Territorial damselfishes as determinants of the structure of benthic communities on coral reefs

Abstract

This review evaluates the generalisation that territorial, herbivorous damselfishes (pomacentridae) have a major influence on the structure of algal, coral, other invertebrate and fish assemblages on coral reefs. Herbivorous damselfishes are a diverse, widespread and abundant component of reef fish assemblages and their territories take up a significant proportion of the shallow reef substratum. There are several mechanisms by which they potentially affect community structure within territories, including both food consumption and potential "farming" activities, such as "weeding" of undesirable organisms, "killing" coral to grow algae, providing nutrients for algal "crops" and the aggressive "defence" of vital resources. A synthesis of the literature that documents assemblages both inside and outside territories revealed a number of common patterns. Erect filamentous algae often dominate territories, whereas low-lying crustose coralline and prostrate algae characterise adjacent areas. Furthermore, territories consistently support a greater biomass, productivity and species richness of algae than undefended areas. Experimental studies suggest that damselfishes modify regimes of disturbance and succession but the potentially different effects of feeding, farming and territorial exclusion suggest a more complex interaction of processes. There are also substantial differences between defended and undefended areas in coral species composition and densities of small mobile organisms such as cryptofauna and juvenile fishes, whereas larger herbivorous fishes are excluded from territories. However, the larger-scale effects of these interactions on the ecology of “inc1uded" or "excluded" species has yet to be examined. Many of the above generalisations may be premature as the literature is clearly biased towards a few larger, more aggressive species that maintain conspicuous algal mats. Our review draws attention to the numerically more abundant and less aggressive herbivorous species whose effects appear to be less dramatic. Furthermore, the spatial and temporal variability in the structure of damselfish communities. is largely unknown, further restricting our ability to make valid generalisations. While the effects of the territoriality have been tested by damselfish removals, more sophisticated experimental work is needed to assess the relative contributions of selective feeding reduced herbivory, weeding and other farming activities. these, mechanisms will be clearer if we have better understanding of the function territoriality, the actual benefits of algal turfs to the damselfishes and the separate tasks involved with establishing and maintaining territories.