Comparing modelling approaches at two levels of biological organisation - Climate change impacts on selected Natura 2000 habitats

Question: Habitats are characterized by their plant species composition. Therefore, climate change impacts on habitats can be assessed by two complementary statistical approaches: either directly by modelling the climate envelope of the habitat, or indirectly by modelling the habitat in terms of its plant species. How do these approaches differ in their projected habitat distribution? What are the consequences for nature conservation? Location: Europe. Methods: Potential climate change impacts on the distribution of European protected Natura 2000 sites were modelled for five natural and semi-natural grassland habitat types, defined by the EU Habitats Directive, using data from the Atlas Florae Europaeae and reports on Natura 2000 sites. We used random forests (RF) and logistic regression (GLM) to model current and potential future distributions for 2050. Results: All habitats are projected to lose between 22% and 93% of their range in the 'no dispersal' scenario. In the 'unrestricted dispersal' scenario, almost all habitats gain suitable climate space, between 5% and 100% of their current range. In the direct habitat approach, both model algorithms have high discriminatory performance on test data and are well calibrated. In the indirect species approach, only GLM shows high model performance; RF models are overfitted. Projections of occurrence probabilities differ more strongly between model approaches ('direct' versus 'indirect') than between model algorithms (GLM versus RF). Conclusions: Habitats are complex entities. Because of their dynamic nature, particularly in the face of climate change, we suggest modelling the future distribution of habitat types not exclusively based on their current definitions and mapped distributions, but also based on their constituent elements, and in particular their characteristic plant species.