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

priority of rivers and lakes in S Europe, large rivers in E Europe and lakes in N Europe, and of karst/limestone areas in the W Balkans and NW Greece, S France, W Bavaria and the E Alps (Fig 1). Freshwater ecoregions and countries with the highest average catchment irreplaceabil- ity were in S, E and central Europe, with the E Mediterranean and south-central ecoregions ranking slightly higher than ecoregions in the W Mediterranean. Hydrological connectivity had little influence on the spatial distribution of priorities, although irreplaceability values were more balanced throughout Europe and showed higher averages in ecoregions and countries when connectivity was included than when it was not (Fig 2). More importantly, however, the number of threatened species for which representa- tion targets were not met doubled or tripled with connectivity compared to without connectiv- ity in each of the three scenarios (Table 1). This result draws attention to a possible trade-off between the inclusion of connectivity and meeting species representation targets [53]. In prac- tice, this trade-off means that the inclusion of connectivity can compromise the protection of some of the threatened species when the maximum amount of land which can be protected is fixed [54]. Moreover, when the emphasis is on river connectivity in a landscape with different water bodies, lakes might appear as less important. Finally, we found that the inclusion of con- nectivity did not really affect the correspondence between irreplaceability and current protec-

tion, which may suggest that connected rivers per se were not a priority in the designation of protected areas in the past in Europe. Although the inclusion of hydrological connectivity of catchments into prioritization with systematic conservation planning for freshwater biodiver- sity is highly desirable [26], these potential issues should be considered in future systematic conservation planning efforts. The comparison of conservation priority and current catchment protection showed inade- quate protection of many catchments in S Europe and generally high protection of catchments in NW Europe relative to their conservation priority. The main difference was that high-prior- ity catchments in S and E Europe were adequately covered by protected areas in countries with the Natura 2000 network (EU member states) but not in others (former Yugoslavia, Ukraine, S Russia) (Fig 3). This result suggests that existing protected areas, in particular, the Natura 2000 network, are probably well placed in Europe for freshwater biodiversity conservation and that they adequately cover the freshwater groups studied here, especially in countries with the Natura 2000 network. The a priori inclusion of well-protected catchments (Scenario 2) also resulted in geographi- cally more balanced priorities (cf. Fig 1A vs. 1C) and a more efficient solution (slightly better coverage of threatened species: 97.4% or 553 species in Scenario 2 vs. 96.3% or 547 species in Scenario 1). This result indicated that the consideration of current protected areas produced a

similar or better network than a completely new network found by the algorithm. Such an out- come is not evident as similar studies have often reported mismatches between conservation priority and current protection [45, 55]. It also provides hope that the ongoing designation of Natura 2000 protected areas in countries aspiring to EU membership in the Balkans will also better cover freshwater biodiversity than the existing national networks of protected areas. The a priori exclusion of well-protected catchments (Scenario 3) further refined the results of the analysis of correspondence between priority and protection by providing a more detailed map on priority catchments with less than 70% area protection and increased chances of degradation [51], which may thus need further conservation interventions. These in http://project.freshwaterbiodiversity.eu, while the shapefile containing freshwater catchments are available at http://hydrosheds.org/ page/hydrobasins. Spatial information on protected areas were obtained from http://protectedplanet.net and http://www.eea.europa.eu. The maps were created with ArcGIS 10.0 by ESRI. https://doi.org/10.1371/journal.pone.0267801.g003 PLOS ONE Spatial priorities for freshwater biodiversity in Europe PLOS ONE | https://doi.org/10.1371/journal.pone.0267801 May 17, 2022 10 / 15