Plant invaders: same difference?

To paraphrase Tolstoy, is each plant invader alike, or is each invasive in its own way? Among the hundreds of papers on this subject that have been published in the last two decades, a paper by Bezeng and colleagues stands out as an unusually thorough investigation in a flora of great interest to invasive species biologists and plant ecologists in particular: southern Africa. The paper, Revisiting Darwin’s naturalization conundrum: explaining invasion success of non-native trees and shrubs in southern Africa, by Bezeng, Davies, Yessoufou, Maurin, and Van der Bank, is the Editor’s Choice for the next issue of Journal of Ecology.

Predicting which introduced species become invasive, and which don’t, is one of the prime objectives of invasion biologists, but successes have been few. Some might point out that invasive species risk assessments often have high predictive ability—a model for U.S. Great Lakes fish introductions was especially good (Kolar & Lodge 2002)—but, particularly for plant introductions, the models overly rely on the trait of ‘is the species invasive elsewhere?’, which tells us little about the ‘how’ of invasions. Although use of plant traits to statistically separate invaders from non-invaders remains a cottage industry, there exist only a handful of traits available for most species, so ecologists are increasingly looking to other types of data for inferences about what, if anything, makes invaders special. Chief among these are genetic data that allow an examination of relatedness between native species and potential invaders.

Even before the invention of modern genetics, the relatedness of invasive and non-invasive species to natives was suggested as ‘risk assessment’ tool of sorts, and by none other than Charles Darwin. Darwin made the original argument for the association of niches and common descent that we now call phylogenetic signal (Losos 2008): all else equal, 1) more closely related species should be more ecologically similar, and 2) consequently, compete more intensely than less related species. Both of these claims are today hotly contested, and as data have accumulated Darwin’s Naturalization Hypothesis has given way to ‘Darwin’s conundrum’ (Diez et al. 2008), with no emergence of general patterns of native-invader relatedness or consensus about what exactly such patterns mean with respect to competition.

Bezeng et al. don’t solve the issue of whether genetic similarity maps neatly on to ecological similarity, but they do conduct one of the largest phylogenetic analyses of native-invader contrasts to date, using a complete two-locus phylogeny across 1400 woody southern African species. Results were clear: introduced woody species that have become invasive are, on average, less related to native woodies than those that haven’t. And in an interesting twist, simple traits like maximum height, seed mass, and flowering and dispersal characteristics that have shown to be invasiveness predictors in other studies did not separate invaders from non-invaders, although some traits were significantly different for non-natives compared to natives. This supports the idea that traits promoting initial establishment are different than those that promote competitive superiority and rapid spread (Richardson & Pyšek 2012), and further suggests that successful ecological strategies of invaders needn’t be reflected in simple traits, but rather be multidimensional—and, unfortunately, difficult to quantify.

Is there hope for a general theory of species invasiveness? Studies like Bezeng et al.’s suggest there is much to be gained from large floristic contrasts of native and introduced species. But ultimately, costly, multi-faceted studies of phenotype are required to make real progress on invasion mechanisms. In the context of invasions between populations of Homo sapiens, Tolstoy wrote that we can only know that we know nothing. In the case of biological invasions, we at least know that tools like phylogenetics tell us something—but what?

Jason Fridley
Associate Editor, Journal of Ecology

References

Diez, J. M., Sullivan, J. J., Hulme, P. E., Edwards, G., & Duncan, R. P. 2008. Darwin’s naturalization conundrum: dissecting taxonomic patterns of species invasions. Ecology Letters, 11: 674-681.

Kolar, C. S., & Lodge, D. M. 2002. Ecological predictions and risk assessment for alien fishes in North America. Science 298: 1233-1236.

Losos, J. B. 2008. Phylogenetic niche conservatism, phylogenetic signal and the relationship between phylogenetic relatedness and ecological similarity among species. Ecology Letters 11: 995-1003.

Richardson, D. M. & Pyšek, P. 2012. Naturalization of introduced plants: ecological drivers of biogeographical patterns. New Phytologist 196: 383-396.