Phenotypic plasticity and plant invasions: is it all Jack?

Phenotypic plasticity has long been suggested to facilitate biological invasions where a general purpose genotype or 'Jack-of-all-trades' strategy could facilitate invasion should native species be, on average, more specialized. Current understanding of the importance of phenotypic plasticity is limited by methodological difficulties yet glasshouse experiments, paralleling those used in evolutionary biology, are increasingly being used to assess whether invasive species have high phenotypic plasticity or not. Unfortunately, these studies have several major limitations. In general, glasshouse experiments have quantified relatively labile means of expressing phenotypes, such as plant biomass, rather than changes in plant development. As some environments favour plant growth and reproduction more than others, simply quantifying allometric or physiological responses relating to differential plant growth may not reveal much about phenotypic plasticity. Plasticity is often a comparative rather than an absolute measure. A range of comparators have been used, for example, unrelated natives, congeneric natives, other congeneric aliens or conspecifics from the source region. Thus, unlike many life-history traits used in analyses of invasion, phenotypic plasticity is strongly context-dependent and limits comparison across different studies. Phenotypic plasticity is assumed to lead to a greater breadth of environmental conditions across which a species can maintain positive population growth and increase the likelihood of invasiveness. Yet, most studies examine only a partial subset of the full environmental range experienced by the species. If the reaction norm of a target species and its comparator vary independently across an environmental gradient, this partial approach can present different interpretations of phenotypic plasticity. Rather than simply quantifying greater phenotypic plasticity in invasive species, research questions should be directed at better understanding its role in the geographic distribution, successful colonization, population persistence and/or high local abundance of invasive species in the introduced range. These issues require integrated measures of plant performance rather than crude differences in individual traits across an environmental gradient. As yet, there is only limited appreciation of the role of phenotypic plasticity in any one of these areas and there is a need to extend studies beyond glasshouse experiments.