The natural drivers and the effects of landscape transformation for dragonflies of the Cape Floristic Region

[摘要] ENGLISH ABSTRACT: The Cape Floristic Region (CFR) is a biodiversity hotspot. The region has three established biosphere reserves, which all aim to alleviate the impacts that land transformation has on ecosystem integrity, without jeopardizing basic human needs. In addition to its unique plant diversity, the CFR has high endemism levels of other taxa, including dragonflies. Dragonflies are useful bioindicators of freshwater quality, which has led to the development of the Dragonfly Biotic Index (DBI), a biomonitoring tool for freshwater. The combined pressures of urbanisation and agricultural expansion in the CFR are a major concern for rare, endemic dragonfly species, as well as for overall river ecosystem integrity. In view of this, my study aims to determine which variables drive lotic dragonfly diversity in the CFR, and to assess the effects that land transformation has on this diversity.I first determined which environmental parameters were consistently important so that they could be used as mesofilters to conserve dragonfly diversity (Chapter 2). Dragonfly assemblages and various environmental variables were recorded along the untransformed reaches of three CFR rivers. Heterogeneity of water parameters was found to be the most crucial variables for dragonfly assemblages and for affecting species richness. Here, heterogeneity is defined by the natural spatial and temporal variation of water temperature, dissolved oxygen, conductivity and pH. This differed from previous studies, which strongly suggest vegetation-related variables are the primary drivers of dragonfly diversity. However, these studies took place in transformed landscapes where the strong effects related to anthropogenic disturbances could override the importance of other more subtle natural variables. The maintenance of a gradient of water parameters, which accounts for the natural range of each of the selected water variables, would thus aid in the conservation of dragonflies in the CFR.I also investigated the effects of urbanization and agricultural development on dragonfly diversity and DBI scores. Land transformation homogenized dragonfly assemblages as some endemic species could not persist in these areas. However, species richness was not always reduced, because disturbance allowed for additional widespread, generalist species to enter the system. Dragonfly assemblages differed between agricultural and urban sites but these sites were more similar to each other than to undisturbed sites. Each river supported a unique dragonfly assemblage, making it important to conserve each individual river. Mitigating the adverse influences of landscape transformation is essential for the conservation of rare and endemic taxa, particularly in areas of high conservation value, and the DBI provided an effective way to assess ecosystem integrity in the region.In conclusion, land transformation negatively affects dragonfly diversity and ecosystem integrity in CFR rivers. Conservation efforts should aim to rehabilitate the natural heterogeneity of riparian ecosystems. However, conservation plans should not only focus on restoration of riparian vegetation, but also incorporate variation in water quality parameters. There is not a high possibility of reducing land transformation, with the requirements of an ever-increasing human population. An important alternative option, as I show here, is to protect ecological integrity within a biosphere reserve. The proclamation of more biosphere reserves in the CFR, that include other, additional river catchments, will allow for the conservation of more rare, endemic dragonflies and other taxa. Dragonfly assemblages and the DBI should be used in future monitoring programs and to guide conservation actions.

[发布机构] Stellenbosch University