2.7 Conclusio ns
2.7.1 Temporal changes
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Chapter Three Significance of medium-term temporal changes when designing a reservoir for conservation of dragonfly diversity
3.1 Abstract
A study was undertaken at a reservoir formed from an impounded stream in 1988 for the purpose of insect and plant conservation at the National Botanical gardens,
Pietermaritzburg. While little information exists in the southern hemisphere on longer- term changes in dragonfly assemblages,this study was important in determining whether a species in a conservation area is being afforded protection from varying local
anthropogenic impacts over time, especially in an El Nifio-prone area like KwaZulu- Natal. The overall aim here was to assess the reservoir created 13 years earlier to determine the changes (if any) that have taken place, and specifically to document the physical biotope changes and also how these have affected the dragonfly assemblage over time.The reason for this is to understand how concepts of residency and succession underpin conservation decisions.A total of30 dragonfly species wAS 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 high species richness, including endemics, it is essential to maintain a variety ofbiotopes 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.
*
This chapter is in press with Biodiversity and Conservation58
3.2 Introduction
After spatial selection of areas and sites for conservation, the next question becomes to what extent is a site managed to maintain a particular assemblage of organisms in the successional process.Furthermore, the success of conservation programmes often
depends on developing an approach tailored to the particular culture and circumstances of a place and its inhabitants (Primack et al. 2001).Artificial dragonfly ponds are a
significant addition to Japanese and more recently, British landscapes.While dragonflies are a major component when assessing ecological composition of aquatic biotopes
(Chelmick et al. 1980; Watson et al. 1982, Buchwald 1993; Samways 1992; 1993; 1999), the assemblage composition at anyone locality can change over time (Moore 1991;
2001). The significance of this is that short-term inventory of species may not present a faithful picture of the dragonfly assemblage which is dynamic over time (Schmidt 1985). Nevertheless, quantitative short-term studies on dragonfly colonisation and vegetational succession for newly-established ponds yielded information needed to understand habitat requirements of species associated with specific stages of succession, and to derive appropriate management practices (Corbet 1999).Also, such assessments form an essential starting point for the applied science of restoration ecology, which, in turn, requires recognition of the changes to be expected in species abundance and diversity as ecological succession proceeds.Information is being accumulated on medium to long- term changes in dragonfly populations after conservation ponds have been established, especially in some European countries (Schmidt 1985; Moore 1991,2001;Chovanec
1994; Chovanec and Raab 1997).
In South Africa, Samways et al. (1996), Steytler and Samways (1995) and Osborn and Samways (l996), have documented early changes at an urban reservoir which was constructed in 1988 for insect and aquatic plant conservation.Nevertheless, little information exists in the southern hemisphere on longer-term changes in dragonfly assemblages. Such longer-term studies are crucial for determining whether a species in a conservation area is being afforded protection from varying local anthropogenic impacts over time, especially in an El Nifio-prone area. The overall aim here therefore, is to assess a reservoir which was created 13 years earlier to determine the changes that have taken place. The reason for this is to understand how concepts of residency and succession u~derpinconservation decisions. This study specifically aims to document the physical biotope changes and also how these have affected the dragonfly assemblage over time and to ascertain how the assemblage has responded to the altered ecological conditions.
As this reservoir is also part of a dragonfly conservation awareness trail (Suh and Samwa~s2001), the study also makes recommendations for future management of the reservoir.