The meaning of the term ‘function’ in ecology: A coral reef perspective

Abstract

The inherent complexity of high‐diversity systems can make them particularly difficult to understand. The relatively recent introduction of functional approaches, which seek to infer ecosystem functioning based on species’ ecological traits, has revolutionized our understanding of these high‐diversity systems. Today, the functional structure of an assemblage is widely regarded as a key indicator of the status or resilience of an ecosystem. Indeed, functional evaluations have become a mainstay of monitoring and management approaches. But is the heavy focus on broad metrics of functional structure grounded in empirical research? On tropical coral reefs, the ocean’s most diverse ecosystems, remarkably few studies directly quantify ecosystem functions and the term ‘function’ is widely used but rarely defined, especially when applied to reef fishes. Our review suggests that most ‘functional’ studies do not study function as it relates to ecological processes. Rather, they look at easy‐to‐measure traits or proxies that are thought to have functional significance. However, these links are rarely tested empirically, severely limiting our capacity to extend results from community structure to the dynamic processes operating within high‐diversity ecosystems such as coral reefs. With rapid changes in global ecosystems, and in their capacity to deliver ecosystem services, there is an urgent need to understand and empirically measure the role of organisms in various ecosystem functions. We suggest that if we are to understand and manage transitioning coral reefs in the Anthropocene, a broad definition of the word ‘function’ is needed along with a focus on ecological processes and the empirical quantification of functional roles. In this review, we propose a universal operational definition of the term ‘function’ that works from a cellular to a global level. Specifically, it is the movement or storage of energy or material. Within this broad definitional framework, all functions are part of a continuum that is tied together by the process‐based unifier of material fluxes. With this universal definition at hand, we then present a path forward that will allow us to fully harness the power of functional approaches in understanding and managing high‐diversity systems such as coral reefs. A plain language summary is available for this article.

Historical biogeography of herbivorous coral reef fishes: The formation of an Atlantic fauna

“…These parrotfishes are the largest‐bodied herbivorous fishes in the Atlantic, with two species ( S. guacamaia and S. coeruleus ) growing to much larger sizes than any other Indo‐Pacific congeneric (Siqueira et al, ). Since size is related to numerous functions (Bellwood, Streit, Brandl, & Tebbett, ), these species were probably important for Atlantic ecosystem functions prior to human‐induced declines.…”

Patchy delivery of functions undermines functional redundancy in a high diversity system

“…The abundance, quality and patchiness of algal resources are known to drive space use in fishes on multiple spatial scales: from reef‐wide feeding territories (Carlson et al, ; Nash, Graham, Januchowski‐Hartley, & Bellwood, ) to microhabitat bite locations (Brandl & Bellwood, ). However, to‐date we have very limited knowledge of the spatial distributions of resources within turfing algae, or indeed how selective different “how” functional groups are in targeting these resources (Bellwood et al, ). Future research is needed to elucidate whether resource distribution and fish feeding are indeed patchy on congruent spatial scales.…”

“…All herbivore functional groups contribute to the overarching ecosystem process of removing algal biomass. Algal removers thus form a “what” functional group, sensu Bellwood et al (), defined by what they do. However, species perform this function in different ways (i.e., differing in “how” they deliver this function).…”

“…If all feed in the same locations, different “how” functional groups would overlap and collectively deliver a comprehensive version of the “what” function, that is removing algae. Hence, overlapping feeding would offer both redundancy (within the “what”) and the potential for response diversity (across the “hows”), potentially supporting reef resilience (Bellwood et al, ; Burkepile & Hay, ,; Graham et al, ; Tebbett et al, ). By contrast, if different “how” functional groups feed in different locations, delivery of the “what” function may be either less comprehensive or subject to much greater than expected heterogeneity; and assumptions about how the “how” groups influence ecosystem resilience need to be revisited.…”

Coral reef ecosystem functioning: eight core processes and the role of biodiversity

“…Although each of the eight key processes – and their importance to coral reefs – has been understood for decades (Done ; Hatcher ), our definition of coral reef ecosystem functioning is timely in that it identifies a set of reciprocal fluxes that can be explicitly measured and compared to facilitate the structured, quantitative exploration and potential conservation of coral reef ecosystem functioning, a concept that has to date lacked clarity (Bellwood et al . ). Using this framework, we can begin exploring the potential effects of biodiversity on coral reef ecosystem functioning, which remain only rudimentarily explored beyond the roles of coarse functional groups of fishes or corals (Bellwood et al .…”