Ecosystem-based modelling of a marine protected area

Résumé 0

## WHAT IS IT?This model was developed as a part of a project addressing trade-offs between ecosystem services in a french marine protected area with science-policy prospective experiments. It is made to explore some effects of climate change, fishing and management on the biomass of each (group of) species, and specifically on the distribution of biomass between each group of an ecosystem. Then simulation results can be used to build ecosystem services indicators. It consists in a simplified ecosystem-based representation of the social-ecological system of our case study, mapping 4 major ecosystems representative for the area: "sand&mud" (31 species), "rock" (18 species), "posidonia" (17 species), "coralligenous" (15 species). Here species are represented in aggregate form (biomass density) and linked together with diet ratios.## HOW IT WORKSTo simulate ecosystem dynamics, we use the food-webs corresponding to the selected ecosystems as transmission chains for the following types of controlling factors: bottom-up control (climate, management), top-down control (fisheries, management). Climate change is represented through the possible variation of primary production (phytoplankton biomass). Fishing is represented through four fishing gears whose effort can evolve under management measures. Management is represented through fully protected areas development, facilities planning (especially artificial reefs) and ecological engineering (reintroduction of species). For each (group of) species, biomass variation results from the equal combination of two potential drivers on a yearly basis: the abundance of prey (bottom-up control, positive feedback) and the abundance of predators (top-down control, negative feedback). The food-web model is located at the cell level with previous year's outputs as input data for each new year. Other human and non-human agents are also represented at the cell level. At this stage we model temporal dynamics but we lack spatial dynamics such as adaptive behaviors of human and non-human agents resulting in transfer effects between sites or towards new sites. For now, interactions between agents are mostly made of spatial-temporal co-occurrence with restricted mobility. In this respect, the model simulates the variation of any group of species in terms of biomass density in case of a change in primary production, fishing effort, artificial reefs planning or reintroduction of species. In accordance with the project specifications, the size of each patch is related to the average size of an artificial reef village (0,25 km2), and simulations run by 2050 with an annual time step.