Impact of avian predators on high intertidal limpet populations

Abstract

Black turnstones ( Arenaria melanocephala (Vigors)), black oystercatchers ( Haematopus bachmani Audubon), and gulls of the western/glaucous-winged complex ( L. occidentalis Audubon and Larus glaucescens Naumann) were observed to feed on limpets ( Collisella spp.) in the high intertidal zone on the coast of Oregon, U.S.A. Black turnstones were the major predators of small limpets (< 10 mm), whereas oystercatchers were the major predators of large limpets (> 20 mm). Intermediate-sized limpets were eaten by all of these species. Visitation by avian predators to marked plots was infrequent; but, in the case of turnstones, the cumulative time spent foraging by a visiting flock was sometimes substantial. Two bird-exclusion experiments in the high intertidal zone indicated that birds decreased densities of 5–10 mm limpets ( Collisella spp.), but not densities of smaller limpets. The effects of avian predation on large limpets (> 10mm in length) could not be tested because the latter were rare on exposed high intertidal mudstone beaches. An experiment in which 6–10 mm limpets were added to caged and uncaged plots demonstrated that avian predators (probably turnstones) were capable of drastically reducing limpet densities in unvegetated high intertidal areas. A second trial, conducted 10 wk later, showed no evidence of avian predation suggesting that avian predation varied temporally.