Replenishment of populations

Resilience is the ability of populations to replenish losses incurred as part of norma! population fluctuations or as a result of exploitation or other human impact. Given the key functional roles of many coral reef' populations in bioconstruction and protein production, resilience benefits not only the individual populations concerned, but also the maintenance of key ecosystem outputs of framework and protein.

However, resilience in a cora! reef population is as much a function of the location of the cora! reef in relation to other reefs as a property of the population itself (Johnson and Preece 1993). Individuals usually exist in partially isolated sub-populations linked by pelagic dispersal of larvae (Hughes et al. 1992; Knowlton and Jackson 1994). Both year-to-year population replenishment, and genetic diversity within and among the local sub-populations, depend on the strength of these links. Within large and dense archipelagos arranged along major current systems (e.g. the Great Barrier Reef), most reefs are assailed regularly by dense aggregations of the larvae of fish, corals and other invertebrates released from upstream reefs (Oliver and Willis 1987; Doherty and Williams 1988). In this setting, high degrees of gene flow have been demonstrated in a number of invertebrate taxa (Benzie 1993, 1994; Benzie el at. 1995>. There is, however, enormous interannual variation in larvae supply and recruitment success, over scales from patch reefs to regions (Doherty and Williams 1988), and unexpected restrictions to gene flow can occur (Benzie 1993, 1994; Benzie el al. 1995).

At reefs separated from their neighbors by great distances, unfavorable currents, or both (e.g. French Polynesia), larvae from other reefs are more likely to arrive as a dribble than as a torrent, and at intervals of many years, decades or even longer. For their year-to-year replenishment, populations on isolated reefs must depend much less on larvae from other reefs and much more on retention of their own reproductive output (Planes 1993). This includes locally settling larvae (e.g. Stoddart 1983), and asexually produced buds (e.g. Sammarco 1981) and fragments (e.g. Done and Potts 1992). Both larval-retention rates and conditions for survival are a function of the reefs shape and hydrodynamie setting. The residence times of water and the rates of delivery for water-borne materials (larvae, nutrients, suspended sediments) depend on the presence or absence of features such as a lagoon, its depth, the number and width of passages, the continuity and height of the reef rim, tidal characteristics, and so on (Black 1993, Wolanski 1994). Such considerations affect both the probability that larvae will be carried to a particular part of a reef, and the likelihood that it will survive and grow once there.

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