It is hypothesized that invasive species are doing something fundamentally different in their introduced ranges – they seem to grow faster and larger, spread more aggressively, and outcompete native species, lowering biodiversity. However, these assumptions have surprisingly little evidence to back them up. In my last post, I wrote about Powell et al. 2013’s article, that found invaders may not be as bad for native biodiversity as we think. In this post, I’m presenting an article by Parker et al. 2013, looking at the assumption that invaders really are better performers in their introduced ranges.
Many hypotheses for the success of invasive species are based on altered ecological or evolutionary pressures driving increased performance in the invaded range: enemy release, evolved increased competitive ability, novel weapons, invasional meltdown, and the list goes on. It seems intuitive that species in a new community context will experience either reduced or increased interaction strengths with competitors, diseased, predators, and mutualists. However, it could be that invasiveness is instead determined by traits – indeed many invaders seem to share high fecundity, rapid growth, and long distance dispersal. In this case, the traits that help a species perform well in the native range would be the same as those in the introduced, and we would not expect to see performance differences between the two ranges.
Therefore, if we find that invaders have higher performance in their introduced ranges, that would be good evidence for an evolutionary or ecological driver behind invasiveness [(b) in figure]. If, however, invaders are similar in size and performance in their introduced ranges, it could just be driven by inherent traits that help them perform well in both ranges [(a) in figure].
What they did: Parker et al. performed a meta-analysis of 221 studies looking at performance (individual size, fitness, and population abundance) of invasive species in their native and introduced ranges. They found data for 53 species across plants and animals, 36 listed among the “worlds worst” invaders. Along with performance data, they also collected information on whether the introduction was intentional or accidental (people may have selected for increased performance in intentional introductions), and the time since introduction (invaders may evolve increased performance over time).
What they found: Their findings are straight forward – on average, they found that invaders do indeed outperform their conspecifics in the native range. However, at the species level, this pattern was only true for about 50% of species. That means, that while some invaders are larger, more fecund, and grow denser in their introduced ranges, about half do not. They did not find any differences between mode of introduction or time since introduction.
Why it’s cool: This paper provides strong evidence to support an old assumption – that invasive species perform better in their introduced ranges. It also points out the very important distinction that, while some species do perform better, it is not a feature shared by all invasives. It would be interesting to compare species that fell into the two categories: species that perform better in their introduced range would be those predicted to be experiencing altered ecological and evolutionary pressures. Those that do not may share a common set of traits that lead to invasiveness. These might suggest two pathways by which we could predict and prevent future invasions.
Parker, J. D., M. E. Torchin, et al. (2013). Do invasive species perform better in their new ranges? Ecology 94(5): 985-994.