The resilience of coral reefs

Many people are justifiably concerned with the potential impacts of climate change and ocean acidification on coral reefs. But, coral reefs have been declining for at least the last 25 years and probably much longer, overwhelmingly due to threats that are unrelated to climate change. If we do not address these impacts we will continue to lose coral cover and reefs will be more vulnerable to climate change and ocean acidification.

A coral outcrop on the Great Barrier Reef (photo Wikipedia)

A new paper serves as an illustration of how resilient coral reefs are to climate impacts when they are isolated from other anthropogenic impacts, such as overfishing and agricultural runoff. James Gilmour and other researchers from the Australian Institute of Marine Science and some from the Centre of Excellence for Coral Reef Studies followed the recovery of the Scott Reef system after a catastrophic bleaching event in 1998 that reduced coral cover from 50% to 10%. There was great concern for the reef system because it was isolated from other reefs that could supply coral larvae to fuel recovery.

The Scott Reef system. The crescent shaped reef at the bottom is Scott Reef South, the small reef above the left arm of the crescent is Scott Reef and the pear shaped reef is Scott Reef North (photo Wikipedia).

It turns out that, on balance, the isolation was a good thing. The supply of coral larvae reaching the reef was less than 6% of what it was prior to the bleaching event for six years. But, the reef was also isolated from chronic anthropogenic pressures, particularly overfishing. The number of herbivorous fish was already high at the time of the bleaching and jumped afterwards. As coral cover increased the numbers of herbivorous fish declined back to what they were prior to the bleaching.

The daisy parrotfish, Chlorurus sordidus, is an important herbivore on coral reefs (photo Dennis Polack, EOL).

The herbivorous fish kept seaweed and other organisms that compete with coral from taking over. Remnant corals that survived the bleaching were able to grow quickly and the small numbers of coral larvae reaching the reef had unexpectedly high survival. The fast growth of existing coral drove the initial recovery of the reef. Once young corals became established and began reproducing the supply of larvae increased and the recovery of coral cover accelerated.

Ten years after the bleaching event the supply of coral larvae had returned to the levels seen before the bleaching. Two years later the amount of coral cover and community structure on the reef had largely been restored. The rate of recovery is made more remarkable by the occurrence of a second more moderate bleaching event, two cyclones and a disease outbreak.

The study highlights just how resilient coral reefs can be to the effects of climate change and other disturbances if chronic anthropogenic stress is low. Overfishing, sedimentation and pollution are causing severe declines in coral cover right now. If we can control these threats, coral reefs might be able to survive in a warmer, more acidic ocean.

Reference:

Gilmour, J., Smith, L., Heyward, A., Baird, A., & Pratchett, M. (2013). Recovery of an Isolated Coral Reef System Following Severe Disturbance Science, 340 (6128), 69-71 DOI: 10.1126/science.1232310