Invasive species is adaptable not adapted

In invasive alien species, evolutionary change can occur rapidly. Although there are several mechanisms that are believed to inhibit adaptation, e.g. low genetic variability, local adaptation of invasive alien plants to distinct habitat types has been demonstrated repeatedly. It has been suggested that invasive alien species can be used as model organisms for studying ecological and evolutionary processes in real time.

A prominent invasive alien plant species that has colonized distinct habitats in Europe over a long time period is Impatiens glandulifera. In the invaded range, this species frequently occurs in near-natural habitats, primarily in riparian habitats, fenland, mesotrophic grassland and deciduous woodland. Impatiens glandulifera is a suitable species to study local adaptation to different habitats because it is an outcrossing annual with potentially fast evolution.

A recent paper in Annals of Botany combines field and greenhouse experiments to study Impatiens glandulifera (Himalayan balsam), which has invaded distinct habitats in Central Europe that show remarkable differences in shade, soil acidity and competition. The authors find no indication of local adaptation in this study. Their results suggest that the species is coping with environmental variation by means of high phenotypic plasticity following a ‘jack-and-master’ strategy, by which it is able to maintain high fitness under a wide range of environmental conditions, but performs particularly well in favourable habitats.

Pahl, A. T., Kollmann, J., Mayer, A., & Haider, S. (2013) No evidence for local adaptation in an invasive alien plant: field and greenhouse experiments tracing a colonization sequence. Annals of botany, 112(9), 1921-1930.
Background: Local adaptation enables plant species to persist under different environmental conditions. Evolutionary change can occur rapidly in invasive annual species and has been shown to lead to local adaptation. However, the patterns and mechanisms of local adaptation in invasive species along colonization sequences are not yet understood. Thus, in this study the alien annual Impatiens glandulifera was used to investigate local adaptation to distinct habitats that have been consecutively invaded in central Europe.
Methods: A reciprocal transplant experiment was performed using 15 populations from alluvial deciduous forests, fallow meadows and coniferous upland forests, and a greenhouse experiment was performed in which plants from these habitats were grown under treatments reflecting the main habitat differentiators (shade, soil acidity, competition).
Results: Biomass production, specific leaf area, plant height and relative growth rate differed between habitats in the field experiment and between treatments in the greenhouse, but not between seed origins. Overall, there was no indication of local adaptation in either experiment.
Conclusions: Since I. glandulifera is a successful invader in many habitats without showing local adaptation, it is suggested that the species is coping with environmental variation by means of high phenotypic plasticity. The species seems to follow a ‘jack-and-master’ strategy, i.e. it is able to maintain high fitness under a wide range of environmental conditions, but performs particularly well in favourable habitats. Therefore, the proposed colonization sequence is likely to be based primarily on changes in propagule pressure. It is concluded that invasive alien plants can become dominant in distinct habitats without local adaptation.