Seminar Series 2003

Cryptic Density Dependence: finding patterns and inferrrng process.

Craig Osenberg, University of Florida

The strength of density-dependence plays a critical role in population dynamics. Ecologists have used a variety of tools (including null hypothesis tests) to detect effects of density on the dynamics of reef fish. However, inferences drawn from null hypothesis tests do not capture variation in the strength of density-dependence. I illustrate this point with results from a meta-analysis of studies of density-dependent survival in reef fishes. Instead, we require a more quantitative (mathematical) approach based on clear description of the biological processes under investigation. Field studies also vary in the methodological approach they take. Approximately half of existing studies rely on natural spatial variation in density, whereas the remaining half employ experimental manipulations of density. Results from observational and experimental approaches can give vastly different results. I illustrate this with data from studies of a gobiid fish in St. Croix USVI (collaborative work with Jacqueline Wilson) and the six barred wrasse in Moorea, French Polynesia (collaborative work with Jeff Shima). Observational studies in both systems consistently underestimated effects of density. We propose that this effect results from factors that are confounded with settlement intensity, and can arise from spatial variation in larval habitat traits that affects estimation of the strength of density dependence. Indeed, cryptic density dependence leads to underestimation of the effects of density and overestimation of the effects of density-independent processes. These effects may help explain existing controversy in the literature of the relative importance of density-dependent and density-independent processes.