Forecasting the combined effects of disparate disturbances on the persistence of long-lived gorgonians: a case study of Paramuricea clavata

The combined effects of disparate impacts on population health are a continuing problem in risk assessment and management for threatened species. Populations of red gorgonian Paramuricea clavata in the NW Mediterranean Sea are threatened both by chronic and localized increases in mortality due to high diving activity and by widespread but episodic mass mortality events related to climatic anomalies. Using demographic data obtained from 3 populations (2 under contrasting levels of diving and one affected by a 1999 mass mortality event), we developed size-structured matrix models to forecast the long-term consequences of both disturbances and their combined effects. When we considered only the effects of diver damage, our results showed that population stability could be achieved with an increase in the annual survival of adult colonies of between 3 and 7%, demonstrating the need for diving reduction in the study locations, where there are estimated to be between 30000 and 70000 dives yr(-1). Modeling the effects of mass mortality events alone showed a low annual population growth rate (0.886) and near certain extinction risk over even short time scales. Considering these 2 types of impacts together, we found that the effects of mass mortality events aggravate the decline in gorgonian populations subjected to high diving impact, reaching the extinction threshold after 36 to 55 yr under the actual frequency of mass mortality events. Simulated reduction of diving effects dramatically increased the time to quasi-extinction for populations subjected to realistic frequencies of mass mortality events. Our simulations reveal the need of management actions to ensure the gorgonian viability in the face of climate change, and suggest that management of more controllable impacts, such as diving, can help buffer populations against the less controllable effects of climate change.