Habitats on the grid: The spatial dimension does matter for red-listing

Besides species Red Lists, recently, a variety of frameworks have been proposed for assessing higherlevels of biological organisation, i.e. ecosystems, habitats, plant communities. Most of these protocolsrefer to ‘plant species assemblages’ or ‘vegetation types’ as proxies for ecosystems or habitats. Indeed, thehabitat concept based on plant communities has acquired a central role as a key approach for biodiversityconservation above the species level. Plant communities, like every complex biological system, hold scale-dependent ‘emergent’ properties which vary as a function of the scale of observation. With reference tored-listing, these scale-dependent properties have far-reaching consequences for both identification andclassification, as well as for representation and evaluation, and become particularly challenging whendealing with criteria regarding decline in distribution or restricted distribution. The recent discussionon the red-listing protocols has evidenced several aspects that claim special efforts for a suitable use.In the present paper, starting with the analysis of some recently proposed protocols for the red-listingof habitats and ecosystems, we discuss and test some ‘emergent’ properties of species assemblages,providing cues for reflection. Based on a variety of theoretical models and scientific outcomes in literaturefrom the last decades, we theorise that plant communities own some intrinsic, ecologically based andscale-dependent spatial features, which give rise to different types of pattern of spatial occupancy. Wediscuss a model where, in natural conditions, the possible patterns of spatial occupancy are referred to3 basic types: areal, linear and point. This approach is here proposed as a tool to discriminate amongdifferent broad categories of plant community-based habitat types and optimise their assessment inthe red-listing process. Starting from a homogeneous data set, the proposed case studies prove that thechoice of the scale affects the comprehension of the habitats’ occurrence, with a substantial relapse onthe estimates of their distribution size. In particular, habitats with linear and point distribution, oftennaturally small in size and dispersed, are more susceptible to biased evaluation of their actual distributionand consequently of their threat status. The intrinsic spatial attributes of plant communities should notbe neglected in a red-listing process and claim for a ‘habitat-tailored’ approach. The use of different grid-cell sizes and thresholds for the three main patterns of spatial occupancy here proposed, might certainlyavoid inaccurate statements.