Water temperature, predation, and the neglected role of physiological rate effects in rocky intertidal communities

Ecologists and physiologists working on rocky shores have emphasized the effects of environmental stress on the distribution of intertidal organisms. Although consumer stress models suggest that physical extremes may often reduce predation and herbivory through negative impacts on the physiological performance of consumers, few field studies have rigorously tested how environmental variation affects feeding rates. I review and analyze field experiments that quantified per capita feeding rates of a keystone predator, the sea star Pisaster ochraceus, in relation to aerial heat stress, wave forces, and water temperature at three rocky intertidal sites on the Oregon coast. Predation rates during 14-day periods were unrelated to aerial temperature, but decreased significantly with decreasing water temperature. There was suggestive but inconclusive evidence that predation rates also declined with increasing wave forces. Data-logger records suggested that thermal stress was rare in the wave-exposed habitats that I studied; sea star body temperatures likely reached warm levels (>24°C) on only 9 dates in 3 yr. In contrast, wind-driven upwelling regularly generated 3 to 5°C fluctuations in water temperature, and field and laboratory results suggest that such changes significantly alter feeding rates of Pisaster. These physiological rate effects, near the center of an organism's thermal range, may not reduce growth or fitness, and thus are distinct from the effects of environmental stress. This study underscores the need to consider organismal responses both under "normal" conditions, as well as under extreme conditions. Examining both kinds of responses is necessary to understand how different components of environmental variation regulate physiological performance and the strength of species interactions in intertidal communities.