Population dyanamics of rhabdomys bechuanae (Rhodentia, Muridae) in two localities of the Free State province

Kotze, Leon
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My study aimed at investigating the population dynamics of the African striped mouse Rhabdomys bechuanae, in two semi-arid localities, Tussen die Riviere (TDR) and Soetdoring (SO) Nature Reserves in the Free State Province, South Africa, and analyse the influence of extrinsic factors on population dynamics of these populations. Live trapping and habitat assessments were conducted at eight sessions from April 2015 to February 2017. For both study sites, regional temperature and precipitation data were obtained from the South African Weather Service and Florisbad Quaternary Research Station. The R. bechuanae habitat was highly seasonal with clear fluctuations in temperature, precipitation and plant cover. A total of 586 R. bechuanae were trapped at TDR compared to 120 individuals at SO. An equal number of female to male R. bechuanae were captured at TDR, indicating parity in the sex ratio of this species, at least where sample size is adequate. Contrary to R. pumilio of the Succulent Karoo, no sexual dimorphism was apparent in R. bechuanae body mass. Potentially reproductively active individuals were observed in seven out of eight trapping sessions. Peaks in breeding activity were expected in Summer 2015 (prior to the first trapping session) and Autumn 2016. I used simple capture-markrecapture models in program MARK to estimate R. bechuanae population size (full-likelihood model) and survival rate (Cormack-Jolly-Seber). Population size and survival rates fluctuated seasonally for R. bechuanae at both reserves. Peak density was lower than compared to studies of other Rhabdomys species and may be attributed to methodological differences between studies and / or differences in species ecology, specifically life-history. I analysed associations between four population parameters, i.e. population size, survival rate, reproductive activity and body mass, and five extrinsic factors, i.e. mean monthly minimum- and maximum temperature; total rainfall for one month and three months preceding trapping sessions as well as median total plant cover. Different combinations of extrinsic factors were associated with variation in each population parameter. According to the literature, weather (temperature and rainfall) could act both directly and indirectly, although an indirect effect is more likely for R. bechuanae. Plant cover likely acted directly on population parameters and was associated, among other factors, with reproductive activity and body mass. I posit that the lack of correlations occurring between R. bechuanae population parameters and extrinsic environmental factors may point to non-linearity in the response of R. bechuanae to environmental variation. Additionally, the extrinsic factors could act additively on R. bechuanae dynamics most likely through determining the availability of resources. Future research on the population dynamics of R. bechuanae should aim to analyse density dependence in population dynamics; the effects of interaction between causal factors, both intrinsic and extrinsic as well as the effect of dispersal, through immigration and emigration, on population growth.
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg in fulfillment of the requirements for the degree of Master of Science. Johannesburg, 2019