Characterising invasive species

Functional traits are commonly used in invasion studies due to their potential to generalise mechanisms to become invasive across species, which can inform both border screening and post-border management tools. Despite decades of research, however, finding general rules of traits that confer invasiveness has proven challenging.

Trait-based invasiveness studies often compare discrete groups of invasive and non-invasive (or native) species and attribute their differences in invasiveness to their functional differences. Yet no universal definition of ‘invasiveness’ exists, and thus different criteria can be used to decide which species are invasive and non-invasive. Less often, invasiveness studies present more nuanced, continuous metrics of invasiveness, avoiding the need to define invasive/non-invasive boundaries. Unlike categorical classifications of invasiveness, continuous metrics have the potential to reflect individual demographic processes – i.e. species growth, reproduction and spread. Hence, they acknowledge the possibility that different ecological mechanisms enable invasions and that those mechanisms may relate to traits differently.

Initial excitement for the use of functional traits in invasion forecast has turned into realisation that invasions are highly context-dependent processes; i.e. traits promoting invasiveness are strongly linked to the ecological and historical context. Accordingly, risk assessment protocols often include information on human use, species’ known invasion history and species’ environmental preferences, along with species’ traits. Robust prediction of species invasiveness based on functional traits, and its application for biosecurity protocols, requires critical thinking on which traits promote distinct mechanisms of invasion, consideration of the context and validation of findings across contrasting case studies.