Evidence-based restoration of freshwater biodiversity after mining: experience from Central European spoil heaps.
Post-mining freshwater habitats can serve as secondary habitats for threatened species. Being globally plentiful, their efficient restoration should be based on detailed evidence, especially under the current global decline of freshwater biodiversity. Synthetic studies focusing on the effects of restoration on freshwater communities are surprisingly rare, even for the most common restoration approaches. Such a lack of evidence limits the efficiency of habitat restoration in post-mining landscapes. We combined a landscape-scale field study of aquatic beetles and hemipterans in 29 freshwater pools at lignite spoil heaps in Czechia with a comparative synthesis of all 14 available datasets on freshwater communities at habitats created by coal mining in Central Europe. We compared the diversity and conservation value of freshwater habitats created by the main restoration approaches, including technical reclamation, spontaneous succession and their combination. Additionally, we analysed the detailed influence of key habitat descriptors on their conservation potential. Communities of aquatic beetles and hemipterans had more species in spontaneously and semi-spontaneously developed pools, which were also preferred by more red-listed species of both studied groups. The reviewed studies showed the same trend for amphibians, odonates, dipterans and macrozoobenthos, although some technically created pools and drainage ditches harboured some threatened species of aquatic plants, odonates and macrozoobenthos as well. Relatively independent of the restoration approach, the conservation value of freshwater communities was related to the particular characteristics of the restored habitats. The key beneficial features were as follows: smaller area of restored water bodies, heterogeneous littoral vegetation with suppressed dominants, heterogeneous bottom substrate with low and gradually sloping banks, absence of fish, sun-exposed water surface and some forests in the surroundings. Synthesis and applications. Efficient restoration of freshwater habitats at post-mining landscapes can create important secondary refuges for threatened and common species. Freshwater communities of high conservation value can be supported by all main restoration approaches if heterogeneous habitats are created. A combination of technical reclamation with spontaneous succession seems to be the most efficient approach to maximise biodiversity in freshwater habitats at coal-mining spoil heaps.