Predation, Competitive Exclusion, and Diversity in the Soft-Sediment Benthic Communities of Estuaries and Lagoons

Abstract

A review of experiments in which large, epibenthic predators are excluded from soft-sediment marine benthic communities in unvegetated portions of estuaries and lagoons and a comparison of unvegetated areas with nearby grassbeds, where predators on the infauna are less effective, demonstrate that such soft- sediment systems, when freed from predation, usually exhibit 1) an increase in total density, 2) an increase in species richness, and 3) no tendency toward competitive exclusion by some dominant species. The currently accepted model of community organization, developed from experimental work in marine rocky intertidal communities, would predict that significant simplification of the community should occur as a consequence of intense competition in such a system where density had increased substantially following the removal of predators.

Four general types of explanation are developed to account for this anomalous behavior of the soft-sediment benthic communities. Firsts the experimental exclusions of predators may have been carried out for an insufficient length of time. Second, interference competition which produces rapid mortality on hard substrates, may be much less common in soft substrates because crushing of individuals is made difficult by the lack of adhesive contact in soft substrates and because the three- dimensionality of soft sediments permits some segregation along another resource axis. Interference in the form of overgrowth is also less common in soft sediments because the competitors best suited to utilize this strategy¿ colonial forms, are excluded by the mobility of the sediments and because the infaunal mode of existence prevents colonization onto “secondary space,” the surface of other organisms. Third, intense negative interactions between densely established adults and potentially colonizing larvae may be sufficient in the absence of predation to maintain the community density below carrying capacity and thereby prevent competitive exclusion. Fourth, if interference competition is rare among soft-sediment benthic organisms exploitation may be the dominant mode of competition in infaunal systems. Sluggish, cold-blooded marine invertebrates are tolerant of the stress of low food concentration because they have low basal metabolic rates and an ability to restrict growth. This low energy requirement may make competitive exclusion through exploitation competition an ineffective or extremely slow process.

Thus, although the process of interference competition as mediated by predation and physical disturbance evidently does not govern the structure of soft-sediment marine benthic communities in estuaries and lagoons, intense biological interactions, particularly selective predation and disturbance by epibenthic predators and predation by benthic organisms on colonizing larvae, retain an important organizing role. Interspecific competition also plays a significant role, but only through the preemption of resources by established individuals: competitive exclusion is apparently not achieved by invaders’ causing the mortality of established adults. However, additional observations on the natural history of infaunal organisms are needed to confirm the significance of adult-larval interactions and the ineffectiveness of adult-adult competitive interactions.