Temporal Alpha and Beta Diversity of Diving Beetles (Coleoptera: Dytiscidae) Reveals Biotic Heterogenisation in Urban Ponds

Urbanisation has led to decreasing quantity and quality of both terrestrial and aquatic habitats, resulting in biodiversity loss in urban ecosystems. Current research has focused on spatial patterns of biodiversity along urban gradients, while temporal patterns have received little attention. Here, we investigated how alpha and beta diversity of beetles changed through time, and if pond structural connectivity (i.e., clustering of the ponds) and the presence or absence of fish affected the temporal change. We trapped diving beetles (Dytiscidae) in 20 ponds in the Helsinki Metropolitan Area, Finland, for six years and analysed the data using generalised linear mixed models and temporal beta diversity index. We found that (a) dytiscid alpha diversity, i.e., species richness and total abundances, decreased through time, while dytiscid temporal beta diversity increased. (b) Clustered ponds had higher alpha diversity than isolated ponds, but temporal beta diversity did not differ between clustered and isolated ponds; clustered ponds had lower species losses and higher species gains than isolated ponds. (c) Ponds with fish had little change in alpha diversity during the study period, but ponds without fish experienced declining alpha diversity over time; ponds with fish experienced lower species losses and higher species gains than ponds without fish. Our results indicate that dytiscid assemblages experienced biotic heterogenisation during the six years, but the heterogenisation processes are different in the presence and absence of fish. Dytiscids in clustered ponds are more likely to maintain stable populations due to immigration via the rescue effects than in isolated ponds. Urbanisation has long-lasting negative effects on biodiversity and may replace urban-sensitive species with urban-adapted species. The temporal dynamics of aquatic invertebrates suggest that improving landscape connectivity and local habitat conditions can mitigate species losses and support biotic communities in urban ponds.