Masters Degrees (Physics)

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Browsing Masters Degrees (Physics) by Subject "Assemblage organization"

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    Modelling assemblage organisation of African Odonata
    (Stellenbosch : Stellenbosch University, 2020-12) Basel, Ashleigh Megan; Hui, Cang; Midgley, Guy F.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: Climate change is expected to cause large range shifts of species assemblages, especially for climate-sensitive taxa, such as dragonflies and damselflies (Insecta: Odonata). I investigated odonate species under two spatial extents - South Africa and the entire African continent. At the scale of South Africa, I firstly quantified the major drivers of Odonata compositional turnover along key environmental gradients using pairwise beta-diversity metrics, and secondly mapped the predicted spatial variation in species composition as both a continuous gradient of assemblage similarity and also as discrete bioregions, depicting major areas of odonate endemism based on clustering analyses. Using space-for-time substitution, I thirdly estimated and mapped the magnitude of expected change in species turnover in response to climate change under two emission scenarios, for the years 2050 and 2070, respectively. Lastly, I estimated bioregion instability by assessing how the present-day odonate bioregions may shift under these future climate change scenarios. At the continental scale of Africa, I extended beyond pairwise metrics by considering zeta diversity metrics for measuring multi-site assemblage similarity. Specifically, I firstly explored the community assembly processes for dragonflies and damselflies subgroups separately. Secondly, I differentiated the environmental and geographical drivers behind compositional turnover of widespread versus rare species. At the regional scale of South Africa, I found that with adequate sampling, Odonata are effective bioindicators of climate change. Present-day bioregions correspond to climatic zones and are clearly separated by transitional zones (ecotones) with rapid spatial turnover. Present odonate bioregions are projected to undergo extensive reorganisation by 2050. Furthermore, depending on the emission scenario, temporal turnover in species composition is projected to reach up to 80% in the large arid interior and 64% along the coast. Such sharp spatial and temporal turnover threatens the persistence of 12 narrow-ranged species. At the continental scale of Africa, in terms of the form of zeta diversity declining with increasing orders, both suborders were fit by a power law form relationship indicating that communities have a non-random co-occurrence, indicating there are niche or range differences among species. This was supported by the spatial patterns of the suborders, which differed greatly. For damselflies, geographic distance between communities was consistently one of the major community drivers. For dragonflies, environmental filtering was the major assembly driver with little dependence on geographical isolation. My findings suggest that differences in species dispersal mechanisms play a role in the determinants of commonness and rarity for damselflies, whereas for dragonflies niche differentiation relating to environmental preferences is more likely to be causing the shift between commonness and rarity. These discrepancies may reflect the ecological differences between suborders. The results further suggest that damselflies showed a largely more nested structure (i.e. species assemblages in species poor sites are subsets of assemblages in more species rich sites) than dragonflies, which is an important consideration for site selection in future conservation planning. Taken together, the compositional turnover of Odonata species suggests an interplay of geographic distance and different environmental gradients that produce distinct drivers for the different suborders, spatial scales (South Africa vs. African continent), and common vs. rare species. Notably, my results suggest climate change could cause drastic range shifts in this bioindicator taxon.