Indirect Effects, Prey Susceptibility, and Habitat Selection: Impacts of Birds on Limpets and Algae

I experimentally manipulated avian predation pressure in an intertidal community to determine its direct and indirect influence on the abundance and distribution of three limpets and their algal food source. Birds reduced the overall abundance of Lottia digitalis only; L. strigatella abundance increased and the abundance of L. pelta, the species most frequently consumed, did not change. Through crypsis, the goose barnacle Pollicipes polymerus and the mussel Mytilus californianus indirectly affected the abundance of L. digitalis and L. pelta in opposite ways by changing the risk of succumbing the bird predation. These limpets also exhibited strong habitat selection for their cryptic substrates. By altering the amount of preferred habitat, birds indirectly influenced limpet abundance: gull predation reduced the area covered by P. polymerus, releasing M. califonianus from space competition. By consuming limpet grazers and reducing space competition, birds indirectly enhanced the abundance of algae. L. strigatella sizes generally fell below the range of limpet sizes consumed by birds. Consequently, birds indirectly increased L. strigatella density by reducing the intensity of exploitative competition with other limpet species; L. strigatella biomass declined significantly with increasing biomass of the other two limpets, and with decreasing algal cover. These results demonstrate that indirect effects and apparently adaptive behaviors can counteract (or reinforce) direct interactions between species pairs, suggesting that conclusions from short—term experiments emphasizing species pairs can be tenuous.