Why studying the response of trait coordination to insularity matters?

Functional island biogeography can provide eco-evolutionary insights into which main drivers contribute to shaping the distribution of organisms' forms and functions on islands. It does so by examining trait patterns. As a result, traits are increasingly studied on islands, either along insularity gradients or by comparing patterns of island versus mainland biota. So far, functional island biogeography has investigated trends of trait values (i.e., average, functional diversity), whereas coordination between pairs of traits remains unexplored along insularity gradients. Yet, trait coordination analyses constitute the foundational tool to detect main functional spectra and strategies of organisms. In this perspective, we set out to offer a conceptual and analytical framework that should facilitate the inclusion of trait coordination (i.e., the co-variation of traits both at the intra- and interspecific level) in functional island biogeography. We illustrate, with a case study focused on persistence traits of edaphic island plant specialists, what type of insights can be gained by examining the response of trait coordination to variation in insularity. We asked two questions, namely whether, with increasing insularity, the strength of the relationship (R2) increases (Q1), and the direction of the relationship (slope) decreases (Q2). We positively answered our research questions, with lines of evidence suggesting a selective "forcing" towards tighter and more strongly coordinated strategies (Q1), and functional trade-offs (Q2). We infer which ecological and biogeographic drivers could be behind the observed patterns, while acknowledging possible drawbacks. We conclude by identifying three main take-home messages and related future directions for integrating trait coordination in functional island biogeography to further advance the field.