Habitat red-listing: the pattern of spatial occupancy does matter.

Starting with the analysis of recently proposed protocols for the red-listing of habitats and ecosystems, we discuss and test some emergent properties of species assemblages, providing cues for reflection. Each habitat and vegetation type owns intrinsic, ecologically based, spatial features, which affect its spatial distribution in natural conditions. We hypothesize that the cogent relationship between vegetation and environmental heterogeneity accounts for an intrinsic property of any plant community, i.e. its “pattern of spatial occupancy” (pso), which might have a role in red-listing when applying quantitative criteria – above all those based on the “Area of Occurrence” (AOO). We discuss a model where the possible types of pso are referred to three basic types: areal, linear and point. Some case studies are then discussed using, as assessment units, the Annex I habitats as defined by the European Habitat Directive 92/43/EEC. Our approach is proposed as a tool to discriminate among different broad categories of habitat types and optimize their assessment in red-listing processes.