Fish diversity has changed dramatically on the Great Barrier Reef

Life on the Great Barrier Reef is experiencing significant shifts caused by climate change and various human-generated factors, according to a recent study. 

Researchers warn that reef fish, which are crucial to marine ecosystems through their roles in food provision, seaweed control, and sand production, are undergoing notable changes.

These developments could have broad consequences for both people and coral reef habitats.

Transformations in fish communities

The research was led by an international team of marine scientists from the UK and Australia, led by Lancaster University and working with the Australian Institute of Marine Science (AIMS).

The team found that the diversity and makeup of fish communities across the Great Barrier Reef (GBR) have changed dramatically since the 1990s. The findings indicate that the pace of change has accelerated in recent years, raising questions about evolving geographic patterns of biodiversity.

The researchers examined three decades of data from 1995 to 2022, spanning an extensive stretch of more than 1,200 kilometers between 14°S and 24°S on the GBR.

Historically, one of the best-documented global biodiversity patterns is the “latitudinal diversity gradient,” where species richness increases closer to the Equator. This pattern, previously thought to be stable over long timescales, now appears to be shifting as multiple pressures intensify.

Pressures on the reef

Coral bleaching events, occurring six times between 1998 and 2022 – and again in 2024, after the study concluded – are among the most severe disturbances affecting the GBR. 

Other stressors, such as cyclones, water pollution, and outbreaks of crown-of-thorns starfish, have also altered the reef by reducing coral abundance, changing coral assemblages, and ultimately affecting fish diversity.

In the northernmost GBR, closer to the equator, certain groups of fish (including omnivores, plankton feeders, and herbivores) have decreased. In contrast, these groups have increased in the southern sectors, raising important questions about what these trends mean for ecosystem functioning. 

The researchers noted that fish species richness in the southern reef has fluctuated dramatically – shifting between high diversity and very low diversity over different periods.

Long-term changes in reef fish diversity

The study also highlights how fish communities themselves are undergoing continuous turnover, with some species replacing others more frequently as disturbances become more common.

This ongoing turnover shows no sign of stabilizing, indicating that fish diversity patterns in the world’s largest coral reef ecosystem could continue to transform.

Javier González-Barrios is the study’s lead author and a PhD researcher at Lancaster University. He noted that the 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,” said González-Barrios.

“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 stabilization.”

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

Coral composition and reef fish diversity

A key insight from the research is that shifts in the types of coral species on a reef are more influential in driving fish diversity than the overall percentage of hard coral cover. 

While reefs can recover coral cover after disturbances, the specific mix of coral species can change drastically – and that variation in coral composition can alter the three-dimensional structures that fish rely on for shelter and food.

“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 Mike Emslie, a scientist at 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 supports 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.”

According to Emslie, the 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.

Future implications for coral reef ecosystems

The study highlights the importance of reef fish in maintaining essential ecosystem processes. These fish not only keep seaweed in check, preventing it from overtaking seabeds, but also generate sand for beaches – assets integral to tourism and local livelihoods. 

Understanding how and why fish communities are changing is critical for predicting the future benefits that reefs can provide to people.

Nick Graham, a professor of marine ecology at Lancaster University, noted that 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,” said Professor Graham. “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 is published in the journal Nature Communications.

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