Browsing by Subject "Biological invasions"

Now showing 1 - 4 of 4
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    Open Access
    A fine-scale assessment of the ecosystem service-disservice dichotomy in the context of urban ecosystems affected by alien plant invasions
    (2019-10-28) Potgieter, Luke J; Gaertner, Mirijam; O’Farrell, Patrick J; Richardson, David M
    Abstract Background Natural resources within and around urban landscapes are under increasing pressure from ongoing urbanisation, and management efforts aimed at ensuring the sustainable provision of ecosystem services (ES) are an important response. Given the limited resources available for assessing urban ES in many cities, practical approaches for integrating ES in decision-making process are needed. Methods We apply remote sensing techniques (integrating LiDAR data with high-resolution multispectral imagery) and combined these with supplementary spatial data to develop a replicable approach for assessing the role of urban vegetation (including invasive alien plants) in providing ES and ecosystem disservices (EDS). We identify areas denoting potential management trade-offs based on the spatial distribution of ES and EDS using a local-scale case study in the city of Cape Town, South Africa. Situated within a global biodiversity hotspot, Cape Town must contend with widespread invasions of alien plants (especially trees and shrubs) along with complex socio-political challenges. This represents a useful system to examine the challenges in managing ES and EDS in the context of urban plant invasions. Results Areas of high ES provision (for example carbon sequestration, shade and visual amenity) are characterized by the presence of large trees. However, many of these areas also result in numerous EDS due to invasions of alien trees and shrubs – particularly along rivers, in wetlands and along the urban edge where tall alien trees have established and spread into the natural vegetation (for example increased water consumption, increased fire risk and reduced soil quality). This suggests significant trade-offs regarding the management of species and the ES and EDS they provide. Conclusions The approach applied here can be used to provide recommendations and to guide city planners and managers to fine-tune management interventions at local scales to maximise the provision of ES.
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    Current patterns of habitat transformation and future threats to biodiversity in terrestrial ecosystems of the Cape Floristic Region, South Africa
    (2003) Rouget, Mathieu; Richardson, David M; Cowling, Richard M; Lloyd, J Wendy; Lombard, Amanda T
    The formulation of an effective strategic plan for biodiversity conservation in the Cape Floristic Region (CFR) requires an assessment of the current situation with regard to habitat transformation, and an explicit framework for predicting the likelihood of remaining habitat (i.e. that potentially available for conservation) being transformed. This paper presents the results of a detailed assessment of the current and future extent of three important factors that threaten biodiversity in the CFR: cultivation for intensive agriculture (including commercial forestry plantations), urbanisation, and stands of invasive (self-sown) alien trees and shrubs. The extent of habitat transformation was mapped at the scale of 1:250,000, using primarily satellite imagery. We compared models derived from a rule-based approach relying on expert knowledge and a regression-tree technique to identify other areas likely to be affected by these factors in future. Cultivation for agriculture has transformed 25.9% of the CFR and dense stands of woody alien plants and urban areas each cover 1.6%. Both models predict that at least 30% of the currently remaining natural vegetation could be transformed within 20 years. There was an overall accuracy of 73% between both models although significant differences were found for some habitat types. Spatial predictions of future agriculture threats derived from the rule-based approach were overestimated relative to the statistical approach, whereas future alien spread was underestimated. Threat assessment was used to derive conservation targets for subsequent stages of conservation planning for the CFR. The importance of integrating vulnerability knowledge into conservation planning is discussed. The choice of vulnerability analysis (future habitat degradation and/or impact on biological entities) and methods will depend on the complexity of the threatening processes and the availability of spatial data.
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    Open Access
    The ecology of invasions by Pinus (pinaceae) and Hakea (proteaceae) species, with special emphasis on patterns, processes and consequences of invasion in mountain fynbos of the southwestern Cape Province, South Africa
    (1989) Richardson, David Mark; Cowling, Richard M
    The fire-prone mountain fynbos of the southwestern Cape Province of South Africa has been severely invaded by introduced trees and shrubs. These invasions have transformed fynbos shrublands to dense thickets of trees and shrubs in many parts of the region, thus disrupting various natural ecosystem processes. The ecology of invasions by species of Pinus and Hakea (the most successful genera) was studied using a series of natural experiments in conjunction with autecological studies. The study was divided into main four parts: (i) case studies to elucidate major patterns and processes of invasion; (ii) studies of the life history and population ecology of selected invaders; (iii) studies of the determinants of invasibility; and (iv) assessments of the consequences of invasion and of control programmes. Bibliography: pages 235-272.
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    Open Access
    Spontaneous succession of riparian fynbos: Is unassisted recovery a viable restoration strategy?
    (2008) Reinecke, M K; Pigot, A L; King, J M
    The invasion of alien trees is a major threat to the freshwater resources and biodiversity of South Africa. The Working for Water (WfW) Program was initiated in 1995 in order to control the growth and spread of woody alien species in riparian zones, but the extent to which the indigenous vegetation naturally recovers following alien clearance remains poorly understood. In this study spontaneous succession of riparian vegetation following wild fires and alien clearing was monitored over a number of years at two sites on the Cape Peninsula: a pine plantation in an upland plateau and an Acacia spp.-invaded valley floodplain. After clearing, the vegetation at the pine site was successfully recovering along a trajectory towards Afromontane forest and as a result it is suggested that no active restoration is required. By contrast, our results show that areas cleared of Acacia spp. may be less resilient, with extensive regeneration of woody aliens and only a negligible recovery of indigenous trees. We propose that the absence of riparian trees may have been responsible for precipitating the transition to a community dominated by weedy nitrophilous grasses, and find evidence that this may be perpetuated through the continued removal of Acacia spp. It is hypothesized that this grass-dominated state may be resilient to natural restoration and thus represents an additional constraint to the recovery of riparian communities. Under such circumstances, we argue that active restoration would be required in order to re-instate the riparian community.