Browsing by Author "Jordaan, M. S."

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Food web properties vary with climate and land use in South African streams
    (Wiley Online, 2020) Jackson, M. C.; Fourie, H. E.; Dalu, T.; Woodford, D. J.; Wasserman, R. J.; Zengeya, T. A.; Ellender, B. R.; Kimberg, P. K.; Jordaan, M. S.; Chimimba, C. T.; Weyl, O. L. F.
    Land use intensification and climate change are two prominent drivers of variation in biological communities. However, we know very little about how these two potential environmental stressors interact. Here we use a stable isotope approach to quantify how animal communities respond to urban and agriculture land use, and to latitudinal variation in climate (rainfall and temperature), in 29 streams across South Africa. Community structure was shaped by both land use and climatic factors. The taxonomic diversity of invertebrates was best explained by an independent negative effect of urbanization, while abundance declined in summer. However, we could not use our variables to predict fish diversity (suggesting that other factors may be more important). Both trophic functional diversity (quantifed usingisotopic richness) and food chain length declined with increasing temperature. Functional redundancy (quantifed usingisotopic uniqueness) in the invertebrate community was high in wet areas, and a synergistic interaction with urbanization caused the lowest values in dry urban regions. There was an additive effect of agriculture and rainfall on abundance-weighted vertebrate functional diversity (quantified usingisotopic dispersion), with the former causing a decline in dispersion, with this partially compensated for by high rainfall. In most cases, we found that a single dominant driver (either climate or land use) explained variation between streams. We only found two incidences of combined effects improving the model, one of which was amplified (i.e. the drivers combined to cause an effect larger than the sum of their independent effects), indicating that management should first focus on mitigating the dominant stressor in stream ecosystems for successful restoration efforts. Overall, our study indicates subtle food web responses to multiple drivers of change, only identified by using functional isotope metrics-these are a useful tool for a whole-systems biology understanding of global change. A freePlain Language Summarycan be found within the Supporting Information of this article.
  • Thumbnail Image
    Item
    Variation in sex determination and the application of the YY male technology for the production of all-male populations of the tilapia Oreochromis mossambicus
    (Stellenbosch : Stellenbosch University, 2004-12) Jordaan, M. S.; Brink, Danie; Stellenbosch University. Faculty of Agrisciences. Dept. of Genetics.
    ENGLISH ABSTRACT: Tilapia is presumed to have a well-defined genetic mechanism of sex determination, but not all sex ratios are compatible with a monofactorial sex determination model. A theory of autosomal gene influence, as well as temperature sex determination (TSD) has been proposed in order to explain large variations in sex ratios. This study assessed the variation in progeny sex ratio in O. mossambicus as a basis for the application of YY male technology in the production of all-male progeny groups. Three populations of O. mossambicus were sampled that are representative of the geographical distribution of the species in Southern Africa. Progeny groups were produced from randomly selected parents and maintained at constant temperature during the labile period of sex differentiation. Variation in sex ratios between different families of the same strain as well as between different strains was calculated. No significant differences were observed in sex ratio between strains, though a significant intra-group variation was identified. This study identified both male and female-biased sex ratios. The data in general conform to a monofactorial sex determination model. Male-biased sex ratio observed in one strain can possibly be ascribed to temperature sex determination (TSD). This strain of O. mossambicus originated from an area with different annual temperature patterns and the possibility of TSD having an adaptive advantage is discussed. This thesis also presents the results of a program to produce monosex male tilapia through the application of the YY male technology in O. mossambicus. Viable XY female and YY male genotypes were produced. XY females sired progenies ranging from 68-94% male ofspring, while YY males sired a mean progeny of 94% male phenotypes. From these results it is concluded that YY male technology provide a viable method for the production of all-male progeny in O. mossambicus. Once available on a commercial scale, the technology can be made more reliable through the application of the appropriate selection methods.