Invasive alien plants in South Africa: how well do we understand the ecological impacts?

Introduction This account provides a brief overview of alien plant invasions in South Africa, with special emphasis on what is known about their consequences, including those affecting the delivery of ecosystem goods and services. We draw on published and unpublished sources, and highlight some important research challenges in invasion ecology that need to be met if we are to address critical gaps in our understanding. Although South Africa has problems with invasive alien organisms from most major taxonomic groups, here we deal only with alien plant invasions in natural and semi-natural terrestrial and freshwater ecosystems. Human communities and natural ecosystems worldwide are under siege from a growing number of destructive invasive alien species (including disease organisms, agricultural weeds, and insect pests). These species erode natural capital, compromise ecosystem stability, and threaten economic productivity. The problem is growing in severity and geographic extent as global trade and travel accelerate, and as human-mediated disturbance and increased dissemination of propagules makes ecosystems more susceptible to invasion by alien species. Besides their effects on agriculture, forestry and human health, biological invasions are also widely recognized as the second-largest global threat (after direct habitat destruction) to biodiversity. In many parts of the world, the most challenging and time-consuming tasks of conservation biologists and managers are those relating to controlling alien species, preventing impacts and, increasingly, repairing systems damaged by aliens. South Africa has a long history of problems with invasive alien species, and of research and management of biological invasions (Table 1). The Working for Water programme was started in 1995 to conduct and coordinate alien-plant management throughout South Africa. The programme initially worked only in watersheds and riparian areas, but now leads alien-plant management initiatives in all natural and semi-natural ecosystems. It has grown into one of the world’s biggest programmes dealing with invasive alien species. The enterprise’s success has been attributed to its multi-faceted and cross-disciplinary nature that has enabled it to leverage local and international funding and continuing political support. The programme is driven by multi-disciplinary ecological, hydrological, social and economic goals. In practice it has focused on hydrological and social concerns (as embodied in the name of the programme), and its ecological goals are less clearly defined. The extent to which the aim of improving the ecological integrity of natural ecosystems through the control of invasive alien plants has therefore not always been clear to both programme participants and other stakeholders. In this paper, we examine the ecological evidence for the impacts of invasive alien plants on South African ecosystems. We begin with a brief review of what is known about the extent of invasions and influences of these plants, and then discuss the consequences for the delivery of ecosystem goods and services to people. However, the emerging field of invasion ecology addresses issues beyond the effects of invasive species. The different aspects of invasion ecology can be related to the critical stages of invasion, and these stages also provide a useful framework for classifying the management interventions that are required to deal with the problem (Box 1). Our understanding of many of the broader aspects of invasion ecology needs to improve, and we use this framework to suggest the main challenges for research that will address critical gaps in knowledge and that will serve explicit management needs.