Résumé
Global changes affect the entire planet at all scales, terrestrial and marine. The impact of anthropogenic pressures on marine ecosystems has been relatively little studied because of the lack of data and the difficulty to acquire it. Fine resolution cartographic data (1:10 000) of marine habitats on French Mediterranean coasts have recently become available (Andromede Oceanologie, 2014). This allows using landscape ecology approaches to study the spatial configurations of these habitats, their evolution through time and confront that to anthropogenic and environmental pressures. Here we take advantage of this new data to study Posidonia oceania meadows, a marine habitat of great ecological and economic importance. Posidonia oceanica is an endemic species to the Mediterranean Sea. Posidonia oceanica is also an indicator of the ecological quality of coastal waters since changes in its spatial distribution reflect changes in environmental conditions. The spatial cover of Posidonia oceanica meadows has declined sharply in the last 50 years, in particular because of increased human activities along the coast. In this study, we tested the relative influence (and possible interactions) of anthropogenic coastal pressures and environmental variables (Previmer data) on the decline of Posidonia oceanica meadows in the French Mediterranean sea. Different composition and configuration landscape descriptors were calculated for different grid cells (400m x 400m; 200m x 200m, 100m x 100m), between 0 and 40 m depth over 1800 km of French Mediterranean coastline (Mainland + Corsica). An approach using Random Forest was applied to explain these indices with the anthropogenic and environmental factors and to identify the factors that contribute the most to the decline of Posidonia oceanica meadows. With a model that explains 87% of overall variance, our results show a very important effect of environmental factors on the decline and the patchs cohesion of Posidonia oceanica meadows. Anthropogenic pressures also have a destructive effect on the posidonia, because if the intensity of the pressures is high, the decline is strong and the cohesion between the patches is weak.