From Small to Large: What Small-Scale Experiments on Tropical Gobies Tell Us about Large-Scale Population Dynamics

Although field experiments allow rigorous tests of ecological hypotheses, they are usually limited to small spatial scales. We often want to know if their findings extrapolate to larger scales, especially when seeking to apply their results to conservation and management. Experiments on small coral reef fishes (bridled gobies) occupying small habitat patches reveal that locally density-dependent mortality is inflicted by predators. As prey become crowded, they suffer a progressively increasing shortage of structural refuges. Consequently, goby mortality at small-scales is well described by a model in which vulnerability is proportional to the ratio of gobies to refuges. A manipulation of refuge abundance on entire reefs, which are the size of small marine reserves and approach the scale at which some reef fisheries operate, suggests that a similar interaction occurs at this much larger spatial scale. This result is in accord with a scaling model which indicates that the effects of refuge shortage on entire reefs can be extrapolated from the model describing goby vulnerability as proportional to the ratio of fish to refuges. In simple terms, the biological interactions causing density dependence scale up. Careful extrapolation from small-scale experiments identifying species-interactions may thus be possible, and so should improve our ability to predict the outcomes of alternate management strategies for coral reef fishes.