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

Introduction Freshwater ecosystems (rivers, lakes and wetlands) cover less than 1% of the surface of Earth yet harbor 10% of the species described thus far [1]. They provide important ecosystem ser- vices such as food production, carbon sequestration, water purification, flood and erosion con- trol [2]. Biological diversity is declining much faster in freshwater ecosystems than in terrestrial and marine ones [3], mainly due to habitat loss, e.g. wetland destruction [4], hydro- morphological alterations for flood control and hydropower development [5], habitat frag- mentation due to dams and bridges [6, 7], flow alteration [8], impacts of exotic or invasive species [9], pollution [3] and climate change [10, 11]. Yet freshwater ecosystems, habitats and species are still poorly protected globally [12–14] and there is an urgent need for conservation interventions to face these threats [15]. Due to the global and omnipresent nature of threats, the limited resources, and the necessity for interdisciplinary and multi-sectoral approaches to conservation, these interventions need to be prioritized in a scientifically sound manner [16, 17]. Although protected areas are a major foundation of biodiversity conservation, their desig- nation has traditionally been guided by socioeconomic or aesthetic criteria rather than by sci- entifically sound principles [17]. Protected areas were often designated for terrestrial biodiversity, or, more recently, for marine biodiversity but rarely for freshwater biodiversity

[18–20]. Scientifically sound methods such as systematic conservation planning also rarely focus on freshwater ecosystems [16]. Prioritizations for freshwater biodiversity were typically based on river sections or grids as planning units that have little ecological meaning and practi- cal usability, typically on modelled ranges of species of one taxonomic group (most frequently fish) and on the regional spatial scale [21–24]. The uptake of these studies in conservation pol- icy and practice is slow as they are limited by scale and are rarely based on water management units such as catchments [9]. There are few published exercises at larger scales and where catchments are used as planning units [e.g. 25, 26]. In such studies, hydrological connectivity is typically not considered, even though river catchments are inherently connected, which should be accounted for in the prioritization [27, 28]. Conservation planning is also used to evaluate the capacity of existing protected areas to conserve biodiversity [26]. However, few studies investigated the spatial correspondence between hotspots of freshwater biodiversity and protected areas [20, 21, 29]. Similarly, there are only a few prioritizations that consider threat status or degradation [27, 30, 31]. These stud- ies suggest that management should be allocated to catchments that have high levels of biodi- versity, are well protected but are vulnerable to future threats, whereas restoration is necessary in catchments that have high levels of biodiversity, are not adequately protected and/or are degraded [22, 32].

The aims of this study were to establish spatial priorities for river and lake catchments in Europe by assessing their importance in the conservation of freshwater biodiversity based on the representation of threatened species, and to evaluate the role of the level of catchment pro- tection (‘catchment protection’ hereafter) and connectivity on conservation priorities. We used an extensive, continental-scale database on the distribution of four ecologically relevant taxa of freshwater ecosystems mapped to catchments as ecologically meaningful planning units in spatial prioritization by the Marxan systematic conservation planning tool. We used selection frequency as a proxy for irreplaceability or the conservation priority of catchments and data on protected areas and connectivity of the catchments to address four questions: (1) Where are the priority areas for the conservation of freshwater biodiversity in Europe? (2) Does the inclusion of well-protected catchments improve the efficiency of prioritization? (3) Where are areas with high biodiversity and low protection, that need further conservation PLOS ONE Spatial priorities for freshwater biodiversity in Europe PLOS ONE | https://doi.org/10.1371/journal.pone.0267801 May 17, 2022 2 / 15 the Hungarian Academy of Sciences. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.