A new study suggests that reef fish species may be hitting a ‘glass ceiling’ as water temperatures raise while reef predators receive substantial energy subsides from sources outside the reefs. An international team of researchers led by scientists at Macquarie University offers a fresh perspective on threats to the health and biodiversity of reef ecosystems by synthesising energy expenditure data for individual fish with abundance and biomass data collected from reef fish communities all over the world. Their calculations indicate that individual fish exhibit a general trend of increasing energy use with temperature, but only up to about 33°C, at which point energy use begins to decline. This finding suggests that reef fishes in the warmest regions of the tropics may be limited in their physiological capacities to respond to future warming, which may have knock-on effects on entire reef ecosystems. In addition, the total rates of energy expenditure by top predators, such as reef sharks, appears to be far higher than would be predicted if these predators obtained all of their energy from the resources available on the reefs. Lead researcher Diego Barneche says the implication of this finding is that protecting the reefs themselves is necessary, but insufficient to protect reef biodiversity from harmful processes like overfishing outside reefs. “Scientists have long recognised that these fishes play key roles in maintaining the overall health and biodiversity of reef ecosystems by consuming a variety of foods, including plants, invertebrates, and fish on the reef bottom and in the water column. Energetics provides a useful currency for quantifying these roles, and how they may be altered by global change,” said Diego. “As this research highlights, we need to be aware that processes such as ocean warming and overfishing may prove harmful to reef fish and their ecosystems by disrupting energy dynamics.” The full study ‘Scaling metabolism from individuals to reef-fish communities at broad spatial scales’ has been published by Ecology Letters.