As this is my first crack at an explanatory post, I’d appreciate feedback on both the level and quality of the writing.  Thanks!


One of the things I plan to do on this blog is delve a little deeper into ecological stories that arise in the news.  A great opportunity popped up today in a front-page story in the New York Times about coral reef bleaching.  It seems that record temperatures this year are causing one of the largest coral die-offs in history.  I urge you to read the whole thing, but I’ll focus on this part:

If temperatures drop, the corals’ few remaining algae can reproduce and help the polyps recover. But corals are vulnerable to disease in their denuded condition, and if the heat stress continues, the corals starve to death.

Even on dead reefs, new coral polyps will often take hold, though the overall ecology of the reef may be permanently altered. The worst case is that a reef dies and never recovers.

This is an example of hysteresis in an ecological system.  This is an important concept in ecology and central to my own research.  An ecosystem that shows hysteresis can undergo rapid declines in response to small environmental changes, but large environmental changes are needed to return the system to its original condition.  I’ll illustrate with a graph:

From: Scheffer et. al.

Let’s say that the x-axis here, “Conditions”, represents temperature, and the y-axis, “Ecosystem state”, represents coral reef health.  When we start out at the upper left, the reefs are healthy, but as temperatures rise, they get worse slowly.  When we reach a critical point (F2), a bleaching event occurs and the health of the reef rapidly declines. 

To get the coral reef back to the point where it started, though, temperatures have to drop much further than to the level where the bleaching event occurred.  They have to go lower, all the way back to point F1, for the system to recover.

Hysteresis presents a vexing problem for envioronmental protection, and it occurs in many ecosystems.  Lakes polluted with fertilizer experience a rapid growth in algae that persists even when pollution is reduced, droughts can turn grasslands into deserts that don’t recover with increasing rainfall, and it is believed that the melting of polar ice caps will cause extreme global warming that will be hard to reverse.

Usually, we don’t know exactly when the rapid change (popularized by Malcolm Gladwell as the “tipping point”) will occur, and once it does it is very difficult to get things back to where they used to be.  Often the best approach is to improve resilience, meaning improving a system’s ability to recover from small impacts so that they don’t become large ones.  Resilience is another big part of my work, and I will write more about it in a future post.

For further reading on hysteresis in ecosystems, see Scheffer et. al. “Catastrophic shifts in ecosystems.” Nature. 413:591-6.  For more on coral reefs, see Mumby, P. “Phase shifts and the stability of macroalgal communities on Caribbean coral reefs.” Coral Reefs. 28:761-773