In the beginning: phenotypic change in three invasive species through their first two centuries since introduction

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Previous studies have demonstrated that most introduced species go through rapid phenotypic change during their first decades to centuries of being introduced to a new range. However, little is known about the trends these phenotypic changes follow through time. Using herbarium specimens we track changes in the leaf area, leaf shape and plant height of three species (Epilobium ciliatum Raf., Senecio squalidus L. and Veronica persica Poir.) through their first ~200 years since introduction to the United Kingdom. All three species showed fluctuating direction and strength of phenotypic change through time. Then, we asked whether invasive species showed an initial lag phase in phenotypic change. Contrary to some expectations, we found that none of our species underwent a lag phase for the first 100 years after introduction. This is probably because the strongest selective pressures are exerted on them at the very beginning of their invasion, when the mismatch between their traits and the optimum for the new environment is greatest. Finally, we asked whether invasive species were still changing, or whether they had reached a new equilibrium, finding that all three species were still changing in at least one trait more than a century after their introduction. 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.