ZSL scientists have shown the devastating effects of back-to-back heatwaves in the region surrounding the Chagos Archipelago. Coral cover was reduced by 60% in 2016 and by a further 30% in 2017. Nonetheless, this also indicates that some coral species have a higher resilience to rising temperatures thus offering some hope for the future.

Although representing les than 1% of our marine habitats coral reefs harbour a quarter of marine life. There are strong economic, conservation and scientific reasons for us to safeguard these reefs in a changing world.

In 2015 an eight weeklong heatwave struck the Chagos Archipelago, causing the seawater of the area to be unusually hot for a prolonged period. Researchers compared surveys of the reef before and after the heatwave in order to map the changes and harm it caused to the reefs. Evidence from 2015’s observations shows a 60% reduction in coral cover in the MPA. This included a dramatic 86% decline in the branching Acropora species that was previously a dominant reef building coral.

A year later, and before any recovery was possible, this region was subject to another bleaching event caused by a four-month long heatwave. Using data from Peros Banhos atoll an estimated 68% of the remaining corals had bleached. This suggest that around 70% of the reef area covered by corals was lost between 2015/2017.

Although more severe than the first heatwave, the second did less damage than the previous one. The first one killed corals most vulnerable to heat stress, leaving the more resilient colonies to try and survive the second. The fact that a few species have a higher heat tolerance is a glimmer of hope. According to Dr. Catherine Head the increasing severity and occurrence of heatwaves will inhibit shallow reefs ability to regenerate from bleaching events. It previously took 10 years for coral reefs to regenerate in BIOT, there is a high probability that the more vulnerable coral species will completely die off in the near future if these bleaching events become more frequent.

Preliminary reports from 2019 suggest a grim future for the Indian Ocean Region’s reefs as another heatwave was recorded and observations made of the start of yet another bleaching event of unknown impact. Research is key to understanding how corals adapt to rising sea surface temperatures and how we can positively impact their recovery.

Researchers in the Bertarelli Programme in Marine Science studied both islands, with and without rats, to see what effect the presence of rodents has on the health of the surrounding coral reefs.

The presence of back rats on islands in BIOT results from human exploration of our seas. Many islands around the world have been completely taken over by black rats that we accidentally introduced through ships years ago. Upon arrival on these seabirds-filled islands rats began to eat their eggs, chicks and even full-grown adults. This had disastrous consequences on native seabird populations on these invaded islands, 90% of tropical islands saw their bird population crash.

Thankfully, a small number of islands were spared and could act as a haven for the remaining frigate birds, boobies, terns and shearwaters. In BIOT, 18 of the 58 islands remained rat free.

Professor Nick Graham commented: “The rat-less islands are full of noisy birds, the sky is full, and the smell is strong – because of the bird’s guano deposits on the island. If you visit an island with rats, there is almost no seabirds. “ 

On rat-free islands, seabirds, including boobies, frigate birds, noddies, shearwaters and terns, roam for hundreds of miles to feed out in the open ocean. Upon their return to the island, they deposit rich nutrients from the fish they eat. These nutrients then spread into the surrounding waters and affected the biology of reef systems.

The results, published in Nature, are clear: the fish on reefs adjacent to the rat-less islands grew faster and bigger than that of the fish living in islands infested by rats.

Therefore, rat-filled islands have an indirect impact on the already fragile reef, since seabirds provide nutrient rich guano to the surrounding reef.

Since coral reefs are constantly affected by changes in ocean temperature and with coral bleaching frequency increasing, this research suggests that the elimination of invasive species such as rodents could be an important way to increase the resilience of these critical habitats while also restoring native populations of seabirds.

To date 500 islands have been gradually de-ratted and, according to Dr. Graham, 2-3M$ is necessary to fully de-rat BIOT.