Biodiversity, population regulation, and the stability of coral-reef fish communities

Abstract

Unprecedented population declines and extinctions because of human activities, combined with a growing recognition that such losses affect the stability of ecosystems, underscore the need to better understand how populations persist naturally. We provide field experimental evidence that high biodiversity-in particular, the combined effects of predators and competitors-acts in a way that regulates the size of local fish populations within their coral-reef community. These results indicate that complex interactions among multiple species are necessary for the stability of a highly diverse community, and so forewarn that overexploiting such species may have cascading negative consequences for the entire system.

Fig 1.

Relationship between cohort per capita mortality and initial recruit density on patch reefs in the absence of both predators and competitors (older territorial damselfishes) (A), absence of predators and presence of competitors (B), presence of predators and absence of competitors (C), and presence of both predators and competitors (D). These relationships indicate low and density-independent mortality of recruits in the absence of both predators and competitors (A, r² < 0.001, P = 0.78), higher and density-independent mortality in the presence of competitors only (B, r² < 0.001, P = 0.42), still greater density-independent mortality in the presence of predators only (C, r² = 0.22, P = 0.24), and strong density-dependent mortality only in the combined presence of both predators and competitors, which is the unmanipulated natural state of the system (D, r² = 0.75, P = 0.036). Linear regression analyses are based on arcsin(square root) transformed per capita mortality rates.

Fig 2.

Relationship between cohort per capita mortality and initial recruit density on plots of continuous reef in the presence of predators and absence of competitors (older territorial damselfishes) (A) and presence of both predators and competitors (B). These relationships indicate qualitatively similar patterns as observed on patch reefs: density-independent mortality of recruits in the presence of predators and absence of competitors (A, r² < 0.001, P = 0.95) and density-dependent mortality in the combined presence of both predators and competitors (B, r² = 0.25, P = 0.08). Note the similarity between Figs. 1D and 2B. Linear regression analyses are based on arcsin(square root) transformed per capita mortality rates.