Browsing by Author "Van Blerk, Justin"

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    Can hydrological niche segregation explain species distributions in an aquifer-dependent wetland?
    (2025) Havhi, Mpho; West, Adam; Reinecke, Karl; Van Blerk, Justin
    Aim: To investigate post-fire succession in fynbos wetland plant communities, test whether these species segregate along fine-scale hydrological gradients and to identify species potentially most sensitive to groundwater abstraction. Intro: Hydrological gradients, which form between permanently inundated wetlands and adjacent dry habitats, are key drivers of vegetation community composition and structure, promoting opportunities for adaptive speciation and community coexistence. In the seasonally arid fynbos region of South Africa, the occurrence of permanently inundated, groundwater-fed wetlands suggests that fine-scale hydrological gradients may significantly shape floristic diversity in this region. The extent to which fine-scale hydrological gradients influence community composition and species segregation requires further testing in fynbos but has important implications for understanding the potential impacts of groundwater abstraction in these fire-prone systems as well as identifying potentially sensitive species. Methods: I analyzed plant community composition and abundance data, along with soil volumetric water content collected across three transects in an aquifer-fed wetland, from 2011 to 2022. Non-parametric multivariate tests were used to quantify spatial and temporal changes in community composition, highlighting general patterns of community clustering across transects and changes related to post-fire succession. The hydrological niche space was quantified across an aquifer-dependent wetland by representing soil moisture data along two hydrological axes: the sum of exceedance values for flooding (SEVa) and drought (SEVd). Using a subset of persistent species, species locations were fitted to these hydrological axes based on their proximity to soil moisture probes. Species were ranked in accordance with their position on this gradient, indicating a preference for flooded or drier sites on the wetland. Results: Successional changes were observed in the vegetation at different ages post-fire as well as shifts in early post-fire community composition between fire events. A robust test of the hydrological niche concept using several species that persisted over the post-fire successional trajectory shows that wetland community composition is spatially structured along hydrological gradients. It was also determined that species display some degree of hydrological niche segregation, with preference for either waterlogged or dry niches within the wetland. Discussion: The results suggest that species segregate along hydrological gradients defined by the number of days under flooding and dry conditions and can thus be ranked according to their preference for inundated or dry hydrological niches. The occurrence of several species with a strong preference for, or dependence on, permanently inundated niches, highlights the potential impacts of groundwater abstraction on species composition, abundance and distribution, which may affect this hydrological niche. The study takes proactive steps in monitoring ecologically sensitive ecosystems before groundwater abstraction, to ensure that appropriate vegetation management and conservation strategies are in place and can be implemented when necessary.
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    Rodents and Restionaceae : sex-specific plant-animal interactions in dioecious plants
    (2016) Van Blerk, Justin; West, Adam; Midgley, Jeremy J
    Differing plant canopy structures between the sexes of the dioecious Cannomois congesta (Restionaceae) were found to be the result of the destructive pre-dispersal seed predation methods used by the four-striped grass mouse (Rhabdomys). Mature, reproductive culms on female plants were felled down (mid way along the culm) in large numbers by Rhabdomys in order for it to access the seeds held terminally on culms. Male plants were largely unaffected due to their lack of seeds. Seeds acquired through culm-felling by Rhabdomys were all eaten shortly after their discovery, leading to significant seed mortality each year and reducing the rate of seed dispersal by ants (myrmecochory). I studied the physiological effects of culm-felling by Rhabdomys in C. congesta. Firstly I investigated nutrient resorption from senescing culms and considered the effects of reproductive effort (higher in females) and sex-specific nutrient losses. I found that nutrient resorption from mature culms was highly efficient in both sexes. Culm-felling by Rhabdomys lead to the loss of pre-senescent culm tissue in females and therefore lead to the loss of a significant proportion of potentially resorbed nutrients. Nutrient resorption was found not to be flexible as a strategy to compensate for higher reproductive costs/nutrient-losses in females. This was possibly due to nutrient resorption occurring near is physiological maximum. Plant photosynthetic performance was also investigated using stable isotopes and gas exchange and was found to be similar in both C. congesta sexes throughout the year. The influence of culm-felling by Rhabdomys on female micro-climate also did not appear to have a significant influence on photosynthetic performance between sexes. There was a general lack of evidence to suggest compensatory physiological changes or reductions in fitness (photosynthetic capacity/growth/reproduction) in females due to higher reproductive costs/nutrient-losses. The lack of physiological differences and the fact that females supported a greater number of culms relative to males suggested that other compensatory mechanisms could exist. Phenological differences were discovered and could be important in allowing flexibility between sexes to best use available resources for their specific sexual requirements. Another possibility is that compensatory growth occurred in females at the cost of rhizome-stored nutrients. This has been shown to occur in graminoids after high levels of herbivory (Bryant et al 1983).