A More Nuanced Approach is Needed to Manage Coral Reef Ecosystems

For many years, conservationists have tended to focus on one key parameter when assessing coral reef health: the biomass of coral reef fishes. But according to a new study of more than 500 coral reefs around the world, what constitutes healthy or "functional" goes far beyond this single metric. Reporting in the journal Nature Ecology and Evolution, an international team identified five key functions that fish provide to a reef. Together, they paint a clearer picture of reef health.

"This work really changes the way we need to think about coral reef conservation," said Simon Brandl, an assistant professor at the University of Texas at Austin Marine Science Institute (UTMSI) and co-author of the study. "Our study shows that we cannot maximize all aspects of functioning. Instead, we need to develop a more nuanced approach to conserving coral reefs that considers local species, ecosystem dynamics and stakeholder needs."

Instead of focusing entirely on biomass and one-size-fits-all solutions, the researchers recommend a more nuanced approach to reef conservation. Their advice: find which fish provide the most useful functions in each reef system and protect them.

The project was led by researchers at several organizations in the U.S. (including UTMSI), France (including the École Pratique des Hautes Études (EPHE) and the French National Centre for Scientific Research (CNRS)), and elsewhere.

The researchers demonstrated that five key functions performed by fish communities — the removal of algae, predation, biomass production and the cycling of nitrogen and phosphorus — are inherently interconnected. As such, while the performance of these processes is influenced by the community structure of reef fishes on any given reef, no reef can maximize each of the five processes simultaneously.

"Imagine a coral reef fish community swirling with small fishes that feed on algae," said Nina Schiettekatte, the lead author. "This community will be characterized by high algal consumption and high biomass production, but it will have low phosphorus cycling because these species excrete very little phosphorus."

This also means that conservation managers have to determine which functions are most important for each reef, both for ecosystem and human stakeholders.

"If our goal is to manage reefs so that they are the most functional version of themselves, we're going to have to ask ourselves on a case-by-case basis, what is the process that we value the most?" Brandl said.

The researchers gained this knowledge by collecting data from individual fishes and combining that with a large dataset on fish communities worldwide.

"Throughout this project, we collected thousands of fishes across more than 100 species to gain detailed biological information on how they acquire and use energy and nutrients," said Jordan Casey, an assistant professor at UTMSI. This information can then be projected onto fish communities to understand how they collectively move biomass and nutrients through the food web.

"Our work is novel because it quantifies multiple functions for the first time," said Valeriano Parravicini, professor at EPHE. "Previously, most researchers have used fish biomass as a proxy for coral reef functioning, but we show that it is critical to look beyond biomass and really disentangle the different components of functioning to understand how reefs work."

Knowing that no reef can excel in all functions, the researchers asked whether a certain set of species are more important than others globally. Surprisingly, they found that no single species was consistently important across its range, but half of all species were important in at least one location.

"This means that there are no global superhero fish species for ecosystem functioning," said Sébastien Villéger, researcher at the CNRS. "But there are many local superheroes."

This research was funded by the BNP Paribas Foundation (Reef Services Project) and the French National Agency for Scientific Research. This research is the product of the SCORE-REEF group funded by the Centre de Synthèse et d'Analyse sur la Biodiversité of the Foundation pour la Recherche sur la Biodiversité and the Agence Nationale de la Biodiversité.