Distribution of Wintering Common Eiders over Mussel Beds: Does the Ideal Free Distribution Apply?

Abstract

The preferred habitat of common eiders (Somateria mollissima) wintering in the Gulf of St. Lawrence, eastern Canada consists of submerged reefs distributed in patches of different sizes which are composed of kelp beds and urchin barrens. In this habitat, the blue mussel (Mytilus edulis), a preferred prey, is found only inside kelp beds and is considered to have a high carrying capacity to be depleted slowly over the course of the winter. We examined the effect of population density on the number of mussel patches simultaneously used by eiders. Our rationale was that if interference competition was occurring, we should observe a spacing of eiders and the number of patches used should increase with an increase in population density. We used the number of eiders counted in these patches as an index of density. The data show that the eiders crowd into one or two patches as population density increased. We thus discarded the possibility that interference competition was driving the distribution of wintering eiders. We also hypothesized that prey depletion could influence eider distribution over the winter and we predicted that patch use was correlated with food availability through time and follows a simple ideal free distribution. Our data supported this hypothesis. Although population density and prey depletion act at two different time scales and result in two contrasting distributions, we show that these two factors can play a role in determining eider distribution.