SPECIES-DIVERSITY - PREY REFUGES MODIFY THE INTERACTIVE EFFECTS OF PREDATION AND COMPETITION

We explore the effects of interactive variation in predation, interspecific competition, and the availability of absolute, spatial prey refuges on the local diversity of a sessile prey assemblage potentially competing for space. Prey diversity responds to increasing predation intensity in a characteristic pattern, called a "prey-diversity response." Previous theoretical and empirical studies have demonstrated three prey-diversity responses: negative, unimodal, and bimodal. Using simple simulations involving two to five prey species, we examine four scenarios: (1) Given a negative response in a simple habitat, such that diversity only decreases as predation intensity increases, increasing the proportion of the habitat that is safe from prédation should result in a gradual flattening of the negatively sloped diversity-response curve. (2) Given a unimodal prey-diversity response where the dominant competitor persists over most predation intensities, such that diversity initially increases and subsequently decreases as predation increases from zero, the effects of refuges are more complex. As the proportion of prey-refuge space increases at low prédation intensities, prey diversity should decrease monotonically. However, at high predation intensities, prey diversity should initially increase and subsequently decrease as refuge space increases. Consequently, the shape of the prey diversity-response curve should change progressively from a unimodal function, with the peak shifting to the right, to a monotonically increasing function which gradually flattens. (3) Given a system where the dominant is extirpated at low prédation intensity, but a unimodal prey-diversity response occurs nonetheless, increasing refuge space should result in diversity initially increasing and subsequently decreasing at most predation intensities. In this case, the position of the peak of the unimodal curve should not shift along the abscissa. (4) Combining the previous two patterns, prey refuges can transform a bimodal response into a unimodal response. Although no explicit tests if these models are currently available, data from three published experimental studies of sessile marine benthos are consistent with some of these predictions. © 1991.