Mechanistic insights into the role of functional response in apparent mutualism observed in tundra ecosystems

Andréanne Beardsell, Dominique Berteaux, Jeanne Clermont, Pascal Royer-Boutin and Joël Bêty, Biologie, chimie et géographie, Université du Québec à Rimouski, Rimouski, QC, Canada, Andréanne Beardsell and Joël Bêty, Quebec Center for Northern Studies, QC, Canada, Dominique Gravel, Départment de Biologie, University of Sherbrooke, Sherbrooke, QC, Canada, Gilles Gauthier, Department of biology and Center for Northern Studies, Université Laval, Québec, QC, Canada, Vincent Careau, Biology, Université d'Ottawa, Ottawa, ON, Canada, Claire-Cécile Juhasz and Nicolas Lecomte, Canada Research Chair on Polar and Boreal Ecology, Université de Moncton and Quebec Center for Biodiversity Science, Moncton, NB, Canada, Claire-Cécile Juhasz, Nicolas Lecomte and Pascal Royer-Boutin, Centre d'études nordiques, QC, Canada

Background/Question/Methods
Functional responses link predator acquisition rates to prey densities, underpinning drivers of predator-prey population dynamics, and leading to a better understanding of indirect interactions in natural communities. Despite their relevance, field-based studies of the functional response of generalist predators are extremely rare in vertebrates. Functional responses are generally evaluated using statistical models, which can fail to discriminate between predetermined shapes of functional responses and do not allow identifying proximal mechanisms of species interactions. We developed a mechanistic model of arctic fox functional response to three prey species (lemmings, passerines and sandpipers) coexisting in the Arctic tundra ecosystem. These species form a predator-prey system in which short-term apparent mutualism between lemmings and birds has been documented. The originality of our approach was to assess functional response i) by breaking down the components of predation (searching, chasing, capturing, and handling prey) and ii) by using extensive empirical data collected over the past 20 years on Bylot Island (Nunavut) to parameterize each component of the model. To identify the main mechanisms that could explain apparent mutualism, we extended the Holling's multi-species functional response model to include density-dependent changes in components of the functional response.
Results/Conclusions
Our study illustrates how mechanistic models based on empirical estimates of the main components of predation can generate functional response shapes specific to the range of prey densities observed in the wild. Our results also highlight the importance of predator searching components in predator-prey interactions, especially the time spent foraging and the distance traveled by the predator, while predator acquisition rates were not limited by handling processes. Predator satiation or saturation cannot explain the strong positive effect of high lemming densities on the reproductive success of birds. However, changes in time spent foraging and distance traveled by the predator induced by fluctuations in lemming density can generate variation in bird success that are consistent with those observed in the wild. The use of mechanistically derived multi-species functional responses can strongly improve our ability to untangle proximate drivers of predator-prey dynamics and understand indirect species interactions in natural communities.