A comparison of plastic responses to competition by invasive and non-invasive congeners in the commelinaceae

Evidence supporting an association between phenotypic plasticity and invasiveness across a range of plant taxa is based primarily on comparisons between invasive species and native species whose potential invasiveness is typically unknown. Comparison of invasive and non-invasive exotic species would provide a better test of whether plasticity promotes invasion. Such comparisons should distinguish between adaptive and non-adaptive plasticity because they have different consequences for invasiveness. Adaptive plasticity is expected to promote the invasion of multiple habitats, but non-adaptive plasticity may reflect specialization for invading more favorable habitats only. We grew four invasive and four non-invasive species of the Commelinaceae with and without competitors and compared their putatively adaptive plasticity of three traits related to competitive ability and non-adaptive plasticity in performance. The invasive species grew significantly more than the non-invasive species only in the non-competitive environment. The invasive species had greater plasticity of performance (total biomass) in response to competition than non-invasives, but there was no consistent difference in the plasticities of the traits related to competitive ability. These results are consistent with specialization of these invasive taxa for invading the more productive non-competitive environment rather than a superior ability to invade both competitive and non-competitive environments. A comprehensive understanding of the relationship between plasticity and invasiveness will require many more comparisons of the plasticity of invasive and non-invasive taxa in a range of traits in response to a variety of environments.