Technical Report No. 23

SPECIES-SPECIFIC RESPONSES TO ENVIRONMENTAL GRADIENTS DETERMINE REGIONAL SCALE PATTERN IN AN INTERTIDAL COMMUNITY

Christopher D. G. Harley

December 1998

Bibliographic Citation
 Harley, Christopher D. G. 1998. Species-specific responses to environmental gradients determine regional scale pattern in an intertidal community. Washington State Department of Ecology (Publication No. 00-06-01), Padilla Bay National Estuarine Research Reserve Technical Report No. 23, Mount Vernon, Washington. 31 pp.

Abstract
Although the factors that control the local patterns of distribution and abundance of species are well studied, their relative influences on regional patterns remain poorly understood. In this study, I manipulated a biological interaction (predation) at either end of a regional scale environmental gradient in the Strait of Juan de Fuca. From west to east, wave action decreases, air temperature increases, and the upper limits of intertidal organisms retreat down the shore. Because of this redistribution of sessile species, a larger percent of their intertidal range becomes vulnerable to intertidal predators, such as the sea star Pisaster ochraceus, that are able to track the tide. Thus, at the west entrance of the Strait, the barnacle Balanus glandula is rarely encountered by Pisaster. However, at the eastern terminus of the Strait, the effective vertical ranges of Pisaster and Balanus overlap almost completely, and barnacle populations are restricted to refuge habitats such as crevices. It is the differential impact of the environmental gradient on the vertical ranges of predator and prey that creates the dramatic differences in assemblage structure seen on a regional scale. An understanding of ecological pattern over multiple spatial gradients will aid in our ability to predict the outcome of temporal changes over an environmental gradient.