Agricultural pond restoration enhances species richness in non-biting midges (Diptera: Chironomidae)

Agricultural ponds are biodiversity hotspots, especially when restored to open-canopy conditions. Studies comparing pre- and post-restoration pond diversity are rare, however, and have widely focused on “easy-to-identify” taxa. Chironomids have so far been neglected in pond restoration studies, but are a highly speciose group critical to aquatic ecosystem functioning. This makes them good potential indicators of ecological change. Despite a reputation for being taxonomically challenging, the final molts of chironomid pupae, the exuviae, can be reliably sampled and identified using the Chironomid Pupal Exuvial Technique (CPET). For a lowland farmland pond landscape in eastern England, we used CPET sampling in a Before-After Control-Impact study encompassing nine ponds restored by major removal of woody vegetation and sediment, four terrestrialized control ponds and two open-canopy control ponds. Species richness was significantly greater in the restored and open-canopy control ponds compared to the terrestrialized ponds, likely due to structurally complex plant communities in the former. Few chironomid species preferred the wood and organic matter-dominated conditions afforded by the terrestrialized ponds, while the restoration of these ponds to open-canopy, macrophyte-dominated condition strongly increased chironomid diversity. To maximize landscape-scale chironomid diversity, agricultural landscapes should ideally contain ponds at different stages of succession, including a high proportion of ponds restored to an early successional macrophyte-filled state. As for many other taxonomic groups, we show that pond restoration is essential to the maintenance of species-rich chironomid communities in farmland pondscapes.