Diversity of small waterbodies sustains aquatic biodiversity in drained forest landscapes

Small freshwater bodies (streams, pools, ponds, ditches) are biodiversity hotspots that are vulnerable to hydrological modification of landscapes. The impacts of landscape-scale modifications are difficult to study because multiple processes combine (loss, creation and modification of waterbodies and their surroundings). For an insight, we sampled sensitive insect orders (Ephemeroptera, Plecoptera, Trichoptera, Odonata) in small waterbodies in naturally comparable more and less drained forested catchments in Estonia. We asked how the landscape-scale species richness, abundance and assemblage composition are formed by contributions of different waterbodies. The mean abundances and assemblages were similar, but overall species richness was higher in more drained landscapes, primarily due to added man-made ponds (with distinct assemblages) and the heterogeneous surroundings of waterbodies. Ditches did not host specialist species and could not compensate for the loss of natural waterbodies. Natural pools supported the fewest species, but at a comparable mean abundance with the other waterbody types. The results indicate that a mid-term landscape-scale impact of forestry drainage on freshwater biota depends on which waterbodies and how abundantly are added and retained. To better regulate hydrological interventions, it is necessary to develop such regionally observed patterns into a functional understanding of long-term effects across different landscapes.