Persisters and Opportunists in an Assemblage of Coral-Reef Starfish

Coral reefs seem to defy many of the paradigms which characterise less complex biological communities. While there is general agreement that the biota of coral reefs exhibit high species diversity, some authors have characterised coral reef assemblages by selecting species with high population densities (Sale, 1974; 1976; 1977; 1984; Sale and Dybdahl, 1975; Connell, 1978). Other authors have included rarer species (Kohn, 1959; 1968; Den Boer, 1971; Grassle, 1973) and Endean and Cameron (1990 a) have emphasised the importance of the role of these rarer species and stated that rarity is virtually ignored in most ecological models of the coral reef ecosystem. They suggest that our understanding of coral-reef ecology is influenced strongly by the constraints of many of the analytical tools being used in reef studies. As a result they believe that most analyses have dealt primarily with species that are sufficiently numerous to provide statistically satisfactory numbers of records and that most studies have excluded rare species which, in fact comprise the majority of coral-reef species.

The complexity of coral reef ecosystems is not surprising given the great length of time that these ecosystems have been in existence. While the shallow water distribution of coral reefs has varied with the alternation of glacial and interglacial periods (Hays, Imbrie and Shackleton, 1976), in their broad biological form, coral reefs have existed since the Precambrian and reefs similar to present reefs have existed for around 50 million years (Newell, 1972). While stating that there is no general rule for coral-reef organisms, Endean and Cameron (1990 a) have suggested that the attribute of persistence possessed by most of the rarer species characterises the majority of coral-reef species and is responsible for both structuring and perpetuating this ecosystem. They regard the coral reef ecosystem as being an ordered and predictable system. However, other authors (Sale, 1977; 1991; Connell, 1978) have different views.

Sale (1991) regards reef fish communities as open non-equilibrial systems with living space determined in a random manner. Connell (1978) regards intermediate levels of disturbance as essential to the maintenance of diversity in this and other highly diverse and complex ecosystems. There has been much discussion of the meaning of stability (MacArthur, 1955; Dunbar, 1960; Leigh, 1965; May, 1972; Jacobs, 1974; Margalef, 1974; Goodman, 1975; Peters, 1976; Pimm, 1984).

Endean and Cameron (1990 a) have put forward the hypothesis that complex, high diversity assemblages of coral-reef animals are characterised by a preponderance of rare but long-lived species that they have termed persisters. These persistent species exhibit low recruitment, low adult mortality and relative constancy of adult population numbers and population structure. They occur in association with opportunist species that have high recruitment, a high adult mortality and varying adult population numbers and population structure. While individuals belonging to opportunist species are more abundantly represented than those belonging to persistent species, Endean and Cameron believe that the majority of species in the coral reef ecosystem are persistent species. This hypothesis has not been tested in the field.

As no general consensus relating to the organisation of coral reefs has been reached in the literature, the persister / opportunist distinction is examined in this thesis, rather than a deep analysis of the opposing views relating to stability. Events that are stochastic and unpredictable at one spatial or temporal scale may be predictable at another. In addition, the stability or otherwise of any system may be determined, amongst other things, by the particular set of species that is chosen to characterise the system.