Abstract
Adult odonates (dragonflies and damselflies) are well-known as models for conservation of aquatic ecosystem health. There are numerous studies on aspects concerning the use of Odonata as bioindicators for aquatic ecosystem conservation, restoration and development. This study undertakes a quantitative and qualitative assessment of adult odonate assemblage composition and diversity over two summer wet seasons in the Magaliesberg region (North West province) of South Africa. It is hypothesised that the odonate species diversity and abundances will decrease as human impacts increase from pristine to impacted sites. The Magaliesberg, a UNESCO Biosphere Reserve, acts as a comparative assessment to see how this region relates to other odonate studies around the world and to studies from the Western Cape region of South Africa specifically. The two focal streams assessed in this study are the Maretlwane and the Sterkstroom, which together comprise the Gwathle catchment draining northwards from the Magaliesberg range into the Crocodile River. The adult odonate species richness and the abundance was determined at different sites along both streams (six sites for the Maretlwane in both seasons and eight sites for the first season and ten sites for the second season at the Sterkstroom). The sites cover a gradient of human land-use impacts with the upstream sites being pristine and the sites further downstream being impacted by agricultural, mining and urban activities. Qualitative and quantitative data on odonate abundance was collected using a combination of visual observations and capture with sweep nets. Environmental factors that are known to affect adult odonate assemblage composition were also gathered such as the water chemistry and riparian plant community structure that indicate habitat suitability. The Dragonfly Biotic Index (DBI), a bioassessment method for determining aquatic health using adult odonates, was also assessed for each site. The DBI index was used and compared to the South African Scoring System (SASS 5) method that counts and assesses freshwater macroinvertebrates. The SASS 5 data were collected concurrently with the odonate data at each site. The SASS 5 scores obtained for all the sites were evaluated to assess whether the macroinvertebrate assemblages decline in a similar fashion to the odonates because of human impacts. Thereafter, the data were analysed using multivariate statistics to compare the odonate assemblage composition between the sites across the gradient of human disturbances on each stream. The Riparian Vegetation Index (RVI) was used to assess the integrity of the riparian vegetation. The riparian vegetation index never scored below a C evaluation, indicating that the riparian vegetation in all the sites was structurally relatively intact with a substantial amount of undamaged biomass present. The pristine sites scored either an A or B categorisation
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indicating the sites were in very good condition and the habitat was not disturbed. It was found that there is partial support for the hypothesis that increased human impacts negatively affect adult odonate abundance and diversity along a gradient from pristine sites to more impacted sites. For the Maretlwane stream, the negative impact was more apparent, but for the Sterkstroom stream there was not a clear response by the odonate assemblages to the human disturbance gradient. It was found that the species diversity metrics, DBI scores, SASS 5 scores and abundance of adult odonates were generally higher in more pristine areas compared to impacted areas but the differences were not always statistically significant. Specifically, low impact sites were richer in species and richer in the number of individuals per species than disturbed sites (though not statistically significant). In line with results from other regions, the most pronounced impacts on adult odonate assemblages in the Magaliesberg were habitat degradation and water pollution. The multivariate statistics showed that there was a significant difference in odonate assemblage composition between non-impacted and impacted sites for the Maretlwane stream. The Shannon Diversity Index for the odonates showed low diversity for all the sites but the diversity of sites in nature reserves was significantly higher than for impacted sites. The results of this study will help inform on the effects of land-use practices on the aquatic health of streams in the region. Decision making about conservation can happen more effectively and efficiently and be seen as a priority.