Ecological role of herbivory on coral reefs of the Saudi Arabian Gulf coast
This study examined the ecological role of herbivory on coral reefs of the Saudi Arabian Gulf coast. Herbivory is a ubiquitous process and important in regulating benthic marine communities. Three reef sites were studied over a 12-month period; one on an inshore fringing reef (1.5 m depth), and two on the fringing reef surrounding an offshore island (3 m and 11 m depth). The inshore site experienced greatest extremes in temperature, salinity and sedimentation. The ecological role of herbivory was determined from algal settlement plates and their selective exclusion from herbivores. The inshore site was naturally dominated by filamentous algae, while both offshore sites supported a higher proportion of crustose forms. Location (i. e. distance from shore) appeared to be more important than seasonality in determining the structure and composition of the epilithic algal community. At the inshore site herbivorous fish (dominated by Siganus spp. ) imposed a uniform, wide-ranging grazing pressure of intermediate intensity. Herbivorous echinoids (Echinometra mathaei) imposed an intensive but localised grazing pressure. At the shallow offshore site, only herbivorous fish (dominated by Scarus sp. ) appeared responsible for grazing impacts, which were also intensive. At the deep offshore site both herbivorous fish (dominated by Pomacentrus spp. ) and echinoids (Diademo setosum) were responsible for limiting algal growth, although other factors (e. g. light penetration) may also inhibit algal productivity at this site. An experiment assessing the effects of extreme perturbations (i. e. removal of the algal community), showed that seasonal life-history strongly affected generic succession and rate of re-colonisation. Perturbation effects were temporary and did not precipitate permanent alternative stable communities. An important secondary effect of herbivory is bioerosion. The mean erosion rate by E. mathaei was comparable to rates recorded elsewhere. Behavioural studies revealed that burrow defence and fidelity were positively correlated with burrow complexity, and that the frequency of agonistic behaviour was low. Foraging range was negatively correlated with burrow complexity. In addition, the risk of mortality by finfish predators at the inshore site was estimated to be very low. The study has shown that Saudi Arabian Gulf reef communities may be particularly vulnerable to the depletion of herbivores, for example by overfishing. Management plans should therefore safeguard the herbivorous community, in order to maintain natural bioerosion rates and other reef processes.