Coral reefs and their sea bird allies

Dr Anjani Ganase, coral reef ecologist, discusses studies that show the land and sea relationships affecting coral reefs.

Mapping coral reefs

Coral reef research has expanded our understanding of coral reefs, their distribution and the extent of biodiversity that utilises the reef habitats. However, the more we discover, it also reveals just how vulnerable these ecosystems are and how much more we need to do to protect them. New satellite technology is revealing more coral reefs to us than ever before.

Scientists from the University of Queensland, Australia have been able to map over 348,000 km2 of shallow coral reefs around the world. This is an area that includes reefs that extend to depths of 30 metres. This new map has also identified at least 80,000 km2 of potential hard bottom habitats, suitable for coral establishment. This global map is an incredibly useful tool for research, management and conservation. Beyond 30 metres down, understanding the deeper reef habitats becomes more challenging as mesophotic reefs that occur between 30-150 metres depth evade satellite detection. Instead, alternative innovative methods have been employed.

The use of eDNA (environmental DNA) which refers to the DNA found in biological cells that are shed from organisms can give insight into the reef communities present at these depths. Scientists from Okinawa Institute of Science and Technology (OIST), Japan, sampled a large area of mesophotic reefs with the use of ROVs to collect eDNA samples. The samples led to the identification of new species of corals based on the genetic information.

As we continue to learn more about the mesophotic reef systems, we also discover how sensitive they are. For the first time, scientists observed coral bleaching on mesophotic reefs along the Chagos Archipelago in the Indian Ocean. Mesophotic reefs were once considered to be a refuge for coral given the cooler water conditions, but now scientists have shown the vulnerability of these systems to temperature changes as a result of ocean circulation patterns.

Fishy behaviour

In the aftermath of natural disasters, human social norms are often suspended in times of desperation from the loss of home or a food source. Scientists have observed similar behaviours in some fish species living on Indonesian reefs. In the aftermath of coral bleaching and die off, butterflyfish that feed on corals, struggled with social cues. They had difficulty in recognising a competitor species, which results in more conflicts and energy loss in defence or from fleeing. The major loss in habitat and food supply drove butterfly fish to explore wider areas in search of food increasing unwanted encounters for the sake of finding food in times of scarcity.

Changes in fish behaviour can happen for less obvious changes to coral reef conditions. Scientists from Lancaster University observed changes in behaviours of damselfish on reefs associated with rat-infested vs rat-free islands of Chagos Archipelago, Indian Ocean. Damselfish is a common reef fish known for their turf algae gardening skills. The fish grow tiny lawns of algae on corals skeletons as a source of food and they defend this territory from other algae grazers.

Turns out that rat-infested islands alter the nutrient conditions of the surrounding reefs, as the rats decimated the bird populations that would typically provide a steady supply of guano (bird poop) to the surrounding reef environment. Under these nutrient rich conditions, damselfish were able to farm small but nutrient rich turf algae gardens that were defended aggressively. Conversely, on reefs of rat-infested islands, damselfish farmed large turf algae gardens that were low in nutrients and also less valued as the damselfish were not as aggressive in these territories.

Seabird allies

On the same island systems, the same group of researchers showed that corals also benefit from the sea bird droppings that were flushed into typically low nutrient reef systems. More specifically, scientists observed faster recovery of coral species after bleaching on reefs adjacent to islands with thriving bird colonies. In fact, transplanted coral from the islands where seabirds were absent to islands with intact bird colonies showed doubled growth rates. These two studies emphasise the importance of land and coastal management for the health of the reefs.