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 fea ture of many rocky shores. The presence of small scale spatial gradients in the demographic structure of prey populations is hypothesized for the regi on adjoining predation haloes. It is also suggested that halo effects in pr ey populations will change in response to environmental constraints on pred ator foraging behaviour. These hypotheses were tested by examining gradient s in barnacle population structure around crevices high and low on the shor e. The probability of encountering a barnacle above the local median size a lways increased with distance from a crevice. Foraging at sites high on a s hore is assumed to be more risky to individual whelks. Initial probabilitie s 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 sh ore. The implications of small scale gradients in prey populations are disc ussed with reference to the role of topographic complexity in mediating pre dator-prey dynamics. (C) 1998 Elsevier Science BV. All rights reserved.