Future large hydropower dams impact global freshwater megafauna

Dam construction comes with severe social, economic and ecological impacts. From an ecological point of view, habitat types are altered and biodiversity is lost. Thus, to identify areas that deserve major attention for conservation, existing and planned locations for (hydropower) dams were overlapped, at global extent, with the contemporary distribution of freshwater megafauna species with consideration of their respective threat status. Hydropower development will disproportionately impact areas of high freshwater megafauna richness in South America, South and East Asia, and the Balkan region. Sub-catchments with a high share of threatened species are considered to be most vulnerable; these are located in Central America, Southeast Asia and in the regions of the Black and Caspian Sea. Based on this approach, planned dam locations are classified according to their potential impact on freshwater megafauna species at different spatial scales, attention to potential conflicts between climate mitigation and biodiversity conservation are highlighted, and priorities for freshwater management are recommended.

Species distribution range scenarios. 'Presence' classification of the megafauna species data provided by IUCN was used to distinguish three scenarios depending on the "certainty" about species occurrence. While for clarity reasons, results of the reference scenario only are given in the main manuscript, results presented in the Supporting Information contain also details on the variability in the results due to different species presence information:  Scenario 1. 'Presence' = "Extant": known or very likely existence of the species in the sub-catchment based on records of the last 30 years, corresponds to the minimum area inhabited by the respective species.  Scenario 2. 'Presence' = "Extant" or "Probably Extant": most probable existence of the species in the sub-catchment. (Note: According to IUCN the category "probably extant" will be phased out in the future but is currently still contained in the data.) Reference scenario, also applied by IUCN.  Scenario 3. 'Presence' = "Extant", "Probably Extant", "Possibly Extant" or "Presence Uncertain": includes sub-catchments in which the species occurred most likely, but recent records are missing or records require verification and are highly uncertain. This scenario corresponds to the maximum possible area inhabited by the respective species.

Fig. S1
Overlap of existing dams and future hydropower dams with numbers of freshwater megafauna species per sub-catchment on a global scale.

Fig. S2
Underlying analysis for derivation of sub-categories for species richness. (i) Distribution of sub-catchment numbers per total freshwater megafauna species number on a global scale as absolute numbers (left) and accumulated percentage (right). Error bars give the range for the three scenarios, which vary in species occurrence data. (ii) -(iv) Results of different species richness categorizations reflected in choropleth maps of sub-catchments (HydroBASINS level 8) according to species richness and threat status (left) and "density" of dams per 10 5 km 2 in different sub-catchments according to richness-threat categories (right), both based on the reference scenario for species occurrence. Species richness categories have been selected as follows: (ii) 5% of the sub-catchments represent highest species richness (> 12 species) and the remaining 95% of sub-catchments with lower species richness are classified with equal distances (1-4 species; 5-8 species; 8-11 species). (iii) equal distances between absolute species numbers (1-5 species: 70% of sub-catchments; 6-10 species: 23%; 11-15 species: 3%; > 15 species: 4%). (iv) equal size fractions in terms of sub-catchment numbers of each category (1-2 species; 3-4 species; 5-6 species; > 6 species). This gives very similar results in comparison to the selected methodology as described in the main text.

Fig. S3
Fractions of freshwater megafauna distribution ranges per freshwater megafauna species number that are directly affected by existing dams and by future hydropower dams. Please note that the total fraction of the distribution range affected in the future might be smaller than the sum of the range fractions of existing and future dams since future hydropower dams partly fall into the same sub-catchment as existing ones and this area is not counted twice. Error bars result from scenarios in species occurrence.

Fig. S5
Major basins with (i) more than 100 existing dams or (ii) more than 100 future hydropower dams and the respective share in overlap with megafauna distribution ranges according to their richness-threat category. Size of the pie chart indicates the number of dams per major basin.