Quantifying threats to biodiversity from invasive alien plants and other factors : a case study from the Cape Floristic Region : working for water

Nearly a third of the area of South Africa's Cape Floristic Region has been transformed by human land use and invasive alien plants. The vulnerability of remaining natural areas to transformation has been assessed, but less attention has been given to characterizing how different types of habitat transformation have affected features of natural communities. A comparison of the effects of invasive alien species with other forms of transformation, including agriculture, forestry and urbanization, can distinguish unique features of invasive alien species and inform responses to their effects. Using Bayesian hierarchical regression models, we predicted what the potential distributions of a set of species of Proteaceae would have been before transformation.We used these predictions to describe and compare the impacts of different forms of habitat transformation. Results show that: 1) agriculture is by far the most important agent of transformation, in area and in severity of species loss; 2) forestry and urbanization cause relatively high species loss where they occur; 3) invasive alien plants are widespread, but have the least severe effects on diversity where present; and 4) agriculture affects common species disproportionately, whereas forestry and invasive alien plants influence species in direct proportion to their prevalence. Invasive alien plants have thus had by many measures a smaller effect on diversity than other forms of habitat transformation. However, they may pose the greatest continuing threat to diversity and rare species if they are allowed to persist and spread to their full potential.