Integrating trait- and niche-based approaches to assess contemporary evolution in alien plant species

Two primary lines of indirect evidence for contemporary evolution in alien species are based on differences between native and introduced ranges in one or more functional traits or a shift in environmental niche. Although the integration of trait and environmental niche perspectives is increasingly recognised as a key to understanding the role of life-history evolution in range-shifting populations, there has been no attempt to bring together these perspectives on the contemporary evolution of alien plant species. We develop a set of scenarios that contrast trait-environment relationships observed in the field for alien species in the native and introduced range, and explore how they might be shaped by contemporary evolution. In each case, the limitations of uniquely trait or environmental niche perspectives are highlighted. The scenarios are examined in relation to long-term trends in covariation between temperature and first flowering date of European plant species introduced into the USA. Support for four of the scenarios is found. Field studies examining how species traits respond to environmental variation along natural gradients cannot by themselves distinguish relationships arising from genetic variation, phenotypic plasticity or genetic variation for such plasticity. This is best assessed by reciprocal transplant experiments. However, trait-environment relationships provide a basis for better targeted common garden studies that are more hypothesis driven and that pinpoint the traits of interest, ascertain the appropriate selection gradients and the range over which they need to be observed, as well as identify candidate species for further study. Synthesis. The need to improve species distribution models through a better understanding of underlying ecological and evolutionary processes makes assessments of trait-environment relationships, in both the native and introduced ranges, significant. They are of paramount importance when explaining the differential success of alien plants in novel environments as well as when predicting the potential for future range shifts following introduction. The current paucity of such comparisons represents a significant gap in our understanding of biological invasions