Allee effects limit coral fertilization success

New study reveals how coral colony proximity drives reproductive success

A groundbreaking study by PICRC and Professor Peter J Mumby from the University of Queensland was published in the journal The Proceedings of the National Academy of Science (PNAS). The study demonstrated the importance of having high densities of corals of a certain species on a reef to allow successful reproduction.

The Allee effect, named after ecologist W.C. Allee, is a biological phenomenon in which the reproductive success of an individual decreases when there are less individuals of its species in the surrounding area. Many coral reef species reproduce during mass spawning events, when eggs and sperm are released in the water and mixed at the surface. If the density of a species – the number of individuals in a certain area – is too low, the probability of egg and sperm mixing and getting fertilized may also become very low.

The study, conducted on Acropora hyacinthus, a common table coral in Palau, revealed that fertilization success significantly decreases as the distance between coral colonies increases. Fertilization success was approximately 30% when colonies were located within half a meter of each other, but this rate dropped to below 10% at a separation of 10 meters and approached zero beyond 15 to 20 meters.

“Our findings highlight how the spatial arrangement of coral colonies plays a pivotal role in their reproductive outcomes.” said Greta Sartori, PICRC researcher. “Increased separation between colonies drastically reduces fertilization success. This becomes particularly important when we consider coral mortality due to bleaching events or other impacts, which may reduce coral density.”

The study also underscored the role of coral spawning synchrony and weather conditions in driving fertilization success. Researchers observed that close proximity of coral colonies enhanced egg and sperm release synchrony, further improving reproductive outcomes. However, adverse weather conditions, such as stronger winds, significantly diminished fertilization rates even when eggs and sperm were released in abundance.

The research also offers valuable guidance for coral reef restoration efforts. Reconnecting fragmented populations by strategically planting corals in determined locations will help conservationists in safeguarding reproductive success and mitigate the decrease in coral populations. The findings also emphasize the importance of maintaining high-density coral patches to ensure resilience against the compounding impacts of climate change. This knowledge can be applied in coral reef restoration efforts in Palau by transplanting corals in areas with low coral density. Some examples are reefs that lost most of their coral cover due to impacts stemming from coral bleaching or typhoons.

“Understanding Allee effects provides a pathway to designing more effective interventions to restore coral reefs and this information will be crucial to develop a framework to understand where restoration will be more helpful” said Professor Peter J Mumby. “It’s a reminder of how interconnected and sensitive marine ecosystems are to even small changes.”

With global warming and human activity continuing to threaten coral reefs, the study highlights the urgent need for further exploration of how spatial and environmental factors interact to shape reproductive success in reef building corals.

For further information about the study, please contact PICRC researcher Greta Sartori at 680 488-6950 or via email at [email protected]. The article can be accessed online at https://www.pnas.org/doi/10.1073/pnas.2418314121.