LOCAL VARIABILITY IN POPULATION-STRUCTURE AND DENSITY OF THE PROTOGYNOUS REEF HERBIVORE SPARISOMA VIRIDE

We compare the (relative) abundance of life phases [juveniles (JU), in itial phase (IF) and terminal phase (TP) fish], social categories (ter ritorial and group adults), and fish following alternative mating styl es, in three local populations of the protogynous reef herbivore, Spar isoma viride, on the fringing reef of Bonaire (Netherlands Antilles). In order to determine the adaptive significance of variations in socia l organization, they are related to the density of conspecifics and ot her herbivores and to the availability of food, shelter and mating sit es. The most striking difference is the high abundance of JU and group fish at one location (Playa Frans) and the total absence of group fis h at another (Red Slave). These differences are coherent with a gradie nt in population density, total herbivore density, scarid grazing pres sure, and reproductive output, all of which are highest at Playa Frans and lowest at Red Slave. Exposure to waves and currents shows an inve rse trend. The differences in the relative abundance of territorial fi sh can be explained by the concept of economic defendability, which is reduced at higher population density. In a life history context, smal l TP group males represent 'bachelors' that sacrifice current reproduc tion for better future prospects. As predicted by life history theory, early sex change is promoted at sites where the future rewards are hi gher (higher spawning rates of large TP males) and where the costs inc urred during the bachelor phase are reduced (more spawning opportuniti es for group TP males). At Red Slave an alternative male mating style ('streaking') appears to be promoted by the lack of a refuge for group TP males and by a dense gorgonian canopy, allowing IP males to reside inside territories. We conclude that most observed differences in pop ulation structure can be considered adaptive in an ecological and in a life history context. Population density is a major factor in both co ntexts. Analysis of the variability in adult density in relation to JU density and the availability of food and shelter indicates that the S . viride populations at Bonaire are not totally controlled by stochast ic processes. Considering the small spatial scale and the high dispers al of the planktonic embryos and larvae, the observed variability in b ehavioural and life history traits of S. viride points to a high degre e of phenotypic plasticity.