David R. Bellwood, Robert P. Streit, Simon J. Brand and Sterling B. Tebbett
Ecosystems ‘function’ as a result of complex interactions between countless species and their environment. To grapple with this complexity, in the face of increasing human-mediated disturbance, many studies have attempted to quantify species’ roles in ecosystems via ‘functional’ traits. But to what extent does this approach demonstrate functionality? We identify the need for a clear definition of the word ‘function’ and question whether the current use of functional traits is consistent with a core definition, where function refers to ‘the movement and storage of energy or material’.
The focal ecosystem for our review was coral reefs, where reef fishes have been the poster-child for functional trait-based studies. For example, management frameworks increasingly highlight the need to protect specific functional groups, especially herbivorous fishes, to maintain reef resilience. Meanwhile back-to-back global bleaching events have left the Great Barrier Reef and other coral reef ecosystems reeling from extensive coral loss. In response, scientists have highlighted the need to maintain the ecosystem functions that sustain reefs, and the services we require from them. This raises the question: how do we identify these ecosystem functions and will functional traits help us find the answer?
Our review of functional studies on coral reefs concludes that the vast majority of functional studies do not study function; most look at easily obtainable proxies and infer functions. Establishing the veracity of functional traits or groupings and their causal relationship with ecosystem functions is a critical challenge for our understanding and stewardship of high-diversity systems such as coral reefs. This requires a primary focus on the functions or processes that are needed, followed by identification of traits that have a bearing on these processes, and lastly, the species that possess such traits. We provide a blueprint for future functional studies with the emphasis being on establishing and highlighting the meaning of the term ‘function’.