Assessing the impacts of invasive alien plants on urban ecosystem services

[摘要] ENGLISH ABSTRACT: The sustainable provision of ecosystem services (ES) is a critical issue in the ongoing and evolving urbanization process that is placing enormous pressures on natural resources within and around urban landscapes. Urban ecosystems provide multiple ES for human well-being, but they can also generate functions, processes and attributes that result in perceived or actual negative impacts on human well-being, termed ecosystem disservices (EDS). Understanding the ES-EDS dichotomy within the context of the urban landscape is important for promoting the development of resilient and sustainable cities.Many alien plants introduced to urban areas around the world specifically to create, augment or restore key ES, are now firmly embedded in urban landscapes and have complex social and economic ties. Urban areas are particularly susceptible to invasion by alien plant species and some have subsequently spread beyond original plantings and have become invasive, causing negative effects on existing ES or creating novel ES or EDS. Complex interactions between humans and alien plants allow these impacts to manifest in many ways. Managing urban plant invasions is particularly challenging given the complex interactions between ecological, economic and social elements that exist in the urban milieu. Management decisions need to give explicit and transparent consideration to divergent stakeholder perceptions to mitigate potential conflicts of interest around alien and invasive plants and their management. Additionally, practical approaches for integrating ES and EDS in the decision-making process are needed to guide urban managers and planners. Given the limited resources available for managing invasive alien plants (IAPs), efficient prioritization of areas for IAP control is crucial for effective land management and ecological restoration.A literature review was conducted to determine the role of alien plants in providing urban ES and EDS globally (Chapter 2). The city of Cape Town was then used as a study system to elucidate the complex interactions between urban residents and managers, and alien and invasive plants in a unique ecological, political and social setting. Questionnaire-based surveys were conducted across the city to determine the perceptions of urban residents regarding IAPs and the capacity of IAPs to provide ES and EDS (Chapter 3). A multi-criterion decision-making analysis (using the Analytic Hierarchy Process approach) was then performed for the city to develop a prioritisation framework for managing areas invaded by alien plants at the city/landscape and at local scales (Chapter 4). The impact of IAPs on safety and security was assessed by exploring the link between urban vegetation and criminal activity through questionnaire-based surveys, analysing criminal court cases, and searching the grey literature (Chapter 5). Finally, an approach, merging several available tools and techniques (such as remote sensing), was developed to assess the role of urban vegetation (including alien and invasive plants) in providing ES and EDS at a local-scale urban context (Chapter 6).A small number of alien plant taxa were recorded as providing multiple ES in many urban ecosystems around the world. Some of these species also generate significant EDS (Chapter 2). IAPs and their management are perceived both negatively and positively by urban residents - these perceptions were shaped by socio-demographic characteristics of individuals (Chapter 3). Factors related to safety and security (such as fire risk to infrastructure) emerged as key features for setting spatially-explicit priorities for IAP management (Chapter 4). The association between vegetation and criminal activity may not always be determined by the biogeographical status of dominant plants (i.e. whether vegetation is dominated by native, alien, or invasive alien species), but rather on the structure they provide. A stronger link between crime and vegetation is likely to occur in areas where plant invasions have drastically altered vegetation structure, as is the case in the Cape Floristic Region (Chapter 5). At the local scale, nodes of high ES provision such as residential gardens and urban green spaces are characterized by the presence of large trees. However, many of these areas also experience 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 natural vegetation. This suggests significant trade-offs regarding the management of species and the ES and EDS they provide (Chapter 6).Using and developing new approaches across disciplines and spatial scales, this thesis provides new insights on the role of alien and invasive plants in providing ES and EDS in urban areas. It highlights the need to integrate public perceptions into the planning and management of IAPs in urban areas to alleviate potential conflicts of interest; it also emphasises the importance of including ES assessments in the decision-making process. The findings will be useful for managers in all urban settings to guide the selection and prioritization of land parcels for IAP management at multiple spatial scales to maximise the provision of ES.

[发布机构] Stellenbosch University