Opinion
A proposed unified framework for biological invasions

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There has been a dramatic growth in research on biological invasions over the past 20 years, but a mature understanding of the field has been hampered because invasion biologists concerned with different taxa and different environments have largely adopted different model frameworks for the invasion process, resulting in a confusing range of concepts, terms and definitions. In this review, we propose a unified framework for biological invasions that reconciles and integrates the key features of the most commonly used invasion frameworks into a single conceptual model that can be applied to all human-mediated invasions. The unified framework combines previous stage-based and barrier models, and provides a terminology and categorisation for populations at different points in the invasion process.

Section snippets

An allopatric model for the diversification of invasion biology

The past two decades have seen an explosion of research interest on human-mediated invasions; that is, invasions by species that are not naturally present in a native assemblage, but have been moved beyond the limits of their normal geographic ranges by human actions. This explosion has, in turn, resulted in substantial development of understanding of the invasion process (e.g. 1, 2, 3, 4, 5). Nevertheless, as the science of invasion biology has grown, some significant anomalies have hindered

Similarities and differences

For it to be possible to merge the Richardson and Williamson frameworks, they must describe the same process. Given that all species, regardless of taxon, have the potential to progress from native to alien invader (although most never reach that endpoint), with the definitions of native and alien invader being applicable to all species, this is certainly true. It follows that the main difference between the two schemes resides in how this common process is described. In essence, the Williamson

Unifying the frameworks

Figure 1 proposes a unified framework for invasion biology that combines the key elements of the Williamson and Richardson schemes, as well as new insights that arise from the process of combining the two. This framework is designed to apply to all human-mediated invasions. As such, the framework includes elements that we would not expect to apply to natural dispersal, or to range expansions occurring indirectly as a result of human activities, such as habitat modification. However, it can help

Further advantages of the proposed framework

A significant advantage of the proposed framework presented in Figure 1 is that it also incorporates important elements of other frameworks that previously have not been integrated with the Williamson and/or Richardson schemes. Thus, the unified framework includes the framework for management interventions proposed by Pyšek and Richardson [32], and (with minor alterations) the terminological framework proposed by Richardson et al. [8]. Inclusion of the latter should help to eliminate much of

Conclusion

The exponential growth of interest in biological invasions has been driven by independent growth in parallel research programs, divided largely along taxonomic [50] and habitat lines (J.T. Carlton and A.M.H. Blakeslee, personal communication). Yet, these different programs share common issues, in terms of the process of invasion, and common consequences, in terms of the ecological and economic impacts of a failure to stem the tide of invaders. There is thus a clear need in invasion biology for

Acknowledgements

TMB thanks the Centre for Invasion Biology, University of Stellenbosch, for hospitality and travel costs. PP and VJ were supported by the projects no. 206/09/0563 (Czech Science Foundation), AV0Z60050516 (Academy of Sciences of the Czech Republic), MSM0021620828 and LC06073 (Ministry of Education of the Czech Republic). PP also acknowledges support from the Praemium Academiae award from the Academy of Sciences of the Czech Republic. JRUW and DMR acknowledge support from the DST-NRF Centre of

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