Ecologists have been speculating about the ability of exotic species to invade new habitats since Darwin (1859) proposed his naturalization hypothesis in The Origin of Species. Readers of Journal of Ecology are likely to be familiar with many of the important studies we publish involving invasive species and efforts to explain their introduction, establishment and geographic spread (e.g. Hierro et al. 2005; Moles et al. 2008). The current issue of Journal of Ecology carries new studies in these areas (see paper by Hejda et al. 2009) and, in particular, the paper by Mark van Kleunen and Markus Fischer, which caught my eye because it follows up an earlier paper seeking to understand the ‘American Invasion’ of North American species into Europe (Bucharova & van Kleunen 2009). In their paper entitled “Release from foliar and floral fungal pathogen species does not explain the geographic spread of naturalized North American plants in Europe”, van Kleunen and Fischer test one of the most commonly-cited hypotheses to explain invasive species success, namely the enemy-release hypothesis. This hypothesis suggests that the success and spread of invasive species in a new habitat is due to release from limitations imposed by pathogens, herbivores or other enemies. The idea is simply that the enemies were left behind in the old, native habitat and pose no threat to the species in the new habitat. Without these enemies around, the invasive species can, it is argued, readily establish, spread and generally run amok.
Despite its popularity, there are few appropriate tests of the enemy-release hypothesis. In their study, van Kleunen and Fischer provide such a test by examining databases which provide information about the occurrence of North American species that have invaded Europe and the fungal pathogens that infect them. Their findings provide some support for the enemy release hypothesis, although it is incomplete. They found that in comparison to data from the plants’ native North American ranges, 58.0% fewer fungal pathogen species were recorded on the 140 North American species that have naturalized in Europe. This is less than the 84% release reported in an earlier study for European species naturalized in North America (Mitchell & Power 2003). However, when van Kleunen and Fischer accounted for sampling biases and the introduction of fungal pathogens in their data, the extent of release was reduced from 58.0% to only 10.3%. Additionally, their analyses showed that enemy release does not apply to all types of enemies; there was significant release from smut and rust fungi, but not from mildews. There was also higher release from rare pathogens compared with common pathogens. This latter observation suggests that any benefit of enemy release may be short-lived if rare pathogens become more common as the invasive plant host spreads.
An important prediction of the enemy-release hypothesis is that the spread of introduced species is facilitated in the absence of native enemies in the introduced range. van Kleunen and Fischer’s survey did not support this prediction. Rather, they found the opposite trend, leading them to conclude that release from foliar and floral pathogens does not explain geographic spread of non-native species from North America in Europe. They suggest a trade-off between the high expansion capacity of some species and their ability to defend themselves against enemies.
Overall, this paper is important because it uses the most recent data available to test one of the key hypotheses proposed to help explain species invasions. The findings show that the enemy-release hypothesis only provides a partial explanation for species invasions, and, as with many things, one size does not fit all. In the case of release from pathogens, the enemy-release hypothesis may apply to some, but not all, plant-fungal interactions. The study does not address the other types of enemy that invasive species may or may not leave behind in their native habitat, such as invertebrate herbivores (MacKay & Kotanen 2008). Further work is needed to investigate these interactions and other invasive species hypotheses including the empty niche, novel weapons and evolution of invasiveness hypotheses (Hierro et al. 2005), and even Darwin’s naturalization hypothesis (Darwin 1859; Duncan & Williams 2002). We look forward to publishing studies addressing these topics in Journal of Ecology.
Editor, Journal of Ecology
Bucharova, A. & van Kleunen, M. (2009). Introduction history and species characteristics partly explain naturalization success of North American woody species in Europe. Journal of Ecology, 97, 230-238.
Darwin, C.R. (1859). On the Origin of Species. Murray, London.
Duncan, R.P. & Williams, P.A. (2002). Ecology: Darwin’s naturalization hypothesis challenged. Nature, 417, 618-619.
Hejda, M. Pyšek, P., & Jarošík, V. (2009). Impact of invasive plants on the species richness, diversity and composition of invaded communities. Journal of Ecology, 97, 393–403.
Hierro, J.L., Maron, J.L. & Callaway, R.M. (2005). A biogeographical approach to plant invasions: the importance of studying exotics in their introduced and native range. Journal of Ecology, 93, 5-15.
Mackay, J. & Kotanen, P.M. (2008). Local escape of an invasive plant, common ragweed (Ambrosia artemisiifolia L.), from above-ground and below-ground enemies in its native area. Journal of Ecology, 96, 1152-1161.
Mitchell, C.E. & Power, A.G. (2003) Release of invasive plants from fungal and viral pathogens. Nature, 421, 625-627.
Moles, A.T., Gruber, M.A.M., & Bonser, S.P. (2008) A new framework for predicting invasive plant species. Journal of Ecology, 96, 13-17.
van Kleunen, M. & Fischer, M. (2009). Release from foliar and floral fungal pathogen species does not explain the geographic spread of naturalized North American plants in Europe. Journal of Ecology, 97, 385–392.
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