Doctoral Degrees (Conservation Ecology and Entomology)

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Browsing Doctoral Degrees (Conservation Ecology and Entomology) by browse.metadata.advisor "Dreyer, L. L."

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    Fungal endophyte assemblages associated with twigs of olives in the Core Cape Subregion, South Africa
    (Stellenbosch : Stellenbosch University, 2022-04) Ngubane, Nombuso Portia; Roets, Francois; Dreyer, L. L.; Slippers, Bernard; Kemler, Martin; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Fungal endophytes are increasingly gaining recognition for their role in plant health. In the face of global change and unprecedented biodiversity loss, it has become an urgent concern to understand these valuable microbes. The main objectives of the work presented here were two-fold, 1) to gain better understanding of the fungal endophytes in a threatened biodiversity hotspot and 2) to improve our understanding of fungal endophyte assemblages associated with ecologically and agriculturally important Olea species. The Core Cape Subregion provides a rare and important study area since it is one of the few areas of olive cultivation with close native relatives, including O. europaea subsp. cuspidata. Many olive (O. europaea subsp. europaea) orchards in the Core Cape Subregion are near natural O. europaea subsp. cuspidata populations. In Chapter 2, I investigated the role of host identity and geographic distance on fungal endophyte assemblages associated with the two Olea europaea subspecies in South Africa. Although many taxa were shared between these hosts, the native host harboured significantly higher alpha diversity. The beta diversity of fungal endophytes also differed significantly between hosts. Geographic distances played a significant role in shaping fungal endophyte assemblages of both hosts, more so in the native host. The native O. europaea subsp. cuspidata is a widely distributed plant growing across a variety of habitats that is also a favoured shade plant, planted in gardens, parks and roadsides. In Chapter 3, the response of fungal endophytes to different levels of disturbance (habitat context) and to differences in surrounding vegetation types (vegetation contrast) were assessed. Endophyte species richness was influenced by habitat context and vegetation contrast. However, fungal endophyte assemblage composition was only affected by habitat context. This suggests that although the host can tolerate different habitat context levels, its fungal endophytes are particularly sensitive to even the mildest of disturbances found in the semi-natural habitat context. In the Core Cape Subregion, two additional Olea species (O. capensis and O. exasperata) are native to South Africa. This made it possible to assess the impact of host identity and relatedness on fungal endophyte assemblages of native hosts (Chapter 4). Fungal endophytes were documented in five native hosts (three Olea and two non-Olea hosts) in the Kogelberg Biosphere. Although fungal endophyte assemblages were significantly different between hosts, this was not correlated to host relatedness (phylogeny). Other factors, other than host phylogeny, were more important to fungal endophytes in this area. The lack of a phylogenetic signal reflected in fungal endophyte assemblages of native Oleaceae hosts suggests that the differences in fungal endophyte assemblages between O. europaea susp. europaea and O. europaea susp. cuspidata are likely due to differences in their histories. The differences in planted African olive trees versus those in the natural context, and the differences between the cultivated and the native olives demonstrate the importance of habitat context. The dynamic nature and diversity of fungal endophytes within the investigated hosts highlights the need to improve our understanding of fungal endophytes in South Africa, especially in native hosts.
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    Mite (acari) ecology within protea communities in the Cape Floristic Region, South Africa
    (Stellenbosch : Stellenbosch University, 2018-03) Theron-De Bruin, Natalie; Roets, F.; Dreyer, L. L.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Protea is a key component in the Fynbos Biome of the globally recognised Cape Floristic Region biodiversity hotspot, not only because of its own diversity, but also for its role in the maintenance of numerous other organisms such as birds, insects, fungi and mites. Protea is also internationally widely cultivated for its very showy inflorescences and, therefore, has great monetary value. Some of the organisms associated with these plants are destructive, leading to reduced horticultural and floricultural value. However, they are also involved in intricate associations with Protea species in natural ecosystems, which we still understand very poorly. Mites, for example, have an international reputation to negatively impact crops, but some taxa may be good indicators of sound management practices within cultivated systems. Their role in natural systems is even less well-understood. In this dissertation I explore the role of mites within Protea populations in both natural and cultivated systems, focussing on assemblages from inflorescences, infructescences and soil. Protea inflorescences and infructescences provide a niche for a unique assemblage of mites that have associations with a group of arthropod-associated fungi, the ophiostomatoid fungi. The mites feed on the fungi and carry their spores to new inflorescences as phoretic partners of Protea-pollinating beetles. As it was shown that some of the fungi have a panmictic population genetic structure over as much as 1000 km, it was assumed that organisms other than beetles must be responsible for this extremely long-range dispersal. Here I present the first concrete evidence of the ability of birds to vector spore-carrying mites to new Protea trees. I also provide evidence for a newly discovered mite-fungus mutualism within ornithophilous Protea neriifolia inflorescences between a Glycyphagus sp. mite and various species within the ophiostomatoid genus Sporothrix. New mite-mite commensalisms between the Proctolaelaps vandenbergi flower mite and the Glycyphagus sp. mite was also discovered and documented. In this intriguing system the Glycyphagus sp. mites have a mutualistic association with species in the fungal genus Sporothrix. These small mites are phoretic on the larger P. vandenbergi mites that, in turn, are phoretic on Protea pollinating birds, explaining genetic evidence for the long distance dispersal of the fungi. It is well-known that flower-associated mites such as Proctolaelaps kirmsei are nectar and pollen thieves of hummingbird pollinated plants in America. These mites reduce nectar and pollen rewards for pollinators, which influences pollinator visitation patterns and decreases available pollen for dispersal, thereby negatively influencing seed-set and plant population dynamics. This phenomenon has, however, not been investigated in similar systems in other parts of the world. I, therefore, set out to determine the possible role of P. vandenbergi flower mites, the most abundant flower mite within Protea inflorescences, as pollen and nectar thieves and as secondary pollinators of P. neriifolia. I provide the first evidence that P. vandenbergi feeds on nectar and pollen and that its reproduction is strongly linked to pollen availability. Nectar consumption rates of P. vandenbergi likely have little effect on total nectar availability for pollinators, but they can significantly reduce available pollen in inflorescences and may ultimately negatively influence seed set. This is exacerbated by the fact that I could show that they do not contribute to Protea pollination. There is rising global concern about the negative impact of land transformation on natural ecosystems. With the increase in land transformation for agriculture, natural flora is replaced by intensively managed exotic crops. This has devastating effects on biodiversity and ecosystem services. Ecologically more friendly management systems are thus urgently required. One proposed such system is the production of native plants as crops, as these can provide known niche space for native organisms including beneficial ones, which may reduce required management inputs. Protea is of high ecological significance and economic value as it is harvested for export within both natural and cultivated systems in South Africa. Although mites associated with these plants can be beneficial, they are usually regarded as pests and/or organisms that pose significant phytosanitary risks. I, therefore, investigated the impact of Protea repens cultivation on the mite assemblages associated with inflorescences, infructescences (the crop products where the presence of mites pose agricultural risks) and the rhizosphere (where most of the agriculturally beneficial mite species would reside). I show that this indigenous crop may well be able to maintain a large native mite biodiversity component in all three of these niches. However, essential environmental services such as the maintenance of sound soil ecology may be hindered even with very low management intensity. Results also indicated that current intensive pest management strategies do not effectively control mites associated with inflorescences. Continued improvement of post-harvest pest management practices, as difficult as these are for sensitive and fresh produce, are urgently needed. Less reliance on intensive management systems during the production phases of Protea inflorescences would also help preserve some natural ecological processes, such as the ones discovered and described in this dissertation.
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    The role of competition and mutualism in shaping microbial communities in Protea flowers
    (Stellenbosch : Stellenbosch University, 2019-12) Mukwevho, Vuledzani Oral; Roets, Francois; Dreyer, L. L.; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Protea is a keystone member of the Cape floristic region and a crucial part of the ecological functioning of the fynbos ecosystem. These plants structurally dominate fynbos vegetation and they maintain large numbers of phytophagous organisms and pollinators. Their iconic inflorescences form the basis of a thriving cut flower industry, but this is under threat from pests and pathogens. Protea inflorescences and infructescences are also colonised by saprobic fungi that are of phytosanitary concern. These are dominated by ophiostomatoid fungi in the genera Knoxdaviesia and Sporothrix that form complex, often mutualistic, interactions with mites, pollinating insects and pollinating birds. How these fungi affect their host plants are not currently known. Also, it is unknown how they are able to dominate fungal communities within an environment optimal also for the dominance of common contaminant saprobic fungi. The fourteen described species of ophiostomatoid fungi from Protea inflorescences have well-defined host ranges and may even be associated with specific tissue types. Here I test various hypotheses related to fungal competitive abilities to explain patterns of association between ophiostomatoid fungal species, ‗environmentally acquired‘ fungal taxa, and their hosts. I showed that host chemistry partially explain host exclusivity of ophiostomatoid fungi, but that differences in the actions of spore vectors may be more important. I found that without ophiostomatoid fungi, infructuscences are dominated by ‗environmentally acquired‘ fungi such as Penicillium, Cladosporium and Fusarium. Even though the ophiostomatoid fungi are comparatively weak competitors, they are able to defend captured space against these when they colonise structures early and when they grow on their usual hosts. Although ophiostomatoid fungi do not increase numbers of viable seeds, they prevent seed release when recruitment will be suboptimal. This is because infructuscences containing ophiostomatoid fungi persist longer on plants. There is therefore mutual benefit for the association between Protea and ophiostomatoid fungi. I also uncovered complex interactions between different ophiostomatoid fungi within individual infructescences. Some species are neutral competitors and they can occupy the same tissue types within individual infructescences, while others are strong competitors on specific tissue types and can exclude competing species. Again the actions of spore vectors likely explain the persistence of weaker competitors in this scenario, but the actions of possible bacterial mutualists or other microbes should not be ignored in future studies. In this work I demonstrated the use of fungal competition studies for investigations into host relations and dispersal ecology of microbes in an atypical ecosystem, but these same techniques can be adapted to investigate associations between microbes in multiple other systems.