Rocky Intertidal Community Organization: The Impact of Avian Predators on Mussel Recruitment

In the Pacific Northwest, mussels (Mytilus spp.) are major prey items of Surfbirds (Aphriza virgata), gulls (Larus glaucescens and L. occidentalis), and Black Oystercatchers (Haematopus bachmani). The impact of this predation is potentially great, as mussels are the competitively dominant members of the mid—intertidal community. I tested the effect of these predators on mussel recolonization of 50 x 50 cm clearings, using bird exclusion cages. Three of five exclosure experiments showed that birds significantly reduced recruitment of juvenile mussels (M. californianus and M. edulis) into clearings in established mussel beds. A sixth experiment indicated that birds were responsible for the absence of mussels from an area with smooth substrate lacking mussel beds. Here clumps of Mytilus edulis 10—20 mm in length became established in all exclosures, but not in any controls. These experiments indicate that the long—term impact of avian predators is greatest in patches where invertebrate predators are uncommon and larval settlement, rather than adult encroachment, is the major form of recruitment. The results of this and other studies suggest that avian predation increases the patchiness of a sedentary prey in heterogeneous habitats, but decreases patchiness where the habitat is sufficiently homogeneous that birds have access to all microhabitats.