Browsing by Author "Zietsman, Johlene"

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    Reproductive biology, ecology and historic biogeography of selected rare and endagered Oxalis L. (Oxalidaceae) species
    (Stellenbosch : Stellenbosch University, 2007-03) Zietsman, Johlene; Dreyer, L. L.; Esler, Karen J.; Stellenbosch University. Faculty of Natural Science. Dept. of Botany and Zoology.
    ENGLISH ABSTRACT: Twenty five percent of all southern African Oxalis species are rare/endangered and highly localized, making them especially vulnerable to threats of extinction (inbreeding, low genetic variation, disrupted biological interactions) associated with small, isolated populations. In addition, Oxalis also displays tristyly, a restrictive breeding system that includes a strong self-incompatibility component that promotes out-crossing within populations. As it requires equal availability of three different floral morph types as well as effective pollinators for seed production, this breeding system can be a further threat to small, isolated populations when it is fully expressed. Breakdown has been recorded in terms of all the components of this complex breeding system, but usually comprises a relaxation in the expression of self-incompatibility. This may significantly alter reproductive assurance regardless of the availability of pollinators and morph types, with positive consequences for at least the short-term future survival of rare species. Potential factors that may contribute towards rarity in Oxalis were investigated by focusing on the reproductive biology and ecology of two widespread and eight rare/highly localized Oxalis species. The expression of tristyly, levels of natural seed production and vegetative reproduction were investigated to identify possible limitations to reproductive success. Several ecological attributes, including habitat type and specificity, population size and density, rainfall patterns, potential pollinators, potential threats, etc. were investigated for the studied populations and compared to the reproductive biological expression in each case. In addition, phylogeographic patterns were assessed by investigating the degree of genetic variation within and among populations of a wellsupported monophyletic clade including two rare and highly localized species and one widespread species. Comparisons of results of different rare species together with comparisons between rare and related common species are presented to reveal the most likely threats to specific populations. Tristyly expression was unstable and extremely variable among the populations studied. In addition to limited reproductive success, most rare species are restricted by their highly specific habitat requirements and are particularly vulnerable to variation in rainfall patterns. Low genetic variation within rare species may also prevent the colonisation of new habitats or the adaptation to a changing environment.