Dragonflies as bioindicators and biodiversity surrogates for freshwater ecosystems
Thesis (PhDAgric)--Stellenbosch University, 2019.
ENGLISH ABSTRACT: Biological indicators (bioindicators) are useful for rapid and cost-effective ecosystem assessments. Dragonflies are valued for their potential as bioindicators in freshwater ecosystems. My dissertation aims to assess and expand on their use as bioindicators in transformed landscapes and as surrogates for other aquatic biodiversity. Of the three bioindicator categories (environmental, ecological and biodiversity), biodiversity indicators and their application are poorly understood. The umbrella species concept is a biodiversity surrogacy method that aims to conserve a large number of species in an ecosystem by focusing on a select group of co-occurring species. I used the umbrella index, which quantitatively identified a group of seven dragonfly species and a group of eight Ephemeroptera, Plecoptera and Trichoptera (EPT) species, any of which could be used as biodiversity surrogates (Chapter 2). Adult dragonflies can only be surveyed on warm, windless days during summer, but are easily identifiable. On the other hand, their larvae can be sampled under any weather conditions and are also sensitive bioindicators. I showed that the interchangeability of the life stages for assessments was dependent on landscape spatial scale, coupled with the specific question asked (Chapter 3). Comprehensive biodiversity surveys at fine ecological scales should sample both adults and larvae. However, at larger spatial scales with coarser ecological questions, either adults or larvae can be used. To mitigate the detrimental effects caused by forestry, ecological networks (ENs) are integrated into plantation landscapes. These comprise grassland corridors connected to protected areas (PAs), which often include rivers. They aim to conserve biodiversity by creating habitats or facilitating dispersal of grassland species. I showed that water quality and adult dragonfly diversity did not differ between EN corridors and PAs (Chapter 4). Therefore, the EN approach is an effective method for conserving dragonfly diversity and river ecosystem integrity in plantation landscapes. In the Pietermaritzburg Botanical Gardens, an insect conservation pond was built along a degraded stream. Dragonfly species richness and abundance significantly increased, as both lentic and lotic species were able to colonize the area. Over time, the pond became overgrown and siltation reverted it back to a stream, which negatively affected dragonfly diversity. Shortly after extensive restoration efforts, the dragonfly assemblage had almost completely recovered and closely resembled that of the original pond. This was linked to alien plant removal, decreased vegetation cover and the inclusion of a range of microhabitats. This indicates that conservation ponds need to be actively maintained to keep their function as biodiversity reservoirs. This highlights the value of dragonflies as indicators of habitat quality in aquatic restoration projects. Throughout the thesis, adult dragonflies continuously demonstrated their success as bioindicators. The umbrella index validated the use of dragonflies as biodiversity indicators and surrogates for some of the most sensitive aquatic taxa (the EPT). Although dragonfly larvae and adults are both indicators of water quality, they are not always interchangeable, in terms of sampling at the species level. Their interchangeability depends on the specific question asked and the scale used. I also successfully monitored dragonfly assemblage responses in agricultural lotic and urban lentic environments, which highlighted their benefits for good aquatic conservation planning in transformed landscapes.