Impacts of predator-mediated interactions along a climatic gradient on the population dynamics of an alpine bird

According to classic theory, species interactions are less important than climatic variation for species’ population dynamics and distributions in climatically harsh regions compared with more climatically benign regions. In boreal ecosystems, the cyclic dynamics of rodents strongly affect many other species, including ground-nesting birds. According to the ‘alternative prey hypothesis’ (APH), high rodent densities positively affect the abundance of ground-nesting birds due to predator-mediated interactions. We analysed a spatiotemporal dataset of willow ptarmigan ( Lagopus lagopus ) and rodents to examine how the role of predator-mediated interactions changes along a climatic harshness gradient in comparison with climatic variation. Ptarmigan and rodent data were collected during a national program of line-transect sampling across Norway during 2007–2017. We build a hierarchical Bayesian model to estimate the sensitivity of ptarmigan populations to interannual variation in climate and rodent occurrence. Ptarmigan abundance was positively linked with rodent occurrence, consistent with the APH. Moreover, we found that both rodent dynamics and temporal climatic variation had stronger effects on ptarmigan in colder regions. Our study highlights how species interactions play an important role for the population dynamics of species at higher latitudes and suggest that they become even more important in the most climatically harsh regions.