Across the river and into the trees: Earthworms in a Central European Floodplain National Park

The Danube Floodplain National Park in Austria (Nationalpark Donau-Auen) protects one of the last remaining floodplain landscapes in Europe. It harbors a variety of habitats and is divided by a flood protection dam. The aim of this study was to assess the diversity of earthworms that inhabit the National Park and its unique habitats. We investigated 15 habitat types attributed to 5 general categories: forests, grasslands, riverine sites, arable sites and shrublands. Earthworms were sampled from soil by a combination of handsorting and mustard extraction, and from dead wood of selected forest types. We recorded 23 of the roughly 60 earthworm species known from Austria, including the first Austrian record of Helodrilus serbicus , rediscovery of Dendrobaena steineri , thirty years after first and only record, and regionally endemic Vindoboscolex hrabei . In soils, species richness was the highest in forest sites (median of 3), with flooded forests showing the highest species richness (median of 7). Earthworm abundance and species richness in soil seemed unaffected by the dam. In contrast, dead wood in repeatedly flooded areas showed higher earthworm density and species richness, suggesting an influence of the dam-altered flooding regime. Earthworm community composition also differed markedly between forest soils and adjacent dead wood. Among all habitat types, dead wood and riverine sites harboured the most distinct assemblages, dominated by specialist species. These findings underscore the importance of floodplain forests and their unique microhabitats for earthworm diversity and indicate that flood-regulating structures can shape earthworm communities in soils and dead wood.