Local- and regional-scale effects of wave exposure, thermal stress, and absolute versus effective shore level on patterns of intertidal zonation

Rocky intertidal ecosystems are characterized by marked zonation patterns in which species replace one another along the vertical gradient of emersion time. Yet, we still do not fully understand the reasons that zonation patterns are variable in space and time. Here, we use effective shore level (ESL), a metric that incorporates the modifying influence of wave splash, to describe the relationship between uninterrupted emersion time and the zonation patterns of two ecologically important species: the mussel Mytilus californianus and the barnacle Balanus glandula. At the local scale (10s to 100s of meters) on Tatoosh Island, the upper limits of both species are closely related to ESL, regardless of substrate aspect or maximum temperature. At larger spatial scales (10s to 100s of kilometers), the upper limit of Balanus is related to ESL at cool sites but not at hotter sites. Thus, although ESL explains most of the local-scale variation in zonation at a cool site, other factors (temperature, desiccation) likely become important as spatial scale increases to incorporate warmer sites. Our results emphasize that an understanding of where and when specific ecological factors are limiting is crucial for our ability to explain and predict large-scale biological patterns in space and time. Recent advances in physiology, ecology, and biological oceanography have considerably improved our understanding of the environmental factors that generate and maintain biological pattern in the marine environment (Bertness et al. 2001; Dahlhoff et al. 2001; Helmuth and Hofmann 2001). Emerging from this recent work is the realization that the importance of any given environmental driver may vary tremendously through space and time (e.g., Bertness and Ewanchuk 2002). The present challenge for ecologists is to understand when and where organisms respond to specific drivers in their environment. Only with such an understanding will we be able to predict the ecological outcomes of perturbations resulting from resource extraction, changes in species composition (e.g., species introductions and extinctions), and changes in climate. In intertidal systems, environmental conditions range from fully aquatic to fully terrestrial over the space of a few vertical meters. The resultant patterns of biological zonation are 1 To whom correspondence should be addressed. Present address: Hopkins Marine Station, Oceanview Boulevard, Pacific Grove, California 93950 ([email protected]).