Coupling the population dynamics and ecosystem function of grazing fishes

Societies are ethically and legally charged with the sustainable management of biodiversity. Most conservation strategies either focus on a particularly important species and assume that the conservation measures also protect other components of the ecosystem or attempt to protect many species by setting aside some of their habitat in reserves. The latter approach assumes that many species can complete their life cycle in the area preserved. An alternative paradigm, albeit one that has been difficult to realize, is that managers focus on the key processes driving the ecosystem. If the ecosystem processes remain intact, then many components of biodiversity should be sustainable. To date, there have been few demonstrations of this approach even though it is explicitly embodied by an 'Ecosystem-based approach to management' to which most governments are now legally obliged to undertake. A process-based approach to conservation will only work if two conditions are met. Firstly, that biodiversity is profoundly influenced by a limited number of processes and secondly that these processes are subject to management intervention. Coral reefs of the Western Atlantic provide an exceptionally compelling case for this approach. The ability of corals to recover from disturbance is highly dependent on the abundance of their seaweed competitor. In turn, the availability of seaweed is determined by grazing parrotfish. Inadequate levels of grazing allow seaweed to bloom and prevents corals from building the complex reef habitat on which much biodiversity depends. This project builds on the success of an earlier NERC grant and enables us to manage a key ecosystem process (parrotfish grazing) explicitly. We recently modelled the importance of grazing in coral reef dynamics. The model allowed corals to compete with seaweeds and the ecosystem was subjected to severe external impacts including hurricanes. Parrotfish grazing was found to exert an overwhelming positive impact on the dynamics of corals. However, whilst we now appreciate the importance of maintaining grazing, we lack the science to achieve it. This project fills this gap by modelling the population dynamics of parrotfish and the impact of exploitation on their communities. The models have two uses. Firstly, they will allow us to test novel ecological hypotheses about exploited communities. Secondly, we will couple the model of parrotfish population dynamics with our existing models of parrotfish grazing and the impact of grazing on coral reef dynamics. Thus, our coupled models allow us to develop the theory of ecosystem-based management because we can answer the question, 'How should parrotfish communities be managed sustainably so that extractive activities do not reduce grazing beyond the level required by the ecosystem?'. Outcomes of this project will have wide interest because we will significantly develop the science behind 'ecosystem-based management' and, by coupling complex ecological models, provide fresh insight into the analysis of complex systems (which lies at the intersection of biology and mathematics).