LATITUDINAL VARIATION IN SPECIES INTERACTIONS: A TEST IN THE NEW ENGLAND ROCKY INTERTIDAL ZONE

Abstract

How species interactions vary as a function of habitat characteristics continues to be an important debate in ecology. Using the barnacle–seaweed assemblage common in New England rocky intertidal habitats, I tested the hypothesis that species interactions switch from negative to positive at sites across the Cape Cod faunal barrier because of latitudinal variation in thermal stress and predation intensity between these regions. I manipulated Ascophyllum nodosum canopies in the high zone of two sites from each region and then determined the consequences for underlying Semibalanus balanoides recruits and adults for two years (January 1995 through January 1997). In both years, algal canopies reduced barnacle recruitment and growth rates at all sites but greatly increased survival rates only at the southern sites. When integrated in a demographic framework, these data showed that the reproductive fitness potential of individuals was facilitated by the algal canopy at southern sites but was reduced under similar conditions at northern sites. At southern sites, this was likely the result of buffering from physical stresses in the absence of predators. At northern sites, any buffering from physical stress was likely offset by an increase in mortality in the presence of predators. Interestingly, this variability in species interactions appeared to be associated with subtle variation in climate. Facilitation was evident only during 1995, the warmest year on record since 1900. In contrast, interactions were entirely negative during 1996, a slightly cooler year. These results suggest that species interactions in the intertidal zone may be sensitive to even subtle changes in climate. Interspecific buffering of neighbors from thermal stress is likely to be common in other systems and suggests that both aquatic and terrestrial vegetation may become increasingly important to local species persistence as climates change during the next century.