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 reefsites were studied over a 12-month period; one on an inshore fringing reef (1.5 m depth), and two onthe fringing reef surrounding an offshore island (3 m and 11 m depth). The inshore site experiencedgreatest extremes in temperature, salinity and sedimentation.The ecological role of herbivory was determined from algal settlement plates and their selectiveexclusion from herbivores. The inshore site was naturally dominated by filamentous algae, while bothoffshore 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 theepilithic algal community. At the inshore site herbivorous fish (dominated by Siganus spp. ) imposed auniform, wide-ranging grazing pressure of intermediate intensity. Herbivorous echinoids (Echinometramathaei) imposed an intensive but localised grazing pressure. At the shallow offshore site, onlyherbivorous fish (dominated by Scarus sp. ) appeared responsible for grazing impacts, which were alsointensive. At the deep offshore site both herbivorous fish (dominated by Pomacentrus spp. ) andechinoids (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 ofextreme perturbations (i. e. removal of the algal community), showed that seasonal life-history stronglyaffected generic succession and rate of re-colonisation. Perturbation effects were temporary and did notprecipitate permanent alternative stable communities.An important secondary effect of herbivory is bioerosion. The mean erosion rate by E. mathaei wascomparable to rates recorded elsewhere. Behavioural studies revealed that burrow defence and fidelitywere positively correlated with burrow complexity, and that the frequency of agonistic behaviour waslow. Foraging range was negatively correlated with burrow complexity. In addition, the risk ofmortality 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 thedepletion of herbivores, for example by overfishing. Management plans should therefore safeguard theherbivorous community, in order to maintain natural bioerosion rates and other reef processes.