A recently published paper on the decline of coral cover on the Great Barrier Reef serves to illustrate an important point; even without climate change we are doing a great deal of damage to some ecosystems. The study by De'ath et al. and published in the Proceedings of the National Academy of Science, finds that coral cover has declined by 50.7% since 1985. They partitioned the losses into 48% tropical cyclones, 42% predation by crown of thorns starfish and 10% to coral bleaching.
|The crown of thorns starfish, Acanthaster planci (image Wikipedia)|
Pollution, sedimentation and overfishing can all change the dynamics of coral reef communities by impairing the ability of corals to recover from other disturbances. Human activities can also increase the mortality of adult coral and reduce the number larvae that survive to become coral. Shifts from coral-dominated communities to seaweed-dominated communities due to these impacts are well documented.
Overfishing, particularly of herbivorous fish, has been strongly linked to shifts in community composition. In direct competition corals lose out to seaweed, which overgrows coral and in some cases uses toxins to kill the coral. Herbivorous fish though, eat the seaweed tipping the balance in favour of the corals. So important are herbivorous fish to corals that some have formed mutualistic relationships with fish, which they signal for help when seaweeds encroach on their space.
Overfishing has also been suggested to reduce predation on larvae of the crown of thorns starfish, allowing it to reach plague proportions when fish would normally control their numbers. A second hypothesis is that nutrient inputs from farms and cities provides the crown of thorns larvae with large amounts of food, increasing their survival. Neither hypothesis is well supported, but there is growing evidence that both mechanisms are playing a role in crown of thorns outbreaks.
For corals, like seagrasses, access to light is critical for their survival. Coral derive as much as 90% of their energy from symbiotic algae growing in their tissues. Nutrient inputs and sedimentation reduce the light available to their algal symbionts, which reduces the energy available to them. This can decrease the resilience of corals to other stressors, such as natural disturbance events. The main sources of sedimentation on the Great Barrier Reef are from human activities, such as agricultural run-off and dredging.
De'ath et al. conclude that there is an urgent need to control crown of thorns outbreaks, especially through improvements to water quality. In the absence of disturbances, the data showed that reefs were able to increase in cover by nearly 3% per year. This is likely to be higher when the full impact of human activities are taken into account. Moreover, their data only go back to 1985, but human impacts on the reef date back to about 100 years before that. The true decline of coral cover on the Great Barrier Reef is, therefore, likely to be far greater than that measured in their study.
De'ath et al. also highlight the impending effects of climate change and ocean acidification. Many people are focused on human emissions of carbon dioxide as the sole problem we need to fix to save the reef. But, it's clear that even without the threats of climate change and ocean acidification the Great Barrier Reef is in great deal of trouble. In order to conserve the reef we need to address the source of these issues now.
De'ath, G., Fabricius, K., Sweatman, H., & Puotinen, M. (2012). The 27-year decline of coral cover on the Great Barrier Reef and its causes Proceedings of the National Academy of Sciences, 109 (44), 17995-17999 DOI: 10.1073/pnas.1208909109