Many seabird species breed in large colonies, from which they make repeated excursions to collect prey for delivery to offspring. This necessarily confines their foraging activities to a region around or near the colony, which is constrained by their powers of mobility. The cumulative impact in this region is hypothesized to be large enough to reduce the density of prey, a phenomenon referred to as “Ashmole’s halo.” Considering its important role in thought about seabird life histories, Ashmole’s halo has received remarkably little formal investigation. Here we model the formation of Ashmole’s halo. In a previous theoretical study, Gaston et al. (2007) derived the size and shape of the halo, using a set of simple assumptions. In that model, prey density was reduced by exploitation alone: prey individuals had no explicit behavioral options with which to evade or elude predators, such as moving away from a zone of high depredation. The objective of the model developed here is to assess the role of prey behavior in the occurrence of Ashmole’s halo. In our model, both seabirds and their prey make strategic choices about foraging location: seabirds do so to maximize the delivery of prey to offspring, while fish do so to maximize fitness (growth and survivorship). The situation is a predator-prey game, because for both seabirds and fish the best choice of foraging location depends on the choices made by all other players. Our simulations show that the halo develops much more quickly and is deeper and stronger when prey individuals are able to adjust their location adaptively (i.e. to maximize fitness), than when a random (i.e. diffusion) process governs prey movement as prey density is altered by exploitation. These results broaden the conditions under which Ashmole’s halo could be pronounced enough to affect seabird biology.
|Publication status||Published - 2014|