Abstract
There has been substantial recent change in coral reef communities. To date, most analyses have focussed on static patterns or changes in single variables such as coral cover. However, little is known about how community-level changes occur at large spatial scales. Here, we develop Markov models of annual changes in coral and macroalgal cover in the Caribbean and Great Barrier Reef (GBR) regions. We analyzed reef surveys from the Caribbean and GBR (1996–2006). We defined a set of reef states distinguished by coral and macroalgal cover, and obtained Bayesian estimates of the annual probabilities of transitions between these states. The Caribbean and GBR had different transition probabilities, and therefore different rates of change in reef condition. This could be due to differences in species composition, management or the nature and extent of disturbances between these regions. We then estimated equilibrium probability distributions for reef states, and coral and macroalgal cover under constant environmental conditions. In both regions, the current distributions are close to equilibrium. In the Caribbean, coral cover is much lower and macroalgal cover is higher at equilibrium than in the GBR. We found no evidence for differences in transition probabilities between the first and second halves of our survey period, or between Caribbean reefs inside and outside marine protected areas. However, our power to detect such differences may have been low. We also examined the effects of altering transition probabilities on the community state equilibrium, along a continuum from unfavourable (e.g., increased sea surface temperature) to favourable (e.g., improved management) conditions. Both regions showed similar qualitative responses, but different patterns of uncertainty. In the Caribbean, uncertainty was greatest about effects of favourable changes, while in the GBR, we are most uncertain about effects of unfavourable changes. Our approach could be extended to provide risk analysis for management decisions.
Highlights
Coral reefs are complex and diverse ecosystems with high economic and ecological value [1]
In the Great Barrier Reef (Figure 3), most observations were concentrated in states A, D, and F (Table 1), with low macroalgal cover and low to high coral cover
In the Caribbean, the probability of persisting in state A, which has up to 25% cover of both corals and macroalgae, (Figure 4A) was lower than in the Great Barrier Reef (Figure 5A), while the probability of transition from state A to state B was higher in the Caribbean (Figure 4G) than the Great Barrier Reef (Figure 5G)
Summary
Coral reefs are complex and diverse ecosystems with high economic and ecological value [1]. There have been substantial changes in structure and functioning of coral reef communities worldwide in recent decades [2,3,4,5], and perhaps over much longer time scales [6,7]. Mass coral mortality has led to phase shifts in which reefs have become dominated by macroalgae [2,13,14] or other organisms [15]. Studies of these changes generally fall into three categories. Studies of one or a few reefs can suggest hypotheses about possible mechanisms, e.g. Analyses of large data sets can be used to evaluate the evidence for patterns of change over large spatial scales, e.g. [21,22,23]
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