Doctoral Degrees (Conservation Ecology and Entomology)

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Browsing Doctoral Degrees (Conservation Ecology and Entomology) by Author "Cheney, Chad"

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    An evidence base for improving management effectiveness of alien plants in protected areas : relationships of scale, efficiency and strategy
    (Stellenbosch : Stellenbosch University, 2019-03) Cheney, Chad; Esler, Karen J.; McGeoch, Melodie A.; Foxcroft, Llewellyn C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Invasive alien plants (IAP) pose a direct threat to the biodiversity of South Africa. Extensive invasion has occurred in many of the country’s protected areas, requiring direct management. In many protected areas, including Table Mountain National Park, the selected study area for this investigation, IAP control programmes were established more than 20 years ago and are well resourced. However, limited tangible success, in terms of reduction of overall alien distribution and density has been achieved. It therefore became necessary, both from an economic and conservation perspective, to investigate the likely future trajectory of control programmes and to determine the main drivers of management success. This dissertation aims to provide a scientific rationale for improving management of IAP programmes in four key management areas. Firstly, the current accuracy of distribution and abundance data that is used in management decision making, is determined (Chapter 2). This is an important starting point for understanding management effectiveness as decisions to assign resources and treat areas are based on knowledge of IAP species presence and their associated densities in an area. Secondly, the long-term implications of suboptimal treatment quality is examined through modelling the expected density of IAP invasion after 50 years of treatment at 38 levels of clearing efficacy (Chapter 3). Thirdly, the choice of management clearing strategy is thought to play an important role in determining clearing success. Although a number of clearing strategies have been postulated by management and documented in literature, the potential outcomes of these strategies have not been formally tested. These proposed management strategies are modelled to provide insight into the performance of each strategy, also considering the mediating impact of clearing quality (Chapter 4). Last, there exists a management dichotomy between area-based and species-based planning. The shortcomings of these approaches are identified with an alternative invasion-stage-based planning approach that considers a number of scale dependent range properties offered (Chapter 5). To assess these factors, the presence and absence of all identified alien species were mapped at a fine-scale across the Table Mountain National park, producing a systematic sample of species from a total of 10,057 plots. Results indicated that management data used in decision making largely over estimate IAP distribution and abundance, while under estimating IAP species richness. Fine-scale sampling provided estimates of species richness and abundance that differed in many cases by orders of magnitude from the data that are used by managers. Currently there are adequate resources to deal with the IAP problem, but quality of work is identified as the primary driver failure to reduce alien species densities over the long-term. The modelling of treatment quality revealed that small increases in clearing efficacy above 80% result in increasingly large gains in the areas that can be covered for the same amount of resources. Conversely, any decrease in clearing efficacy below 80% results in rapidly diminishing areas that can be treated annually with the same resources. A key consequence of the current efficacy levels observed in the clearing programme, is that up to 75% of the future resource costs will be required to treat new infestations resulting from re-seeding of the current standing infestations. With increased efficacy, this future cost can be greatly reduced. The quality of clearing also mediates the choice of clearing strategy. As clearing quality increased or decreased above or below 75%, the best performing strategy changed. This highlights treatment quality as a primary driver of long-term clearing success, while the choice of implementation strategy is a secondary factor. One of the key factors identified for management improvement was the frequency for revisiting previously cleared management units for follow-up treatment. It was found that historical revisitation to management units has been greater than two years. As many of the targeted species are able to produce seeds within two years, the invasion of such species has been allowed to perpetuate. The last key finding was that through spatial analysis of population data the same species could be at different stages of invasion at different sites with the park. This result suggests that a single management approach for a species is not warranted. The choice of management approach for a species should consider range properties of occupancy, population density and spatial pattern of the species at different sites and scales. In conclusion, key improvements can be readily integrated into the IAP programme that will lead to substantive improvements in the outcomes of IAP programmes. These include improving the quality data on distribution and abundance IAP and implementing rigorous quality control. Some of the suggested interventions from this work are already being trialled, with marked improvements already visible. Through these improvements, eradication of target species by organised local scale extirpation, is possible.