Plastic responses of alien plants to environmental change

Background/Question/Methods

It is a long-standing idea that plants with high environmental tolerance are phenotypically more plastic in physiological and morphological traits, and that these plants are more likely to establish and become invasive when introduced to a new environment.  Case studies on single highly invasive species suggest that frequently these species are indeed highly plastic. Surprisingly, however, large-scale comparative multi-species studies, including both invasive and non-invasive species, have rarely been used to test explicitly if the degree of invasiveness is linked to environmental tolerance and phenotypic plasticity. Therefore, we did two common-garden experiments and one meta-analysis of published studies.  In a first common-garden experiment, we grew 14 pairs of invasive and less invasive plants species that are all native to Europe at low and high nutrient levels and at low and high light intensity. In a second common-garden experiment, we grew 40 species consisting of confamilial groups of invasive and non-invasive alien and rare and common native species in Europe with and without competition and at low and high nutrient levels. Finally, in a meta-analysis of existing studies, including data on 211 species, we tested if global invasiveness of species is related to their biomass increase in response to increased levels of nutrients, water and light.

Results/Conclusions

In our experimental studies, both the one using European species that have been introduced elsewhere and the one using alien species introduced to Europe, we found that highly invasive species consistently produce more biomass than less invasive species under different environmental conditions. This indicates that invasive species have a higher environmental tolerance than non-invasive species. Although invasive species also tended to have higher shoot:root ratios and longer leaves, phenotypic plasticity in these traits was not larger for invasive than for non-invasive species. In the meta-analysis, we found that highly invasive species exhibited greater biomass responses to increases in resource availability. Conversely, invasive species reduced root-shoot ratios to a lesser extent than less invasive species in response to increased water and nutrient levels. In conclusion, these experiments and the meta-analysis provide strong evidence that plant species with high environmental tolerance, and particularly those that can more strongly capitalize on increased resource availability, have become more invasive. Because we did not find evidence that invasive species are more plastic in the physiological and morphological traits that we studied, it remains unclear how invasive alien species achieve high environmental tolerance.