"Alien plant invasions and native plant extinctions: a six-threshold framework"
presented by Professor David M. Richardson
Biological invasions are widely acknowledged as a major threat to global biodiversity. Species from all major taxonomic groups have become invasive. The range of impacts of invasive taxa and the overall magnitude of the threat is increasing. Plants comprise the biggest and best-studied group of invasive species. There is, however, a growing debate regarding the nature of the alien plant threat—in particular whether the outcome is likely to be the widespread extinction of native plant species. The debate has raised questions on whether the threat posed by invasive plants to native plants has been overstated.
Professor Richardson will discuss a conceptual framework that proposed in a recent paper by Downey & Richardson to guide discussion on this topic, in which the threat posed by invasive plants is considered in the context of a progression from no impact through to extinction. The framework defines six thresholds along the ‘extinction trajectory’, global extinction being the final threshold. Although there are no documented examples of either ‘in the wild’ or global extinctions of native plants that are attributable solely to plant invasions, there is evidence that native plants have crossed or breached other thresholds along the extinction trajectory due to the impacts associated with plant invasions. Several factors may be masking where native species are on the trajectory; these include a lack of appropriate data to accurately map the position of species on the trajectory, the timeframe required to definitively state that extinctions have occurred and management interventions. Such interventions are likely to alter the extinction trajectory of some species. The critical issue for conservation managers is the trend, because interventions must be implemented before extinctions occur. The lack of evidence for extinctions attributable to plant invasions does not mean we should disregard the broader threat.
David M. Richardson is a Distinguished Professor of Ecology and the Director of the DST-NRF Centre of Excellence for Invasion Biology, based at Stellenbosch University, South Africa. His research focuses mainly on plant invasions, especially trees and shrubs. He is interested in the biogeography, ecology and management of invasions and in conservation biogeography in general. Richardson has published more than 350 papers, edited or co-edited 6 books and has received several awards, including the Hans Sigrist Prize in 2006 and the 2013 John F.W. Herschel Medal from the Royal Society of South Africa. He served as Editor-in-Chief of the journal Diversity and Distributions between 1998 and 2015 and is Associate Editor of several other journals, including Biological Invasions and Neobiota.