Meeting Abstract

35.4  Tuesday, Jan. 5  Diversity, consumer pressure and resource availability on subtidal rock walls ELAHI, R*; SEBENS, KP; University of Washington elahi@u.washington.edu

Resource competition theory predicts an inverse relationship between species richness and resource availability. More species should more fully exploit available resources, decreasing the rate of colonization by both native and exotic species. Space is the limiting resource for sessile organisms on marine rocky substrata, and the availability of space is limited by recruitment and growth and increased through senescence, disturbance and consumption. This study examined the relative importance of sessile species diversity and consumer pressure on space availability in benthic rock wall communities. For two years, we quantified the seasonal abundance of sessile taxa in 72 permanent quadrats installed on six transects at three sites in the San Juan Islands, Washington. Gastropods and other small consumers were counted in quadrats, while large consumers including echinoderms and fish were counted on transects. Within a multiple regression framework, the density of two common consumers, red urchins and chitons, explained 53% of the variation in available space, while sessile richness explained an additional 6%, suggesting that consumers were responsible for the patterns of available space. To test this hypothesis, we used a combination of consumer gut content analyses and field manipulations of urchin and chiton density. Preliminary data suggest that urchins create space by consuming sessile invertebrates and foliose algae, while chitons maintain space by consuming crustose algae and microscopic algal recruits. Together, these results emphasize the need to understand the role of consumers when trying to predict resource availability and invasibility in communities where space is the limiting resource.