Not all traits are 'functional': insights from taxonomy and biodiversity-ecosystem functioning research

The current definition of a functional trait as any trait that is linked to growth, reproduction and survival is problematic. With this logic all traits within an organism can be defined as functional, because there is no one trait that is not linked to these processes. Such a definition, therefore, hinders the realignment of functional traits with biodiversity-ecosystem functioning relationship (BEFR) and is also out of sync with taxonomy resulting in an unfortunate view of functional diversity as a combination of any available morphological, physiological and life-history traits. This paper argues that functional traits have to be functional, meaning that they have to demonstrably effect or respond to ecosystem processes, and following the insights of BEFR they should experimentally be tested too. The recent crop of studies comparing taxonomical versus functional diversity appears to be conceptually flawed, as they conflate trait-based and species-based approaches. If the focus is on the trait-based approach, then taxonomic diversity should be comprised of trait information too, which is given in the species taxonomic descriptions and not just species richness. Instead of the concept of functional traits being easier and quicker than species-based assessment, it is actually more demanding, and the burden of proof for identifying functional traits is much higher. Demonstrating that a trait plays a crucial role in ecosystem processes (i.e. functional) requires a far greater understanding of the organism and its interaction with the environment. The effort put on the functional traits, however considerable, may yield novel insights that are not possible with species-based approach.