The Hidden Battle on Coral Reefs

Coral reefs are a highly diverse ecosystem, but they are also defined by intense competition for limited space. Most reef surfaces are fully occupied, and organisms compete by growing,  spreading, and interacting with one another. These processes play a critical role in shaping the reef’s structure and its present-day function.

Corals are not just building reefs, they are competing for space, light, and resources alongside a wide range of other marine organisms. Algae, sponges, cyanobacteria, and even other corals are all part of this dynamic system. Under stable conditions, these interactions help maintain balance and diversity. However, environmental change and stress can disrupt this balance, triggering shifts that often lead to coral decline.

This year, funded through the 2021 Pew Fellowship in Marine Conservation awarded to Dr Tries Blandine Razak, Indo Ocean Foundation is launching a new research initiative led by Research Associate Gita Alisa, focused on one of the most overlooked challenges in reef conservation: benthic competitor taxa.

Here, “benthic competitor taxa” is used as a functional category referring to organisms that compete with hard corals for space on the reef surface, including macroalgae, turf algae, cyanobacteria, sponges, and soft corals like Xenia.

While these interactions are a natural part of reef ecosystems, problems arise when certain species begin to dominate. In these cases, competitors can overgrow living coral tissue, take up space needed for coral larvae to settle, or even release chemical compounds that suppress coral growth and survival.

Over time, this can shift the balance of the reef. Areas that were once dominated by corals can become increasingly controlled by these competing organisms, making it much harder for corals to recover, even in locations where restoration efforts are already underway.

Despite the growing recognition of these impacts, there is still a clear gap between scientific understanding and practical management.

Much of the existing research focuses on individual species in isolation or is carried out in controlled laboratory settings. While these studies provide valuable insights, they do not always reflect the complexity of real reef environments, where multiple competitors such as algae, cyanobacteria, and invertebrates are interacting at the same time.

For conservation teams working in the field, the challenge is not just understanding what is happening, but knowing what actions are realistic and effective. Once a harmful species is identified, questions quickly follow. How often should it be removed? Is removal even effective? And can these interventions be sustained over time?

This project is designed to address those questions directly by working in real reef conditions and focusing on solutions that can actually be applied.

Alongside this, coral recruitment tiles will be used to look at the earliest stages of coral life. These tiles allow researchers to measure whether coral larvae are able to settle and survive in the presence of different competitors, offering a direct insight into how organisms like algae and cyanobacteria influence reef recovery from the very beginning.

The project also moves beyond observation by testing management strategies directly in the field. Different removal frequencies will be applied across study sites, allowing the team to compare outcomes and identify what level of intervention is both effective and realistic. 

While coral-eating snails like Drupella are one of the more visible threats on the reef, they are only part of a much larger picture.

These small gastropods feed directly on live coral tissue, leaving behind exposed white lesions that weaken the colony and make it more vulnerable to further damage. During outbreak events, where multiple individuals gather on a single coral, the impact can be both immediate and severe.

However, many of the most influential competitors are less obvious.

Sponges can slowly overgrow coral colonies, in some cases acting as coral-killing species that smother living tissue. Cyanobacteria can form dense mats across the reef surface, limiting light and oxygen exchange while also releasing compounds that inhibit coral settlement. Soft corals such as Xenia can rapidly expand under certain conditions, occupying large areas of substrate that would otherwise be available for reef-building corals.

Macroalgae and turf algae are often among the first organisms to colonise open space after disturbance. Once established, they can prevent coral larvae from settling, effectively blocking the next generation of reef growth.

What makes these organisms particularly challenging is that they rarely act alone. A reef may experience multiple forms of competition at once, with algae, cyanobacteria, sponges, and predators all interacting and reinforcing each other’s dominance.

This project aims to better understand these interactions in the context of coral restoration, moving beyond single-species assessments to evaluate how multiple competitor taxa spread, interact, and collectively influence coral condition, recruitment, and restoration outcomes.

At the same time, targeted investigations such as the work on Drupella allow for a closer look at specific management strategies. By tracking coral colonies before and after manual removal, the research will assess whether this commonly used intervention actually reduces reinfestation and supports recovery over time.

One of the most important aspects of this project is its focus on practical application.

Coral restoration is often associated with planting corals, but successful restoration depends on far more than that. If the surrounding environment is dominated by algae, sponges, or other competitors, newly restored corals may struggle to survive regardless of the effort invested.

Identifying which taxa pose the greatest risk, how quickly they spread, and how frequently they require management allows this research to provide clear and actionable guidance for reef managers, including recommendations on optimal maintenance frequency. This is particularly important in community-based conservation settings, where resources are limited and solutions need to remain both effective and achievable.

Coral reefs are dynamic systems shaped by constant interaction and competition. Supporting their recovery requires more than focusing on corals alone. It requires understanding the full community they exist within and how different organisms influence that balance.

This project will address one of the less visible yet highly influential drivers of reef change and represents an important step toward more informed and effective coral reef conservation in Indonesia.

Over the coming year, we will be sharing updates from the field and insights from the research as they unfold on our social media, with detailed results to come in future publications.