Can animal migration explain the dominance of top-down forces in many Arctic food webs? Insights from empirical and theoretical approaches
A key question in ecology is whether top-down forces can drive the trophic dynamics of ecosystems. According to the exploitation ecosystem hypothesis, the relative strength of top-down vs. bottom-up forces increases with primary productivity. This hypothesis, however, has failed to predict the trophic dynamics of many arctic food webs that are rather controlled by top-down forces. This is likely in part because the hypothesis overlooked that food resources originating from other ecosystems such as migratory birds could sustain higher abundance of arctic predator populations than what is predicted by the primary productivity of tundra ecosystems. Here, we first use empirical data to examine the influence of proximity to a colony of migrating geese on diet and reproductive output of arctic foxes on Bylot Island (Nunavut, Canada) during different phases of the lemming cycle. We then developed a theoretical model to predict the strength of top-down forces relative to bottom-up ones in food webs when increasing the level of consumption of food subsidies such as migrating birds by top-predators. We used differential equations to model the dynamic of nutrients, plants, herbivores, and predators at increasing levels of subsidies consumed by predators.
We showed that both the contribution of geese to the diet of arctic fox cubs and the probability that a fox den was used for reproduction decreased with distance from the goose colony. The effect on reproduction disappeared at high lemming abundance. These results suggest that the availability of migrating birds could allow the fox population to reach larger abundance than expected from the primary productivity of the tundra ecosystem. According to the theoretical model, the abundance of predators and, in turn, the strength of top-down forces relative to bottom-up ones increases with the abundance of subsidies available to predators. This prediction is congruent with an increasing body of empirical work showing that top-down forces drive the trophic dynamics of many tundra food webs at a circumpolar scale. Our study shows how the input of migratory birds affects the functional and reproductive responses of an arctic predator, and how it may explain the dominance of top-down forces in arctic food webs. Further work should be targeted at verifying predictions of our model in other study systems, as well as at investigating the impact of global changes in climate and land use on trophic dynamics.