Jennifer L. Bufford and Philip E. Hulme
Bio-Protection Research Centre, Lincoln University, PO Box 85084, Lincoln 7647, New Zealand
Phenotypic plasticity, a change in a plant’s expressed phenotype in response to the environment, has the potential to promote alien plant invasions. However, not all trait variation is beneficial for maintaining fitness across environmental gradients and therefore measuring trait change alone can be misleading. We develop a conceptual model of the relationship between trait plasticity and fitness homeostasis to distinguish between adaptive plasticity, which should allow plants to maintain homeostatic fitness across a wide range of environmental conditions, from passive plasticity, which is not beneficial and may reflect ontogenetic or developmental constraints. We suggest that fitness homeostasis, rather than plasticity per se, is likely to underpin establishment success and increase spread and range size in alien plants. Therefore, we propose that studies of phenotypic plasticity in invasions need to focus primarily on fitness outcomes and not only on trait change across realistic environmental gradients. We present four scenarios that describe the possible relationships between traits and fitness across an environmental gradient and discuss what each reveals about adaptive and passive plasticity. We demonstrate these scenarios along with an analytical approach to connecting traits and fitness using data on alien plant traits and growth rates from an experimental soil moisture gradient. We also discuss the selection of environmental gradients and traits and argue that our understanding of plasticity will improve with a greater emphasis on multiple points along a relevant environmental gradient, but with analyses limited to traits with clear a priori implications for fitness along the gradient considered. By evaluating phenotypic plasticity in the light of fitness homeostasis, we can focus on adaptive plasticity to more clearly and accurately elucidate the role of adaptive trait change and environmental heterogeneity in alien plant establishment and expansion.