(NASA image of Pelorus Island, located off the northeastern coast of Australia. Image courtesy of Tageo.)
To help answer this question for corals in Australia's Great Barrier Reef, a group of researchers recently surveyed both living and dead coral assemblages at three sites around Pelorus Island, an inshore reef located near the outlets of both the Herbert and Burdekin Rivers. Since the late 19th century, when Queensland was colonized by Europeans (and their associated livestock and agricultural crops), the rivers have delivered altered levels of sediments, nutrients, and herbicides to the Australian shore--and the corals located along it. During the recent study, researchers collected paleoecological data in order to determine the impacts of these anthropogenic influences on coral growth.
Specifically, they sent scuba divers underwater to survey both living and dead coral assemblages. The divers noted the identity, percent cover, and growth morphology of the corals. They also gathered samples of dead corals that could later be examined with computed axial tomography (CAT) scans, which can provide information on the age of each coral colony. Finally, the researchers scoured the scientific literature for information on historical climate, weather, and hydrological patterns that might have been associated with fluctuations in coral communities and growth over time.
(Australia's Great Barrier Reef. Image courtesy of GreatBarrierReef.org.)
Cumulatively, the data suggested that the Pelorus Island corals had successfully weathered a series of environmental perturbations over the years, but that they were unable to combat anthropogenic disturbances. The oldest of the three study sites was dated to the mid-3rd century; its younger neighbors sprang up in the mid-9th and early 16th centuries. While Acropora coral species dominated the reefs for several hundred years, the researchers documented a shift occurring somewhere between 1920 and 1955: Acropora were replaced with Porites, Montipora, Pavona, Millepora, and Echinopora species, with exact assemblages varying depending on the site. The researchers also observed different growth morphologies emerging over time, with modern Acropora appearing much thinner and less branched than their ancestors. This suggests that the corals had tried but failed to regenerate themselves after an environmental disturbance.
This is also indicated by the paleoenvironmental data, which revealed that the coral colonies experienced two decades of cool temperatures just prior to the beginning of their collapse; this was followed by a drought and an increase in both cyclone activity and water flow from the nearby rivers. None of these things would have been pleasant for the corals, but none of them should have been insurmountable, either. Corals are actually fairly good at recovering from "acute" environmental stressors such as weather events--except, that is, when they are also dealing with chronic stressors. In this case, the chronic stressors were influxes of chemicals resulting from human use of fertilizers. The researchers reported that nutrient flows are thought to have increased anywhere from 2.1- to 19.5-fold since European colonization, and, in particular, from 1930 onwards. Thus, in the early 20th century, corals near the Herbert and Burdekin Rivers were exposed to abnormally high amounts of sediment, herbicides, and nutrients, all of which disrupted the delicate balance of the coral ecosystem.
(An Acropora species--also sometimes known as staghorn coral. Image courtesy of Coralpedia.)
These results highlight that even the most resilient and hearty of species have limits on how many, and what type of, environmental stresses they can tolerate. Further, given the current differences observed in coral assemblages growing at the three closely situated study sites, it appears that there can be extremely fine-scale variations in how ecosystems will respond to the same set of environmental conditions. This, in turn, suggests that researchers and managers should be cautious about making generalizations about particular localities--such as "Pelorus Island" or "The Great Barrier Reef"; these might be overly simplistic and fail to take into account important local dynamics. Finally, the scientists point out that this sort of paleoecological work is very useful for more accurately determining the "baselines" to which disturbed systems should be returned. Monitoring at the Pelorus Island reefs only began in the 1980s; thus, researchers were previously unaware of how the coral colonies looked before their early-20th-century collapse. Additional paleoecological work at this and other sites should help conservationists make more appropriate targets for preservation and reclamation plans.
Roff, G., Clark, T.R., Reymond, C.E., Zhao, J.-x., Feng, Y., McCook, L.J., Done, T.J., and Pandolfi, J.M. 2013. Palaeoecological evidence of a historical collapse of corals at Pelorus Island, inshore Great Barrier Reef, following European settlement. Proceedings of the Royal Society B 280: online advance publication.
For more on the natural history corals, check out Episode 18 of my weekly science radio show, the Wild Side (Part I, Part II).
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