Scientists are now beginning to describe what they think of as "ecological thresholds"; that is, the point at which ecological decay becomes persistent, inevitable and irrevocable change.

Take, for example, Antarctica, where the Wilkins Ice Shelf hangs by a thread, threatening to plunge into the ocean.

In and of itself, this threat is minor. Most of the ice shelf is already underwater, displacing more of the ocean in the form of ice as it would if it melted. The top portion, 65 feet tall, represents only one-tenth of the total mass, and even melted would raise sea level by no more  than a few centimeters.

The real danger lies in the fact that a disconnected Wilkins Ice Shelf will permit the main Antarctic ice mass, trapped in glaciers, to begin its ever more precipitous slide toward the ocean. The loss of Wilkins is, in effect, like opening a floodgate. This ice mass, in total, contains enough frozen water to raise sea levels by an additional 187 feet.

Thus Wilkins represents one of those ecological thresholds, and the U.N. Climate Panel's 2007 estimate - that sea levels would rise by 7 to 23 inches during this century - did not account for the possibility of a melting Antarctica.

Elsewhere, these thresholds are represented by insect outbreaks, wildfire, and forest dieback, and a report commissioned by the U.S. Climate Change Science Program, under the auspices of the U.S. Geological Survey (USGS), suggests that such changes are likely to affect humans as much as animals and plants.

According to Susan Haseltine, an Associate Director for Biology at the USGS, "One of our biggest concerns is that once an ecological threshold is crossed, the ecosystem in question will most likely not return to its previous state."

This state of affairs will likely be exacerbated by human activities. In times of drought for example, industrial and domestic water use may represent a "tipping point" for the fish and plants living in lakes in streams. A prime example of a tipping point may be the swarms of jellyfish taking over oceans. Jellyfish are omnivorous and opportunistic, and thrive where other species fail. Unfortunately, once they've taken over ecosystems, they tend to prey on other species to the point of extinction, and only die back when they run out of food as a result.

Most insects thrive in temperate regions. As the earth warms, insects like the ant species lasius neglectus, native to the Black Sea region, are taking over Europe and destroying both native ant species and regional vegetation. Bats, dying by the thousands from a curious virus known as "White Nose Syndrome", open the door to more mosquito infestations. A single little brown bat can eat more than 2,000 mosquitoes in a night. Mosquitoes carry West Nile Virus, which has so far no known treatment. In 2007, 124 people in the U.S. died of West Nile, and as the climate warms the disease is likely to claim even more lives. 

Other insects that have gotten out of hand globally include the pine bark beetle, which has wiped out 3.5 million acres in Colorado alone. The beetle, once restricted to southern forests, is moving northward, and evergreens in the Western U.S, decimated by rising temperatures and drought, succumb readily. Where pines die, they are replaced by deciduous softwoods like aspen and willow, and later these native species are crowded out by tamarisk. These ultimate "regime shifts" change the ecosphere forever for birds, chipmunks, squirrels, and other forest denizens.

An old French proverb says that we never know the worth of a thing until we have lost it. For the generations now living on earth, and threatened with the loss of a natural world they took for granted, these ecological thresholds may become a learning experience. Alas, too late for that world itself.

Related Reading:
From Bees to Bats: The Death of the Natural World
The EPA: Keeping Quiet on Colony Collapse Disorder

Image Credits:
lealabyrinth.typepad.com
tncinvasives.ucdavis.edu