32 years of essential management to retain value of an urban dragonfly awareness pond

Abstract

Pond construction in urban areas can mitigate loss of aquatic insects by providing refuges. Urban ponds are also an interface between civil society and aquatic insects, especially via charismatic dragonflies. Ponds have therefore been constructed specifically for dragonfly conservation awareness in many countries. Yet they require regular management, especially when an inflow to a shallow pond supplies inorganic and organic material, leading to vegetation overgrowth and natural infilling, rendering a pond back into a stream, unsuitable for many lentic species. Here, we assess changes in dragonfly diversity over a 32-year period at a pond constructed for dragonfly conservation awareness, and which underwent system changes: stream→pond→vegetation overgrowth and infilling and then following restoration from stream→pond. Adult male dragonflies and 13 environmental variables were recorded along 31 transects, and compared with previous data collected at the same sites before pond construction and then in the short- and medium-term after. Years when the system comprised a pond had higher dragonfly abundance and species richness than when a stream, and both increased after the pond was restored. Shortly after restoration, the dragonfly assemblage closely resembled that of the earlier pond. Vegetation cover and alien vegetation presence were significant drivers of dragonfly assemblage change and decreased dragonfly abundance and species richness. Lessons learned here for maintaining a dragonfly awareness pond include periodical dredging to remove excessive vegetation overgrowth and infilling from organic and inorganic matter, and aim for high microhabitat heterogeneity using selective management of marginal vegetation, alongside a range of flow regimes.