Abstract

The mid-intertidal zone of the western shores of San Juan Island is dominated by beds of the large barnacle, Semibalanus cariosus (Pallas). Although Semibalanus is a competitively dominant barnacle species that can reach a size refuge from predation and potentially monopolize space, more than 50% of the rock surface is bare. Within the barnacle beds, bare space occurs intermingled among the large adult barnacles, which are a few centimeters apart, and in small patches (0.1-1 m2) devoid of adult Semibalanus. The present study builds on previous experiments in order to investigate spatial and temporal variation in the processes maintaining bare space in this community and to evaluate the benefits and limitations of previous experimental information in this system. Working at the same study site over 23 years ago, Dayton (1971) demonstrated that predation by whelks and bulldozing of newly settled barnacles by limpets were important in maintaining bare space in small patches initially cleared of barnacles. Since the focus of this earlier work was on the factors influencing the provision of bare space after a disturbance (e.g., in areas where large barnacles were initially cleared), no experiments were done in intact beds of large barnacles. We repeated Dayton's factorial manipulations of whelks and limpets both under conditions similar to those described in the original study (i.e., small patches cleared of barnacles), as well as in adjacent intact Semibalanus beds to directly test whether the importance of these consumers in maintaining bare space varied with the presence or absence of large adult Semibalanus. A number of ancillary experiments and observations were conducted to investigate potential mechanisms underlying the observed responses. Our results showed that: (1) Under the conditions of the original experiments (small patches), the importance of whelks and limpets in maintaining bare space was strikingly consistent over time; (2) initial experimental conditions, whether the sessile community was left intact or initially cleared, strongly influenced field measures of consumer-prey interactions: (3) the processes maintaining bare space varied dramatically over small spatial scales. Whelks and limpets played key roles in small patches without large Semibalanus, but they had no effect on the maintenance of bare rock within adjacent beds of the large barnacles. In this case, much of our observational and experimental information on predator densities, behavior, and potential physiological stresses was not useful for predicting community dynamics in Semibalanus beds from previous experimental results.