One of the greatest challenges in managing ecosystems sustainably is the lack of meaningful indicators or measures that can provide "early-warning" of serious degradation. A recent study by McClanahan et al, published in PNAS, has developed such critical reference points to assist the sustainable management of coral reefs.
As the authors write, one of the obstacles in steering towards sustainable management of coral reefs is that managers evaluate the condition of coral reefs based on metrics (e.g. % cover of hard coral cover or fish biomass) that poorly link to the loss of resilience or proximity to critical tipping points.
McClanahan et al compiled data from a huge set of coral reef surveys (>300) from nine countries in the Indian Ocean. They then examined the general relationships between the biomass of target fish species (i.e. the fish that are targeted and caught by local fisheries) and eight different ecological metrics. The biomass of target fish species, i.e the fish that are targeted and caught by local fishermen, gives us an idea of how heavily fished a coral reef is. Thee eight different measures of coral reef state - e.g. algal cover, ratio between macroalgae vs. hard coral cover, sea urchin predation index, fish diversity, sea urchin biomass, coral cover - are all indicators of the condition of different reef processes and reef resilience. So in essence, by crunching through all this data, McClanahan et al were able to generate a more complete picture of where major changes in coral reef condition occur (by looking at how the eight different measures of reef state behave) along a gradient in fishing pressure (a strong human driver). And by doing this, one can begin teasing out which of the different indicators could qualify as early-warning signals of degradation.
Fig. 1 (taken from the paper) shows the state of the eight different ecological metrics
Importantly, the metric used by most managers to gauge the health of reef systems - coral cover - is the last indicator to indicate ecosystem failure. This verifies much of the earlier work that has criticized coral cover as being a poor measure of coral resilience. Overfished reefs can appear healthy, i.e. have a high coral cover, and then suddenly shift to algae dominated seascapes.
So the paper makes a strong argument that measuring the biomass of fish instead of coral cover is a better way to identify the early warning signals of reef collapse.
The study also assessed how the different measures of reef state related to different management schemes - full access fisheries, gear-restrictions and no-fishing. Unsurprisingly, reefs that were closed to fishing had the highest level of fishable biomass. However, even easily enforceable regulations that restrict gear or the types of species that can be caught helped maintain biomass. This is important since in closing reefs from fisheries is difficult in such region were people are so dependent to fish for their livelihoods.