A Test for Shelter Limitation of Reef Fish Populations at Large Spatial Scales: An Integrated Empirical and Theoretical Approach

University Of Rhode Island, Kingston RI

Investigators

Abstract

The project is aimed at understanding the causes of population dynamics in two species of reef fish at the meso-scale (over areas tens of km in extent). Most reef fishes, and other benthic marine species, have a complex life cycle that includes a site-attached adult phase and a larval stage during which individuals may disperse long distances across open ocean. Much debate has centered on whether benthic population dynamics are controlled by the influx of pelagic larvae (larval settlement) or by density-dependent interactions among juveniles and adults in the benthic habitat. Past controversy, based on the notion that density-dependence is not compatible with a strong larval-input signal, has been resolved following recognition that both pre- and post-settlement processes influence the dynamics of most populations. One broadly applicable hypothesis (the recruit-adult hypothesis) predicts that oceanographic processes controlling the influx of larvae create predictable differences among populations at the meso-scale: sites with high settlement are chronically resource limited, whereas sites with low settlement rarely reach their carrying capacity and adult densities track larval influx. The carrying capacity of the benthic habitat, and the post-settlement interactions within it, are assumed to be consistent among meso-scale sites. All experimental evidence on the nature of post-settlement interactions is, however, based on study at small spatial scales (a few m's in extent). Studies in other environments indicate that population dynamics can potentially vary with spatial scale, however, and a major challenge now is to test whether small-scale results in marine systems will extrapolate to larger spatial domains. Past small-scale experiments by these investigators on two reef fishes indicate that competition for shelter sites used as refuges from predation is the cause of small-scale density dependence in both species. The strength of density dependence, however, depends on the local availability of shelter, and is eliminated in habitat patches that provide abundant shelter. They hypothesize that, in addition to oceanographic processes causing differential settlement, the carrying capacity of the benthic habitat will also vary at the meso-scale and cause differences among populations in the strength of density-dependent mortality. In this system, the investigators suggest that carrying capacity is a function of shelter abundance. They will test experimentally for shelter limitation at the meso-scale. To complement the field work, a mathematical scaling model will be constructed to explicitly link local- and meso-scale populations. This model will include a functional description of shelter limitation at local scales and will be used to test specific hypotheses for the congruence (or lack of it) in population dynamics at local and meso-scales. Testing the expanded version of the recruit-adult hypothesis will provide a rare experimental test of the causal interactions shaping population dynamics at large spatial scales. Meso-scale populations are little studied, but a deeper understanding of their dynamics is of practical importance because it at this scale that marine reserves are designated and at which most reef fisheries are managed. Support for the hypothesis that meso-scale carrying capacity depends on shelter provided by the reef would also have important implications for the effects on fishes of the ongoing habitat degradation on coral reefs.