Original Paper: Alvarez-Filip, L., J. P. Carricart-Ganivet, G. Horta-Puga and R. Iglesias-Prieto. 2013. Shifts in coral-assemblage composition do not ensure persistence of reef functionality. Scientific Reports 3: 3486.

The calcium carbonate (CaCO₃) skeletons of stony corals form the basis for coral-reef community structure. As they build the reef, coral skeletons increase habitats for other reef species. Coral communities consist of coral species with two basic functional attributes. Coral species with high reproductive rates and relatively fast growth are more tolerant of environmental change, but these species tend to be small and do little to increase reef size or number of habitats. Massive corals that contribute the most to reef size and number of habitats are slower growing and are less tolerant of environmental change.

Corals are undergoing serious declines because of disease, pollution, and the effects of temperature and ocean acidification related to climate change. These declines can result in rapid changes in the composition of coral communities. As the pace of climate change escalates, changes in coral communities will likely increase.

Researchers developed simplified models consisting of four coral genera from the Caribbean to study the ability of coral reefs to retain functionality with changing species composition (Figure A). They looked at reef rugosity, which is a measure of reef complexity and habitat diversity, and calcification rate, which measures how fast coral skeletons are produced. Rugosity is calculated as a ratio of the contour of the reef relative to its length. A value of 1.00 equals a flat reef; larger values represent higher rugosity.

The table shows reef rugosity and estimated calcification rate for the four coral genera. Figure B shows how reef rugosity (dotted lines) and calcification rate (solid lines) would change in coral communities through time in two models: (i) steady coral cover decline from 45 to 10 percent (red lines) and (ii) steady coral recovery from 10 to 45 percent (blue lines).

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Questions

Question 1

Corals are foundation species because their effects on the coral-reef community are due to their relatively large size and abundance. They are also ecosystem engineering species; as they grow, their skeletons physically modify the environment, increasing habitat for other reef species.

Question 2

The genera Acropora and Orbicella would be most important in building reef structure, and Porites and Agaricia would be least important. Acropora and Orbicella have the highest rugosity, indicating greater complexity, which results in a diversity of reef habitats. They also have the highest calcification rates, indicating that they add reef structure more rapidly than the other two genera.

Question 3

Because of the rapid decline in both reef rugosity and rate of calcification, the genera Acropora and Orbicella are likely declining most rapidly. These are the genera most responsible for the large size and complexity of the reef. The loss of these genera would affect reef function by reducing the number of habitats and thus the number and abundance of many species in the reef community.

Question 4

The reef does not appear to be regaining its original functionality, at least in the time span of the model. Although the coral cover increases from 10 to 45 percent, neither rugosity nor calcification rate shows a corresponding increase. Rugosity varies only slightly over the time span, ending with a slight upward trend. Calcification rate increases near the middle of the time period before showing a continuous downward trend. The low levels of both factors, compared with their very high levels in the original healthy reef (seen at the start of model i), indicate a much lower functionality. The low functionality may be a matter of timing; a reef’s function can be compromised when foundation species are lost (as shown in model i), but because the foundation species (massive corals such as Acropora and Orbicella) are replaced slowly, it takes much longer for the reef to recover. The lower functionality probably indicates that foundation species are being replaced by smaller species such as Porites and Agaricia, both of which have lower rugosity and calcification rates. Their effect may be significant in the short term because of their rapid reproductive rate, but less than that of Acropora and Orbicella because they do not contribute significantly to reef building.

Question 5

Yes, both model coral-reef communities are undergoing succession. For model ii, the recovery could be considered secondary succession, given that it is restoring its structure after a major disturbance (loss of most of its coral structure). According to the concept of alternative states, under similar environmental conditions, succession can result in different assemblages of organisms. That is, the new community does not return to its original state. In model ii, the coral community does not reach the same level of rugosity or calcification as the starting conditions of model i, indicating that either different species are present and/or their abundance has changed. The lower levels of rugosity and calcification will result in fewer habitats and lower biodiversity. Given more time, as the reef continues to undergo succession, more massive, slower-growing corals may again predominate, and the reef may be able to support higher species diversity. But the possibility that the coral-reef communities show hysteresis, or the inability to recover to the original community, is also a possibility if the effects of climate change intervene.