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Quantifying levels of biological invasion: towards objective classification of invaded and invasible ecosystems

Biological invasions are a global phenomenon that threatens biodiversity, and few, if any, ecosystems are free from alien species. The relative “success” of invasion is influenced by inherent features of the species (invasiveness) and recipient ecosystems (invasibility), and factors associated with human activities.

Considerable effort has been made to define, characterize and identify species that are invasive, but much less attention has been devoted to quantifying and characterizing invasibility. In a recent paper published in Global Change Biology, Jane Catford, a postdoctoral researcher from the University of Melbourne in Australia and C•I•B core team member Dave Richardson, along with colleagues Peter Vesk and Petr Pyšek, argue that a prerequisite for characterizing invasibility is the ability to compare levels of invasion across ecosystems. Invasion level refers to the extent or severity of alien invasion observed in an ecosystem.

A growing number of studies examine invasion levels among ecosystems and regions, but the metrics used are often not directly comparable (e.g. alien species richness vs. relative alien species cover). If two ecosystems are invaded by a different suite of species, how can management resources be prioritized objectively if the net impact of invasion on the ecosystems is unknown? If Ecosystem A has twice as many native species as alien species, but alien species make up half the total plant cover abundance, is it more severely invaded than Ecosystem B that has the same numbers of native and alien species, but where alien cover is only 25% of total cover? Identifying standard, comparable metrics will help with such decisions. In the paper, the authors identify the best way to quantify the level of invasion by non-native animals and plants by reviewing the advantages and disadvantages of different metrics. They also explore how interpretation and the choice of these measures can depend on the objective of a study or management intervention. Based on the review, they recommend two invasion indices: relative alien species richness and relative alien species abundance. When the relative cover of alien species exceeds relative alien richness (Fig. 1), this could be considered cause for concern. The authors illustrate the use of these two metrics by applying them to two case studies (e.g., Fig. 2). Establishing a standard metric of invasion is not only essential for determining invasibility, but has inherent value. For example, “Invasion level” scores could be used to gauge the following:

  • the ecological consequences of invasion, e.g. biotic homogenization, competition with native species, alteration of ecosystem structure and disruption of ecosystem function, changes to biotic interactions and ecological networks like pollination and dispersal;
  • economic costs of invasion, e.g. loss of ecosystem services, reduced agricultural production, cost of control interventions; and
  • the potential for control, eradication and recovery post-invasion. The authors also discuss how invasion level can be used to gauge invasibility by accounting for propagule pressure and invader traits.

Establishing standard, transparent ways to define and quantify invasion level will facilitate meaningful comparisons among studies, ecosystem types and regions, and it provides a necessary step towards determining the invasibility of ecosystems.