Interspecific interactions, which influence the distribution, abundance, and ecological importance of species, are modified by environmental gradients which extend through space and time. The implications of environmental modification of interspecific interactions remain poorly understood. In this dissertation, I define a metric for measuring the strength of interactions between species. I apply this metric to a specific predator-prey interaction, and to a broad dataset culled from the literature, to demonstrate the spatial and temporal variability of interaction strength. Given this background, I demonstrate that biological interactions can be altered by the experimental manipulation of a specific environmental variable, namely irradiance. The strength of competition was modified by both the light environment and by the identity and size of the interacting consumers.

I next investigated the combined roles of physical and biological variables on species distributions and abundance at larger spatial scales. The red algal turf, Mazzaella cornucopiae, is restricted to north-facing surfaces on Tatoosh Island. Through a series of transplant experiments, I determined that the upper limit of Mazzaella is set by desiccation stress, and the lower limit is set by herbivory. Moving from north-facing to south-facing slopes, the upper limit shifts downshore, while the lower limit does not. As a result, Mazzaella rarely occurs naturally on south-facing shores. However, Mazzaella can survive in south-facing habitats given one of two conditions: 1) the upper limit is extended upshore by experimental shading, or 2) the lower limit is extended downshore via herbivore exclusions.

Finally, this two-dimensional conceptual model was applied to regional scale patterns of species abundance and species richness in the Northwest Straits. As predicted, the west-east gradient in physical stress was accompanied by a progressive decrease in the upper limits of sessile species. The upper limits of a dominant consumer, Pisaster ochraceus, showed no pattern across the regional gradient. As a result, Pisaster was able to control the abundance of several invertebrate species at the eastern most site, but not at more environmentally benign sites further west. This pattern had dramatic consequences for local species richness at the site where Pisaster was able to eliminate most sessile invertebrate prey.

email: mogzilla@stanford.edu
internet: http://stanford.edu/group/denny/harley.html