Weed adaptation

Oxford ragwort

Invasive species undergo behavioural, phenological and morphological changes when confronted with novel conditions. However, we know remarkably little about the trajectories invasive species follow through time. This limits our understanding of the process of invasion, our power to predict the establishment and ecological effects of invasive species, and our understanding of how species respond to environmental changes. The main aim of this paper is to fill this knowledge gap, by quantifying phenotypic change in three plant species through their first 200 years since introduction.

I stole this abstract from a new study which is likely the first to have tracked the phenotypic change of introduced plant species from the beginning of their invasion to the present day. This was only made possible by the centuries-old  tradition of storing plant specimens in herbaria and the collecting obsessions of many individuals - not only professionals.

The international research team looked at three common weeds: Oxford ragwort (Senecio squalidus), winter speedwell (Veronica persica) and a willow herb (Epilobium ciliatum) all of which were introduced to the UK between 120 and 220 years ago. Oxford ragwort was introduced into the UK from Sicily and was first recorded in the wild in 1794. This yellow daisy has spread widely along the railway lines of Britain. 

Winter speedwell is native to Eurasia and was first recorded in the UK in 1826. The willow herb Epilobium ciliatum is native to the Americas but was first recorded in in the UK in 1891.

The researchers found the weeds are getting increasingly better adapted to life in their new environment, so they will perhaps become even more problematic invaders as time goes on. The team measured changes throughout the centuries in leaf shape, leaf area and plant height, features which reflect how plants adapt to new water, nutrient and light conditions.

The Oxford ragwort underwent about a 20 per cent increase in both leaf area and plant height since its introduction. The leaves of the winter speedwell became rounder and 17% smaller, while plant height increased by 14 %. And the willow herb showed a 50 % decrease in leaf area. All three invasive species showed evidence of change at least in one trait during the last 50 years. Changes in the species' traits seemed to happen in spurts: changes are not in a consistent progression, but rather fluctuate through time.

from geochembio.com

The colleagues are well aware of the fact that the changes and patterns they observed could be due to phenotypic plasticity with little impact on the genotypic variation of the three species. Other plants such as dandelions are textbook examples for the ability of an organism to change  only its phenotype in response to changes in the environment. The only way to prove that the observed changes have a genetic basis is to use common garden experiments, and this approach does not allow researchers to follow the trajectory of change through time.

The authors conclude: Our results suggest that some invasive species are yet to demonstrate their full potential as invaders. Overall, our study shows that species are labile in the face of environmental change. Identifying the long term trajectories of invasive species’ phenotypic change during invasion provides important clues for their appropriate management.