Life on the Great Barrier Reef is undergoing big changes in the face of climate change and other human-caused pressures, a new study reveals.

From food security to controlling seaweed and even making sand for beaches, reef fish are a hugely important part of marine ecosystems providing a range of benefits to humans and coral reef ecosystems.

New research from an international team of marine scientists from the UK and Australia and led by researchers at Lancaster University, published today in the journal Nature Communications, reveals significant transformations in fish communities on the Great Barrier Reef, the World’s largest coral reef ecosystem.

Their findings show Great Barrier Reef fish communities differ substantially from those of the 1990s, and the pace of change has increased. The findings raise questions around how long-known patterns of how life is spread across the Earth seem to be shifting.

And importantly, researchers discovered that coral diversity rather than the amount of coral on a reef plays the most crucial role in driving the diversity of fish that depend upon it.

The Great Barrier Reef ecosystem faces many pressures with some, such as coral bleaching and cyclones, becoming more intense and frequent in recent decades. Extreme heat stress resulted in six mass coral bleaching events on the Great Barrier Reef from 1998 to 2022, and another recently in 2024 after this study was completed. This is compounded by other pressures, such as tropical storms, water pollution and outbreaks of crown-of-thorns starfish. These pressures have huge effects on the coral reefs – impacting the amount of coral, the composition of coral assemblages and the diversity of fish that live on coral reefs.

The research team analysed data from the Australian Institute of Marine Science’s (AIMS) Long-Term Monitoring Program, which surveys fish and coral stretching back three decades between 1995 and 2022. The AIMS surveys span more than 1,200 km (from 14°S to 24°S) along the Great Barrier Reef.

One of the most documented patterns of life on Earth is how the number of species (also known as species richness) increases as you get closer to the Equator. This pattern is known as the 'latitudinal diversity gradient'. A key result from this study is showing how this pattern has undergone significant fluctuations.

Latitudinal patterns of diversity are thought to have been relatively stable over long geological timescales, with these patterns also found in fossil records. The study’s results show that although this pattern remains broadly in line with what we expect for the Great Barrier Reef, there are signs the gradient may be shifting as a result of the increasing pressures.

In the northernmost parts of the Great Barrier Reef, which are closest to the equator, the number of different fish species, in particular species of omnivores, plankton feeders and herbivores, have reduced. In contrast, these trophic groups increased in the southernmost sectors of the Great Barrier Reef, raising questions about other potential consequences on the functioning of this ecosystem.

In the southern part of the Great Barrier Reef, reef fish species richness has experienced big fluctuations – flipping from periods with high numbers of different species, and periods where that richness has been very low.

But the researchers not only show how the number of fish species has changed, they also looked at how the composition of fish communities has changed. They found that fish communities have continuously changed as fish species replace others particularly in response to disturbances. And as disturbances become more common so too do these changes in fish communities.

Javier González-Barrios, lead author of the study and PhD researcher at Lancaster University, said: “Our findings show how long-term and persistent changes are occurring in the biodiversity of the Great Barrier Reef with increasing disturbance from factors such as climate change. We reveal that the fish communities that make their home on the Great Barrier Reef have changed substantially from the beginning of monitoring in the 1990s, both in the number of species and their composition, and continue to change as pressures on the reef system increase.

“We have seen changes in the number and composition of fish species throughout the reef system as well as species turnover – when one species replaces another - are accelerating in recent years, without any sign of stabilisation.

“These changes provide valuable insights into classic geographic patterns, such as the latitudinal diversity gradient, and opens questions as to whether these patterns are decoupling from their original underlying drivers.”

Another key finding from the study is that changes to the composition of coral species is a better predictor of altered fish patterns than simply the percentage cover of hard corals – a widely-used metric to evaluate coral reef condition.

“The Great Barrier Reef has undergone major fluctuations, with hard coral cover on many reefs declining and then recovering in response to large disturbances,” said Dr Mike Emslie of AIMS. “However, just looking at coral cover can mask the underlying changes in the composition of coral species. Corals build the three-dimensional habitat structure that support other organisms such as fish, and the diversity of corals present can greatly influence the extent of that 3-D structure. The recovery of some corals might not provide the same level of habitat complexity resulting in changes to the fish communities that rely on that complexity.

“Our study shows that changes in fish diversity on the Great Barrier Reef were strongly correlated with shifts in coral composition, and to a lesser extent with fluctuations in coral cover, highlighting the vital importance of a diverse coral assemblage for reef fish communities.”

Professor Nick Graham, of Lancaster University said: “Reef fish are important to a range of ecosystem processes, such as controlling seaweed so it doesn’t overgrow and take over areas of the seabed, to producing the sand we sunbathe on in tropical locations. Reef fish are also important to fisheries and are a vital source of food for millions of people. With disturbances becoming more frequent and severe, the patterns in fish diversity and abundance we have come to expect are changing, and this will alter the benefits the fish provide for the ecosystem and for people. It’s important that we improve our understanding of how patterns of biodiversity are changing on coral reefs, and the ecological and social implications of this change.”

The study’s findings are outlined in the paper ‘Emergent patterns of reef fish diversity correlate with coral assemblage shifts along the Great Barrier Reef’ published in Nature Communications.

The paper’s authors are Javier González-Barrios, Sally Keith and Nick Graham of Lancaster University, Michael Emslie and Daniela Ceccarelli of AIMS, and Gareth Williams of Bangor University.

The Australian Institute of Marine Science (AIMS) is an Australian government science agency and provides funding for the Long-Term Monitoring Program. Javier González-Barrios was funded by a Natural Environment Research Council (NERC) studentship with the Envision Doctoral Training Centre.