The colonisation by new corals is a key driver of reef resilience and recovery. The success of coral larvae to grow into adult corals (known also as coral recruitment) is critical to the health of coral reef ecosystems.
Coral reefs are prone to major disturbances
Corals are the building blocks of tropical reefs, and coral reefs provide the structure and habitat for the massive diversity of organisms that inhabit these ecosystems. Coral reef ecosystems provide key services such as tourism, fisheries and coastal protection, valuable to millions of people throughout the world.
Yet coral reefs are prone to major disturbances – both natural and human induced. These disturbances include cyclones, outbreaks of crown-of-thorns starfish, mass coral bleaching events, and sediment deposition. These disturbances often remove most of the live coral from entire reefs.
Following major disturbances, one of the key mechanisms by which a reef recovers its corals is through recolonization by tiny larvae coral from neighbouring reefs. But coral recruitment is a complex process that involves multiple early life-history phases. These phases include:
- Mass spawning of eggs and sperm into the water column
- Dispersal of coral larvae from metres to hundreds of kilometres via oceanographic currents
- Settlement and metamorphosis of swimming coral larvae from the plankton to the reef
- Early post-settlement growth and survival of microscopic corals recruits until they are big enough to drive population recovery.
Untangling how well corals do during each life-history phase requires numerous approaches, which take into account the ecological and environmental challenges that corals face during early life-history stages. These include competition with other encrusting organisms and fleshy algae, predation by fish and invertebrates, response to chemical cues, plus dealing with warming and acidifying water and potential smothering by sediment.
How coral recruitment drives coral reef recovery
We are working to understand how coral recruitment drives coral reef recovery using a variety of complementary approaches. These include field experiments and monitoring, laboratory experiments, as well as systems modelling that combines ecological, network and oceanographic techniques.
We distribute various artificial substrates onto the reef in field experiments to track how many larvae naturally settle and how well they grow and survive. These substrates are placed in a variety of environments so we can understand how water temperatures, light profiles, and levels of sedimentation affect recruitment rates.
We also conduct experiments using coral larvae that we collect in the laboratory. Following the early autumn full moon, corals release their eggs and sperm into the water. We collect these eggs and sperm, cross fertilise them, and then use them for a variety of laboratory experiments to assess rates of survival as larvae, settlement and post-settlement growth and survival as tiny recruits.
Data collected through our field and laboratory experiments are used to create simulation models that predict the recovery capacities of reefs under different ecological and environmental conditions. These models incorporate oceanographic modelling with connectivity networks.
The minimum and maximum thresholds of larvae needed
Using data from the field and in the laboratory, we have discovered the minimum and maximum thresholds of larvae needed to successfully settle and survive early post-settlement in north-western Australia. We have also been able to unravel how different interactions with algae and predators alter survivorship.
Our longer term field and laboratory experiments have shown that recruitment is limited by bottlenecks to different early life-history phases depending on the environmental setting. Sometimes reefs have limited supplies of coral larvae, sometimes when supply is not limited the early survival of settled recruits is extremely low.
Our research shows that larval dispersal is highly sensitive to seasonal and intra-seasonal variability in winds and currents, and this variability can have huge effects on the levels of connectivity between neighbouring regions of corals. We found that a reef's role in system maintenance can differ from a reef's role in system recovery, and that oceanographic variability can greatly alter these relationships. From a management perspective, the choice of which reefs should be considered for protection, or restoration, depends crucially on what type of stressors are prioritised.
Overall, our research confirms that coral recruitment is a key driver of reef recovery, and limitations to recruitment success can severely alter the capacity of coral reefs to remain resilient under continued and increasing local and global disturbances.