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

a strong positive relationship between the proportion of protected areas and irreplaceability. First, we classified catchments into four groups by dividing them into high/low irreplaceability and high/low proportion of protected areas categories at the median values. Second, we calcu- lated residuals from an ordinary least-squares linear regression of irreplaceability as a function of proportion of protected areas. Finally, we measured the deviation of each catchment from an ideal, hypothetical 1:1000 line that is expected if the correspondence between proportion of protected areas (range: 0 to 1) and irreplaceability (range: 0 to 1000) is perfect. In these analy- ses, we used irreplaceability values obtained in Scenario 1. Results Scenario 1 showed that catchments with high irreplaceability were in S Europe (S Spain, S France, W, S and E Balkans), along major rivers (upper and lower Danube, lower Don, Dnies- ter and Volga), and around large or smaller lakes in N Europe (Fig 1A). A list of catchments or catchment groups with a selection frequency of 100% based on Scenarios 1 and 3 along with the country or countries in which they are located is given in (S1 Table in S1 File). The pro- portion of threatened (CR+EN+VU) species for which representation targets were met was 96.3% (547 of 568 species) in Scenario 1 (Table 1). When connectivity was considered in the analysis, the variation in irreplaceability increased and high-irreplaceability catchments were less clumped as many smaller river catchments had higher irreplaceability and lake catchments

(e.g. Lake Ladoga, lakes in E Finland) had lower irreplaceability (Fig 1B). With connectivity, the proportion of threatened species with targets met decreased to 92.6% (526 of 568 species) (Table 1). When well-protected catchments were locked in (Scenario 2), irreplaceability increased for catchments in N and central Europe (e.g. in Finland, Iceland, Norway, Poland, Russia, Swe- den) and parts of southern Europe (e.g. Portugal) (Fig 1C), even when connectivity was con- sidered (Fig 1D). The proportion of threatened species with targets met was 97.4% (553 of 568 species) without connectivity and 90.3% (513 of 568 species) with connectivity (Table 1). When well-protected catchments were locked out (Scenario 3), the number of catchments with high irreplaceability decreased, particularly in N and central Europe and along major river systems, but remained high in the W and S Balkans, S Spain, S France and N Alps (Fig 1E, S1 Table in S1 File). The addition of connectivity further decreased the number of high- priority catchments and increased the number of catchments with intermediate level of irre- placeability (Fig 1F). The proportion of species with targets met was the lowest in this scenario (89.3%, 507 of 568 species) and decreased further when connectivity was considered (79.2%, 450 of 568 species) (Table 1). Freshwater ecoregions with the highest average irreplaceability were in S and E Europe, including the catchments of the Volga delta and the N Caspian Sea, the Dalmatian and Ionian

coasts, Crimea, the Caspian Sea, Crete, western Anatolia, the N and S Adriatic Sea, followed by southern Iberian and other W Mediterranean ecoregions (Fig 2A, S1 Table in S1 File). Add- ing connectivity to the analysis changed this order slightly but did not affect the importance of the E and the W Mediterranean regions (Fig 2A). Accordingly, countries with the highest average irreplaceability also were mostly in S Europe (Malta, Montenegro, Albania, Portugal, Bulgaria, Italy, Spain) and central Europe (Hungary, Slovakia, Austria) (Fig 2B, S1 Table in S1 File).The addition of connectivity led to a lower rank for Hungary and the replacement of Aus- tria by Bosnia-Herzegovina in the top 10 (Fig 2B). Analyses of the correspondence between the irreplaceability and the proportion of pro- tected areas of catchments showed that catchments in the Balkans (countries of the former Yugoslavia: Bosnia and Herzegovina, Croatia, Montenegro, North Macedonia, Serbia; Albania; PLOS ONE Spatial priorities for freshwater biodiversity in Europe PLOS ONE | https://doi.org/10.1371/journal.pone.0267801 May 17, 2022 5 / 15