26 - Cryptic density dependence: integrating supply-side ecology with population regulation

Summary

Ecologists have long been interested in the factors that drive spatial and temporal variability in population density and structure. Studies of coral reef fish have contributed strongly to this subject, and in particular, to our understanding of the role of settlement – the transition of pelagic larvae to a benthic stage – and on density-dependent processes affecting recently settled juveniles. Density dependence plays a critical role in the dynamics and regulation of populations. Controversy and debate over the role and importance of density dependence has been fuelled by observational studies that have been unable to detect density dependence. In at least some instances, the failure to detect density dependence may arise from an overly rigid conceptual framework that does not allow for the possibility that the strength of density dependence may vary. Allowances for such variability in underlying relationships can resolve much of the debate surrounding density dependence, and facilitate new avenues for thinking about the processes that shape the distribution and abundance of organisms. For example, if the strength of density dependence varies among sites, then in certain circumstances (e.g. if fish settle preferentially and in greater densities to higher-quality sites), a positive covariance between site quality and settler densities can mask the existence of density-dependent effects through a phenomenon called “cryptic density dependence”. We review the evidence for cryptic density dependence, describe several probable mechanisms that may contribute to this seemingly common pattern, and discuss the implications for coral reef fish.

Ecologists have long been interested in the factors that drive spatial and temporal variability in population density and structure. Studies of coral reef fishes have contributed significantly to this subject, and in particular, to our understanding of the role of settlement – the transition of pelagic larvae to a benthic stage – and the subsequent influence of post-settlement density dependent survival. Density dependence is widely recognized as a requirement for population regulation ([1124,1811,2526]; but see [2620]) and as a force that can homogenize spatial variation in settlement [425,837,1287,2275].