What makes a good invasive species – genetics or plasticity? Insights from Impatiens glandulifera

In a time dominated by discussions of the threats to many plant and animal species, it is easy to forget that some plant species have thrived in recent times. This includes many invasive plant species. Invasive plant species often grow quickly in new environments, with high reproductive output compared to their native range. Expansion in growth area means that invasive species can relatively quickly find themselves growing across a variety of latitudes and different environments.

Across different latitudes, intraspecific variation (variation within a species) in traits such as flowering onset time, plant size and biomass has been recorded for several invasive species. Whether this variation arises from intraspecific genetic diversity or phenotypic plasticity (the ability of plants to heritably vary their traits in response to environmental factors). In their recent paper in Annals of Botany, Kenny Helsen and colleagues address this and other questions about how invasive species adapt to new environments through a study of the annual plant Impatiens glandulifera Royle, which is invasive to Europe.

The authors sampled Impatiens glandulifera seeds from a variety of European locations across different latitudes and grew them in controlled greenhouse conditions for two generations. Helsen and colleagues find that plant height and biomass decreased with increased latitude, and flowering began earlier in plants from higher latitudes. These patterns remained consistent across the two generations grown in greenhouse conditions, indicating that they are caused by genetic diversity rather than environmentally-responsive plasticity.

This raises the question of whether the genetic diversity was directly a consequence of the different latitudes of the source populations. Recorded variation in the traits at different latitudes indicated that the variation in these traits is not entirely driven by latitude itself, suggesting that other local factors in the source population also influence the genetic diversity underpinning trait variation.

The fact that Impatiens glandulifera plants have reduced height and biomass at increased latitudes and are influenced by local environment factors supports the long-running stress-gradient hypothesis, which predicts that species at higher latitudes experience less competition and so can afford to invest less in vegetative growth. Helsen and colleagues also measured how reproductive traits (seed mass and seed mass per individual) varied with source population latitude, and found no trends of these with latitude. So it would seem that invasive species need to keep up their reproductive traits at whatever latitude they are growing, but can change their vegetative traits according to the environment they experience at different latitudes.

The lack of co-variation between vegetative and reproductive traits indicates that these are ‘genetically de-coupled’ in invasive Impatiens glandulifera. Helsen and colleagues point out that genetic de-coupling of traits has been found to occur in species with high rates of sexual recombination, which is compatible with Impatiens glandulifera being an annual plant species, and with other aspects of its reproductive development. In this study of invasive Impatiens glandulifera, it therefore seems that genetics rule.

Lead author Kenny Helsen told Botany One: ‘First, from an invasion perspective, our study provides some new insight into the different strategies Impatiens glandulifera adopts along its growth gradient to secure its high fitness and invasiveness. These insights will hopefully help us to better understand both invasiveness and potential for eradication of problematic invasive species. Secondly, our study is of interest for evolutionary biologists, as it shows that adaptation patterns along latitudinal (or altitudinal) gradients might be responding to several selective drivers simultaneously, thus resulting in more complex trait patterns than only anticipated by allocation theory’.