Ruckelshaus 9629714 Marine organisms and communities are characterized by strong species interactions, and the intensity and character of these interactions can vary geographically, even when the same species are involved. This research investigates the evolutionary consequences of spatially-varying species interactions, with the ultimate goal of quantifying the likelihood that coevolution has occurred between a marine plant and its herbivores. The plant, Fucus distichus, inhabits rocky intertidal habitats and is widely distributed geographically, extending from Alaska to southern California in the western part of its range. Throughout this range, a number of herbivore species occur, including a number of littorine snails, an isopod, and possibly a small crab species. In order to quantify spatial variation in herbivore intensity, the occurrence of the herbivores and the damage each species imposes on the plant will be quantified throughout the geographic range of F. distichus. Next, the potential for the plant to respond to the selection pressures exerted by the herbivores will be quantified by estimating the size of an interbreeding population of plants using genetic markers. If F. distichus individuals are interconnected by dispersal throughout much of their geographic range, then within-population spatial variation in herbivore intensity is not expected to result in local variation in response to herbivory on the part of the plant. On the other hand, if F. distichus occurs in small, isolated populations, spatial variation in the presence of herbivores and the amount of damage they inflict may result in spatially heterogeneous responses of the plant and genetic divergence among populations. Results from this research will begin to address the question of whether well-known spatial variation in marine species has evolutionary consequences. Does spatial variation in species interactions observed in the sea result in a significantly heterogeneous selective environment? Do long ran ge dispersal capabilities of marine organisms result in large interbreeding populations which "swamp out" the effects of local variation in species interactions? Answers to these questions also will provide valuable data for conservation biologists, whose recent focus on conserving whole ecosystems requires data on the nature of species interactions in natural habitats.
