Besides a general consensus regarding the negative impact of invasive alien species in the literature, only recently has the decline of native species attributable to biological invasions begun to be quantified in many parts of the world. The cause-effect relationship between the establishment and proliferation of alien species and the extinction of native species is, however, seldom demonstrated. We conducted a meta-analysis of studies in Mediterranean-type ecosystems (MTEs) to examine: (1) whether invasion of alien plant species indeed causes a reduction in the number of native plant species at different spatial and temporal scales; (2) which growth forms, habitat types and areas are most affected by invasions; and (3) which taxa are most responsible for native species richness declines. Our results confirm a significant decline in native species richness attributable to alien invasions. Studies conducted at small scales or sampled over long periods reveal stronger impacts of alien invasion than those at large spatial scales and over short periods. Alien species from regions with similar climates have much stronger impacts, with the native species richness in South Africa and Australia declining significantly more post-invasion than for European sites. Australian Acacia species in South Africa accounted for the most significant declines in native species richness. Among the different growth forms of alien plants, annual herbs, trees and creepers had the greatest impact, whereas graminoids generally caused insignificant changes to the native community. Native species richness of shrublands, old fields and dune vegetation showed significant declines, in contrast to insignificant declines for forest habitats.

Aim The biophysical impacts of invasive Australian acacias and their effects on ecosystem services are explored and used to develop a framework for improved restoration practices.Location South Africa, Portugal and Chile.Methods A conceptual model of ecosystem responses to the increasing severity (density and duration) of invasions was developed from the literature and our knowledge of how these impacts affect options for restoration. Case studies are used to identify similarities and differences between three regions severely affected by invasions of Australian acacias: Acacia dealbata in Chile, Acacia longifolia in Portugal and Acacia saligna in South Africa.Results Australian acacias have a wide range of impacts on ecosystems that increase with time and disturbance, transform ecosystems and alter and reduce ecosystem service delivery. A shared trait is the accumulation of massive seed banks, which enables them to become dominant after disturbances. Ecosystem trajectories and recovery potential suggest that there are important thresholds in ecosystem state and resilience. When these are crossed, options for restoration are radically altered; in many cases, autogenic (self-driven and self-sustaining) recovery to a pre-invasion condition is inhibited, necessitating active intervention to restore composition and function.Main conclusions The conceptual model demonstrates the degree, nature and reversibility of ecosystem degradation and identifies key actions needed to restore ecosystems to desired states. Control and restoration operations, particularly active restoration, require substantial short-to medium-term investments, which can reduce losses of biodiversity and ecosystem services, and the costs to society in the long term. Increasing restoration effectiveness will require further research into linkages between impacts and restoration. This research should involve scientists, practitioners and managers engaged in invasive plant control and restoration programmes, together with society as both the investors in, and beneficiaries of, more effective restoration.

Despite intensive research on effects of alien species during the past decade, invasion science still lacks capacity to predict impacts and thus provide timely advice to managers on where limited resources should be allocated. This capacity has been limited in part by the context-dependent nature of impacts, research highly skewed toward certain taxa and habitat types, and the lack of standardized methods for observing and quantifying impact. We review different strategies, including specific experimental and observational approaches, for detecting and quantifying ecological impacts of alien species. These include a four-way experimental plot design that allows comparing impact studies of different organisms. Furthermore, we identify hypothesis-driven parameters that should be measured at invaded sites to maximize insights into the nature of impact. We also present strategies for recognizing highimpact species. Our recommendations provide a basis for developing systematic quantitative measurements to allow comparisons of impact across alien species, sites and time.