Abstract
Many seabirds congregate in large colonies for breeding, a time when they are central place foragers. An influential idea in seabird ecology posits that competition during breeding results in an area of reduced prey availability around colonies, a phenomenon known as Ashmole’s halo, and that this limits colony size. This idea has gained empirical support, including the finding that species coexisting within a colony might be able to do so by foraging on a single prey species but at different distances. Here, we provide a comprehensive mathematical model for central place foragers exploiting a single prey in a two-dimensional environment, where the prey distribution is the result of intrinsic birth and death, movement in space and mortality due to foraging birds (we also consider a variant tailored toward colonial social insects). Prey mortality is the result of bird predation at different distances according to an ideal free distribution that maximizes prey delivery under flight and search costs. We fully characterize the birds’ ideal free distribution and the concomitant prey distribution. Our results show that prey depletion halos around breeding colonies are a robust phenomenon and that the birds’ ideal free distribution is sensitive to prey movement. Furthermore, coexistence of several seabird species on a single prey easily emerges through behavioral niche partitioning whenever trait differences between species entail trade-offs between efficiently exploiting a scarce prey close to the colony and a more abundant prey far away. Such behavioral-based coexistence-inducing mechanisms should generalize to other habitat and diet choice scenarios.
Significance statement Large numbers of seabirds, often belonging to different species, breed on small islands and forage in the surrounding sea to provision their offspring. Due to travel costs, these birds are restricted to a limited area around their colony. Using mathematical modelling, we show that such central place foragers produce a zone of prey depletion around islands. In agreement with empirical observations, our model furthermore shows that different bird species can coexist, despite feeding on the same prey species, due to behavioral segregation into different circular zones around the island. Behavioral segregation emerges when individuals chose foraging distances so as to maximize their prey delivery rate and when different species experience different costs while flying to and foraging at different distances.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
laurent.lehmann{at}unil.ch