Reef fishes: Density dependence and equilibrium in populations?

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Small colorful coral reef fishes have been excellent subjects for ecological field experiments (Sale, 2002). They have provided substantial insight about the strength of densitydependent interactions and their underlying biological causes. In this chapter we first summarize what we know about the biological mechanisms of density-dependent regulation in coral reef fishes, highlighting one pervasive mechanism – competition for structural refuges used to avoid predators. We then summarize the evidence for ongoing coral declines and the progressive loss of architectural complexity on reefs.We argue that as reefs become architecturally simpler they provide fewer refuges from predation, and so the carrying capacity for many fish populations is declining. As a result of ongoing competition for gradually diminishing supply of refuges, we hypothesize that some species of reef fishes will continue to experience density-dependent mortality even as their populations decline globally. This hypothesis contradicts conventional views on the regulation of marine populations, which hold that density-dependent interactions should diminish in importance as populations decline (Figures 1.1 and 1.2). A long-standing controversy in ecology has revolved around the relative effects of density-dependent and density-independent processes on population dynamics (Murdoch, 1994; Cappuccino & Price, 1995). Although it is necessary for long-term persistence, recognizing that density dependence need not have a strong effect at all times on the dynamics of a population was a key step in resolving a major historical controversy over density dependence and density independence as mutually exclusive alternatives (Andrewartha & Birch, 1954; Nicholson, 1957). In fact, both sorts of processes affect most populations (Turchin et al., 1995). Ideas about populations of reef fishes and other marine species have followed a similar path, following early controversy over which processes control dynamics (Doherty, 1991; Jones, 1991).

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The Balance of Nature and Human Impact