Beyond the predation halo: Small scale gradients in barnacle populations affected by the relative refuge value of crevices

Haloes of bare space around crevices used as refuges by predators are a feature of many rocky shores. The presence of small scale spatial gradients in the demographic structure of prey populations is hypothesized for the region adjoining predation haloes. It is also suggested that halo effects in prey populations will change in response to environmental constraints on predator foraging behaviour. These hypotheses were tested by examining gradients in barnacle population structure around crevices high and low on the shore. The probability of encountering a barnacle above the local median size always increased with distance from a crevice. Foraging at sites high on a shore is assumed to be more risky to individual whelks. Initial probabilities of recording a large barnacle near a crevice increased more rapidly over small spatial scales at sites high on the shore than at sites low on the shore. The implications of small scale gradients in prey populations are discussed with reference to the role of topographic complexity in mediating predator-prey dynamics.