MEPS 317:29-39 (2006)  -  doi:10.3354/meps317029

Effects of physical ecosystem engineering and herbivory on intertidal community structure

Christopher D. G. Harley1,2,*

1University of Washington, Department of Zoology, Seattle, Washington 98105-1800, USA
2Present address: Department of Zoology, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

ABSTRACT: Physical ecosystem engineers play dominant roles in a wide variety of communities. While many of the direct, positive effects of ecosystem engineers are readily apparent, the roles of engineers are often mediated by indirect interactions stemming from the facilitation of one or a few key species. Although direct and indirect effects are both critical drivers of community dynamics, they are rarely considered together with regards to ecosystem engineering. In the present study barnacle and herbivorous gastropod densities are experimentally manipulated to investigate the direct positive effects of habitat provision by barnacles as well as indirect effects mediated by molluscan grazers. Molluscan grazers (Littorina spp.) and herbivorous arthropods were positively influenced by the presence of barnacles. Arthropod abundance and species richness were lower when Littorina spp. were present. This pattern was not influenced by barnacle cover, suggesting that competition among herbivore functional groups was strong but independent of biogenic habitat complexity. In addition, Littorina spp. had strong negative effects on the filamentous alga Urospora penicilliformis, but this effect was only seen in the absence of barnacles. Finally, Littorina spp. reduced the recruitment of the principal habitat-forming barnacle Balanus glandula, suggesting that Littorina spp. may mediate a negative feedback loop in B. glandula population dynamics. Given the ubiquity of ecosystem engineers, similar combinations of direct and indirect influences may have far-reaching consequences for community dynamics and species richness in a wide range of systems.

KEY WORDS: Balanus glandula · Biogenic habitat structure · Chthamalus dalli · Competition · Facilitation · Grazing · Littorina plena · Recruitment

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