Building relevant ecological indicators with basic data: Species and community specialization indices derived from atlas data

Species and community specialization have become popular indicators to track the spatial and temporal changes of species and community dynamics during current global changes. However, measuring specialization requires detailed and quantitative descriptions of habitat requirements or resource use, which are difficult to obtain for many species. Here, we propose and test a new method to quantify and map the relative composition of specialist and generalist species in local plots compatible with very basic ecological data, typically used for atlases. We used co-occurrence patterns of 1090 plant species recorded in the French Mediterranean region of Languedoc-Roussillon in a systematic grid of 1225 5 × 5 km atlas cells to estimate species specialization. We then calculated the averaged specialization of each cell and tested several expected relationships of these indices. In particular, we tested the relationship between species richness and average specialization and the relationship between community specialization and landscape disturbance induced by land use. As expected from studies conducted on fine-scale data, we found that specialist species were those with more restricted distributions and occurring in richer species assemblages. We also found that community specialization was maximized at an intermediate level of landscape disturbance. These results suggest that aggregating specialization at large spatial scales provides useful species and community level indicators. Estimating specialization level with co-occurrence data is a good complementary approach to traditional estimations of diversity indices for conservation and landscape planning.