Shifting baselines in coral cover

A great problem for conserving marine ecosystems is that we rarely have a good data on what things were like before human impacts started. In my last post, I wrote about a study that showed that coral cover had declined on the Great Barrier Reef by 50.7% since 1985. At the start of the study coral cover was at 28%, but pristine coral reefs can have over 70% coral cover. This suggests that impacts on the Great Barrier Reef predate the time monitoring started by many years.

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

John Pandolfi at the University of Queensland has been trying to establish the past state of the Great Barrier Reef in numerous ways. One way is to take sediment cores from coral reef and compare the historical diversity and abundance of corals on the reef to the modern community composition. A new study lead by Pandolfi has reconstructed the past coral communities on reefs around Pelorus Island in the Palm Island group. They took cores containing coral remains dating back as far as the mid-third century.

They found that there was a pronounced transition in the coral species on the islands reefs between 1920 and 1955. The transition strongly correlated with a 5 to 10 fold increase in the amount of sediment found in the cores beginning in 1870, but showing several large peaks between the 1920s and 1970s. White settlement and land clearing of the area began in about 1870, the same time that high sediment loads were found in the cores. Prior to that, there was remarkable stability in the coral communities and the amount of sediment reaching the reef.

The new study highlights that reefs in 1985 that were thought to be relatively pristine probably had not been for 50 or 60 years. Therefore attempts to conserve reefs as they were in 1985 is inadequate because these reefs are likely to be already severely impacted by human activities. If we a serious about returning coral reefs to a pristine state, we should be restoring them to what they were like prior to white settlement, not what they were like now after a century of mistreatment.

Roff, G., Clark, T., Reymond, C., Zhao, J., Feng, Y., McCook, L., Done, T., & Pandolfi, J. (2012). Palaeoecological evidence of a historical collapse of corals at Pelorus Island, inshore Great Barrier Reef, following European settlement Proceedings of the Royal Society B: Biological Sciences, 280 (1750), 20122100-20122100 DOI: 10.1098/rspb.2012.2100