Posted by: oikosasa | December 20, 2012

What happens with coral reefs after anthropogenic disturbances?

What are the chances that the reefs recover? And how likely is it that they just turn into seeweed-dominated ecosystems instead?  Important issues that Peter Mumby and his colleagues have studied and modelled in the new Early View paper “Evidence for and against the existence of alternate attractors on coral reefs”.

Here’s Peter’s summary of the study:

Coral reefs have been heavily stressed by local anthropogenic disturbances, like fishing and pollution, as well as global events such as ENSO which can cause coral bleaching and wreak devastation on living coral. Ideally, corals would recover after some kind of disturbance but a number of studies have documented a lack of recovery and even continued decline of corals rather than return to a coral-rich ‘attractor’. This raises the question, ‘do coral reefs exhibit multiple attractors?’. If they do, then it is possible for negative feedbacks to emerge that not only prevent reef recovery but reinforce themselves and trap reefs within an undesirable state, often dominated by seaweed. If reefs do become trapped in an undesirable state it might prove extemely difficult for management to reverse the decline and facilitate the return of a healthy ecosystem. 
 
Ecological models of coral reefs have studied the effects of various disturbances including the fishing of herbivores such as parrotfishes. Theory predicts that Caribbean coral reefs do indeed exhibit alternate attractors particularly in their somewhat degraded states today. However, empirical studies have claimed to find no evidence to support this theory. There is, therefore, a controversy over whether reefs can become trapped in seaweed-dominated systems. In this paper we argue first that the empirical studies were incapable of testing for multiple attractors. We then provide new comparisons between theoretical predictions and field observations, both of which are consistent with multiple attractors. However, it is also possible to fit a simpler model to empirical data that does not exhibit multiple attractors. When we take a careful look at this model we find that it makes several troubling ecological assumptions, which lead us to doubt its veracity.
OIK_262_f6
 
Proving the existence of multiple attractors is extremely challenging and there is, as yet, no definitive proof either way. However, the weight of theory and field observation appears to support the notion for Caribbean coral reefs. Given this, and it’s important conservation implications, we feel that management should proceed on the conservative – and more likely – assumption that reefs can become stuck in seaweed states if stringent steps are not taken to increase their resilience.
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Responses

  1. [...] For the April issue, we chose the following two papers as editor’s choice according to our motto of synthesizing ecology. Mumby et al. (2013) discuss various articles that either support or reject the hypothesis that coral reefs might be able to exist under certain conditions in two alternative stable states (ASS): a coral-dominated and a macroalgae-dominated state. Given the fact that the existence of multiple attractors is controversial, synthesis needs to be created by compiling various forms of evidence. Mumby and colleagues provide such an overview of evidence by providing analyses of the literature and the available empirical and theoretical data. By means of this integrated approach, they conclude that the most compelling evidence, which combines ecological models and field data, is far more consistent with multiple attractors than the competing hypothesis of only a single, coral attractor. This message warns managers that degraded reeds might never be able to be restored once dominated by macro-algae. Read Peter Mumby’s summary of the paper here [...]


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