Spatial-accumulation pattern and extinction rates of Mediterranean flora as related to species confinement to habitats in preserves and larger areas

Species-area relationships (SARs) are still the main basis for all projections of extinction rates of species following habitat loss. To investigate spatial-accumulation patterns of floristic species owing to the degree of species confinement to habitats, we considered 38 parks and reserves in Italy on a wide range of scales, covering about 70% of native flora and more than 21% of the land under legal protection. We used robust methods for multivariate outlier detection to derive the best regression model by checking accordance or lack of accordance with the SAR models even for those observations with no recorded species, which can occur when endemic species are rare. This method enabled us to demonstrate the arbitrariness of omitting observations without endemic species, because only a few such observations proved to be true outliers. As the degree of species confinement to habitat increased, the explained variance in species number and the slope value (z) increased significantly. The stronger the confinement of a species to favorable habitat, the more it was likely to be affected by habitat loss. For species within Italy, those found only within Italy had steeper species-area slopes than those found more widely. When the analysis was extended to 17 larger areas of the Mediterranean region, a stability for the endemic spatial-accumulation rate appeared across 15 to 47,000,000 ha. As area increases, the number of species increases, but the number of endemic species increases for more; therefore, a small preserve is expected to contain a large number of species even if it has few or no endemic species. The relatively few reserves we considered captured the country's general species richness far better than they did that of endemic species. We discuss the conservation implications of such results within the context of the national conservation program of the Map of Italian Nature, which is intended to fill the gaps in the existing reserve networks for preservation of species diversity.