Researchers from the C·I·B, working with colleagues in Australia, have argued in a paper in Trends in Ecology and Evolution1 that taking a strict biogeographical definition of “invasions” is important for research and management. This is because human-mediated extra-range dispersal, unlike natural extra-range dispersal, is often the result of multiple introductions from multiple sources to multiple locations. By understanding key differences in the manner in which organisms move (and are moved) around the planet will enable us to better assist, minimise or prevent future movements of organisms.
“Invasion” is obviously not a specifically ecological term. While “biological invasions” can provide room for vivid imagery (Figure 1), as with any metaphor it has room to confuse and conflate different concepts (e.g. Chew & Laubichler, 2003; Simberloff 2003). Indeed the term invasive is often used, or understood to refer to, at least 3 separate concepts: ecological or environmental impact (i.e. a synonym for pest or weed); competitive dominance (Valéry et al. 2008); and the human-mediated dispersal of species over biogeographic barriers.
The biogeographical definition is the most important. This is because understanding the way in which species move around affects which hypotheses of invasion success are likely to operate, and what control measures will be effective (Figure 2). There are several key aspects of how species move around—propagule pressure, genetic diversity, potential for simultaneous movement of coevolved species, selectivity of what is moved, the duration of the dispersal opportunities, evolutionary distance (time since divergence) between species in the original and new ranges, and the level of human assistance provided in spread and establishment—that determine the success of organism movements both for invasions and how species respond following climate change.
The article shows how humans have moved species around throughout history. Initially hunter–gatherer and early agricultural societies moved organisms in a manner similar to natural dispersal processes. But the advent of rapid and frequent trans-continental transport has lead to an increasingly connected world. This connectivity has ecological and evolutionary consequences.
In the classic model of colonisation, a few individuals are introduced and the colonising population takes time before it becomes large. In this case, the resulting population will have gone through a genetic bottleneck and will likely contain lower levels of genetic diversity (both adaptive and neutral) than the source population. However, biological invasions are often different. Multiple sources in the invasion of the eastern daisy fleabane (Erigeron annuus) into France resulted in admixture between individuals from disparate sources, such that levels of allelic diversity and heterozygosity in the native and invaded range are similar (Genton et al. 2005). Such changes in the genetic diversity introduced can have dramatic consequences to the resulting invasions (Prentis et al. 2008).
Determining how species are moved around is obviously important for quarantine (e.g. Hulme, 2008), but details of the dispersal pathways will also affect how we manage our landscapes for many years to come.
Now, read the paper.
- Chew, M. K. & Laubichler, M. D. (2003) Natural enemies - Metaphor or misconception? Science, 301, 52-53.
- Genton, B. J., Shykoff, J. A. & Giraud, T. (2005) High genetic diversity in French invasive populations of common ragweed, Ambrosia artemisiifolia, as a result of multiple sources of introduction. Molecular Ecology, 14, 4275-4285.
- Hulme, P. E., Bacher, S., Kenis, M., Klotz, S., Kuhn, I., Minchin, D., Nentwig, W., Olenin, S., Panov, V., Pergl, J., Pysek, P., Roques, A., Sol, D., Solarz, W. & Vila, M. (2008) Grasping at the routes of biological invasions: a framework for integrating pathways into policy. Journal of Applied Ecology, 45, 403-414.
- Prentis, P. J., Wilson, J. R. U., Dormontt, E. E., Richardson, D. M. & Lowe, A. J. (2008) Adaptive evolution in invasive species. Trends in Plant Science, 13, 288–294.
- Simberloff, D. (2003) Confronting introduced species: a form of xenophobia? Biological Invasions, 5, 179-192.
- Valéry, L., Fritz, H., Lefeuvre, J. C. & Simberloff, D. (2008) In search of a real definition of the biological invasion phenomenon itself. Biological Invasions, 10, 1345-1351.
- Wilson, J. R. U., Dormontt, E. E., Prentis, P. J., Lowe, A. J. & Richardson, D. M. (2009) Something in the way you move: dispersal pathways affect invasion success. Trends in Ecology & Evolution, 24, 136–144.
- 1 Trends in Ecology & Evolution is the highest–cited journal in Ecology and Evolutionary Biology.