Spatial priorities for freshwater biodiversity conservation in light of catchment protection and connectivity in Europe

catchments are mostly located in S and E Europe (W and S Balkans, S Spain, S France, N Alps; cf. Fig 1A vs. 1E). Scenario 3 also resulted in the lowest coverage of threatened species (89.3% or 507 species without connectivity, 79.2% or 450 species with connectivity) as it ignored occurrences of threatened species in well-protected catchments, demonstrating that the inclu- sion of well-protected catchments is essential to achieve species representation targets. The importance of catchments in S Europe corresponds well with biogeographical patterns and processes. Areas in southern Europe provided refuges during Pleistocene glaciation events, and the post-glacial recolonization of northern areas took place from these refuges [56], mainly from the middle and lower Danube basin in the case of freshwater fish [57, 58] and macroinvertebrates [e.g. 59]. Southern areas separated from northern ones by mountain ranges that funcion as migration barriers (e.g. Pyrene´es in Iberia, Alps in Italy, Dinarides in the western Balkans) evolved isolated faunas rich in endemics [57]. Finally, karst/limestone areas rich in dolinas, underground waterflows and springs could serve both as glacial refuges for some species (e.g. fish) and interglacial refuges for cold-tolerant others (e.g. spring snails), particularly for shell-bearing mollusks dependent on CaCO3, while the geomorphological complexity of karst/limestone areas also increases the chances of isolation and diversification [60]. Our results provide a basis for a more efficient allocation of conservation resources to the

protection of freshwater biodiversity and will thus be of interest to conservation scientists, water management authorities, policy-makers and the general public. For instance, because well-protected catchments made up 2% of the total planning area, Scenario 2 identified the next best 15% of the catchments necessary to meet species representation targets, whereas Sce- nario 3 identified 17% of catchments that are suitable candidates for increased protection other than the already well-protected catchments. It is also important to note that catchments in the western Balkans (Bosnia and Herzegovina, Croatia, Montenegro) had consistently high irreplaceability and further need of conservation interventions (Figs 1, 2) but are also a global hotspot of ongoing or future hydropower development [5]. This study provides novel insights relevant in conservation policy and practice in addition to a previous identification of critical catchments in Europe [17]. This study presents spatial conservation priorities for all catchments in Europe rather than only for critical catchments (that hold threatened species), thus, it exploits the full benefits of complementarity and irre- placeability in spatial prioritization. In addition, this study uses a broader range of species, includes range-restriction as a new criterion, presents summary statistics on conservation pri- ority of ecoregions and countries, and evaluates priorities relative to the current protection and the connectivity of catchments. Finally, this study presents a list of high-priority catch-

ments, based on both Scenario 1 and 3, in (S1 Table in S1 File) that may be of relevance in international conservation policy and practice, e.g. in the allocation of resources to freshwater biodiversity conservation, but they may also be of interest to conservation scientists and practi- tioners as well as water management authorities, managers and other local and regional stakeholders. Finally, our approach to measuring the correspondence between priority areas and existing catchment protection provides a novel contribution to methodology in conservation planning. In most studies, this correspondence is typically measured by the identification and quantifica- tion of overlapping areas between maps of conservation priority on one hand and maps of cur- rent protection on the other [e.g. 61, 62]. This approach typically uses a subset of the entire range of priority, e.g. the top 17% of the planning units, and examines its overlap with existing protected areas. In contrast, our approach uses information from all planning units and from the entire ranges of priority and protection, and it evaluates whether a certain catchment has more (or less) priority given its protection level relative to all other catchments. By quantifying PLOS ONE Spatial priorities for freshwater biodiversity in Europe PLOS ONE | https://doi.org/10.1371/journal.pone.0267801 May 17, 2022 11 / 15