Several models of species coexistence have been developed through empirical studies of coral reef ecosystems, where species diversity at very small spatial scales can be considerably high. How do small (<100 m ) patch reefs support over 300 species of fishes and many more invertebrates and algae? First, many reef species exhibit 'functional versatility' whereby their use of resources (food, shelter) is flexible enough to allow many apparently overlapping species to coexist. The range of microhabitats present on coral reefs is large, also facilitating coexistence. However, the patchy and variable supply of young onto the reef (with parallels in other marine organisms and terrestrial insects) can also promote coexistence. The 'lottery hypothesis' suggests that, should a reef resident be removed (e.g., through mortality) the replacement would be drawn at random from the larval species pool adjacent to the reef at that time. Therefore, even if certain species were competitively displaced from a habitat patch, it could be replenished through the larval pool. Hypothetical successional climaxes of reefs, forests, and many other ecosystems would never be reached, so resulting coexistence would not be stable. One consequence would be enhancement of biodiversity in these situations, through invasion of larvae, seeds, and other propagules or migrants. Such invasion could be facilitated by mortalities or displacement of residents through a disturbance (see the next section).
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