Researchers working at reefs in Moorea, French Polynesia have discovered that the structural complexity of coral reefs can impede coral recovery after disturbances. A network of dead coral skeletons left in place by bleaching events causes critical processes to break down, preventing reefs from recovering. The complex landscape protects seaweed from herbivores, enabling it to quickly colonize the reef and outgrow young coral. This research was published in the journal Global Change Biology, shedding light on the challenges reefs face in recovering from disruptions.
Coral reefs are dynamic ecosystems constantly undergoing change, with disturbances like storms, bleaching events, and influxes of coral predators impacting their recovery. Historically, tropical storms and cyclones have been the biggest disruptors to Moorea’s reefs, scraping coral off the reef and leaving behind a flat surface. However, bleaching events and predation are on the rise, killing coral but leaving the reef’s structure intact. The recovery process of the reef can be drastically affected by small nuances, with the reef’s ability to recover from different disturbances varying based on the impact they have on the ecosystem.
A cyclone in 2010 removed all coral colonies from Moorea’s reef, but within five years, it had recovered back to its original state. In contrast, a bleaching event in 2019 killed half of the corals on the reef without removing the dead coral structure, leading to continued coral decline and proliferation of seaweed. Researchers at the NSF-funded Long Term Ecological Research site at Moorea Coral Reef noticed these differences in the reef’s recovery and sought to understand the mechanisms at work. They published a mathematical model of the system in 2023 and conducted a new field study to delve deeper into the processes affecting reef recovery.
The team prepared patches of the reef by creating a blank slate for their experiment, cementing dead coral skeletons in each patch, and plugging healthy young coral into the reef for monitoring. They found that dead coral skeletons prevent herbivores from removing macroalgae, allowing the seaweed to grow and prevent new corals from settling and surviving on the reef. Protection by dead coral skeletons could potentially help young coral recruits settle quickly after a bleaching event, but the advantage lies with macroalgae that reproduce continually and quickly colonize the reef, outcompeting coral for space and resources.
The researchers compared results from their small-scale experiments to long-term data from the site, revealing different trajectories following different disturbances. Coral cover increased after a cyclone while macroalgae cover decreased, but after a bleaching event, the opposite occurred. This highlights the concept of ecological memory, where past events influence the trajectory of an ecosystem and can lead to misalignments between what the ecosystem is used to and what it is currently experiencing. The presence of vast stands of dead coral skeletons after a disturbance can alter long-standing relationships between herbivores, algae, and coral, impacting reef recovery.
Looking ahead, researchers are exploring the possibility of removing dead coral skeletons from the reef to stimulate coral recovery or mitigate the impacts of bleaching. While this strategy is novel for coral reefs, other ecosystems have successfully used similar approaches, such as prescribed burns in forests to remove dead wood. By manipulating dead structures in ecosystems, there may be innovative solutions to aid in the recovery of coral reefs facing challenges in the wake of disturbances. The team’s findings contribute to a deeper understanding of coral reef dynamics and offer insights into potential strategies for managing and preserving these vital ecosystems.
