Significance of temporal changes when designing a reservoir for conservation of dragonfly diversity

While there has been much focus in biodiversity conservation that transcends place, few studies transcend time. Yet an appreciation of vegetational and hydrological succession is essential for maintaining a habitat that has been created with the aim of conserving a particular group of organisms. This is a study of changes in a dragonfly assemblage over a period of 13 years at a biodiversity-rich, southern hemisphere reservoir. A total of 30 dragonfly species were recorded in this study, compared to 12 species before the reservoir was constructed in 1988, and 26 species in 1993, with 25 species resident in both 1993 and 2001. Two of these are local endemics. One other endemic was lost to succession in 1993 but reappeared in 2001. Three other species never reappeared after succession in 1993, yet six other species appeared after this date. Multivariate analyses identified structural and compositional vegetation, especially marginal forest, percentage vegetation cover, percentage shade, as the most important environmental variables determining dragonfly species composition. Other important environmental variables were grasses of tall, medium and short height categories, submerged vegetation, water flow and amount of open water. Not surprisingly, successional changes in vegetation physiognomy and in water conditions significantly increased Odonata species richness and diversity over the years. More importantly, the study shows that to maintain both high species richness, including endemics, it is essential to maintain a variety of biotopes using selective management of the marginal vegetation without allowing succession to proceed to a point where overgrowth of the bank and silting of the bottom begin to impoverish the fauna.