered by climate change but can also mediate its effects. A predator-prey couple reflecting the dynamics of boreal and alpine ecosystems is the gyrfalcon (Falco rusticolus) and the ptarmigan (Lagopus spp.). To determine how climate change may impact the alpine food web, we investigated how ptarmigan abundance and local weather impacts gyrfalcon diet and feeding behaviour, territory occupancy and reproductive success. Using wildlife cameras, we monitored gyrfalcon nests throughout the nesting period to collect data on diet and feeding behaviour. We quantified the gyrfalcon’s functional response by describing how ptarmigan kill rates relate to ptarmigan density. The gyrfalcon’s numerical demographic and aggregative response were quantified by describing how gyrfalcon reproductive success and territory occupancy, respectively, were related to ptarmigan density, using data from large scale monitoring projects. Ptarmigan were the dominant prey species, occupying 98% of the diet. The proportion of ptarmigan in the gyrfalcon diet, gyrfalcon breeding success, and gyrfalcon territory occupancy increased when spring arrived later, but reproductive success decreased with more snow during the nesting period. Gyrfalcon reproductive success was positively related to ptarmigan density, i.e., a positive numerical demographic response, but gyrfalcon territory occupancy or the ptarmigan kill rate were not related to ptarmigan density. These results indicate that the effect of climate change is not straightforward, and investigating how (a)biotic factors impact both prey and predator is relevant in predicting how a predator will respond to climate change. Following current climate predictions, onset of spring will become earlier, which will change the food web structure through prey availability and diversity, and through interactions with other species. This requires adaptations from gyrfalcons and other predators. We emphasize that the impact of climate change on predators and other species can be more accurately evaluated on a multi-species level, rather than individually.