PARENT SESSION
Friday, August 11, 8:00-10:30 am
Poster Session 26 - Latebreaking and newsworthy posters
Exhibit Hall, Ballroom Level, Cook Convention Center

Group size benefits and the spatial scaling of density dependence in a coral reef fish.

White, Will^*,1, Warner, Robert¹, ¹ University of California, Santa Barbara, Santa Barbara, CA

ABSTRACT- Like many marine organisms, coral reef fishes disperse as planktonic larvae before settling into the sedentary, benthic adult stage. Because larval dispersal decouples local production from subsequent local population size, current descriptions of reef fish population dynamics focus on the role of density-dependent mortality, often occuring very soon after settlement, in regulating local population sizes. All else being equal, ubiquitous density-dependent mortality should select against the formation of large aggregations of reef fish. Nonetheless, the recruits of many species aggregate. This paradox may result from a research bias towards habitat specialist species that compete for access to predator refuges, so we examined post-settlement mortality in a habitat generalist, the bluehead wrasse (Thalassoma bifasciatum). Blueheads recruit to many types of reef habitat and occur both solitarily and in groups. We monitored the settlement, mortality, and foraging behavior of solitary and aggregated blueheads on 5 reefs in St. Croix, U.S.V.I. Mortality was density dependent, but only at the scale of entire reefs. At the scale of individual groups and solitary fish, mortality declined with group size. Consequently, local population dynamics will be determined by the opposing forces of stabilizing, large-scale, direct density dependence and destabilizing, small-scale, inverse density dependence. Such scale-dependent switches in the form of density dependence may be common among aggregating organisms, so we used scale-transition theory to construct deterministic models of this type of system. Numerical solutions indicate that a tendency to aggregate may strongly affect population size, but models were dynamically unstable for only a small region of parameter space. These results sharpen our understanding of the effects of group formation in reef fishes and demonstrate that small-scale aggregative behavior is not incompatible with large-scale population stability.

Key words: density dependent mortality, marine population ecology, coral reef fish