The Human Journey

Turning Points in the Development of Contemporary Society

The Sixth Extinction: An Unnatural History

Vintage, 2006

Pages 12

The Body Snatchers

While loss of habitat is the greatest single cause of the reduction in biodiversity, loss due to competition by invasive species is not far behind. Invaders arrive without their predators, giving them an enormous advantage over native species in the struggle for resources.  Every year more non-native mammals, birds, amphibians, and so forth arrive in the USA than there are native species. In the last five years, white nose syndrome, a fungal infection from Europe has killed 95% of the bats in the eastern US. Since bats devour massive numbers of agricultural insect pests, the infestation is expected to drive up food prices.

Ten years ago the golden frog was the signature species of the Panamanian rainforest.  Today it probably exists only in captivity, a victim of another fungus known as Bd that attacks the frog’s skin, basically suffocating it. There are 7,000 species of amphibians world-wide, mostly frogs, and it’s estimated that 30% of all amphibians are infected with the Bd fungus. They were already stressed by habitat loss and pollution, and with the added stress from Bd, amphibians have become the most endangered class of animal, dying at perhaps 45,000 times their average rate.

Golden frog on a leaf
The Panamanian Golden Frog. Recently thriving, it became extinct in the wild within about 5 years.

Since the Bd fungus lives in the soil it infects animals that are released to the wild, so how can rescued amphibians ever be returned to their habitat? The fungus is spread by people transporting types of frogs that had developed immunity: the African Clawed Frog, for example, is a host for Bd but is itself immune. It was widely used for decades in a test for human pregnancy. Still, how the fungus is in so many remote and formerly pristine locations, like the Panama rainforest, is a mystery. 

Humans have connected the biology of even the remotest places to everyplace else. A count taken during a single summer in Antarctica revealed that tourists and researchers inadvertently brought with them more than 70,000 seeds!  

In Hot Water

We have increased the amount of CO2 in the atmosphere to levels higher than at any time since humans first evolved, greater than any time in the last 15 million years. This will likely increase average global temperatures by 2°C to 4°C, enough to melt the Arctic ice cap and inundate coastal areas in the ensuing rise in sea level. Climate change will intensify all of the existing threats to wildlife and encourage predation. 

Additionally, the change in climate presents its own unique dangers. Coral reefs are under heavy and increasing pressure from a variety of sources. Reefs are the marine equivalent of tropical rainforests, supporting 25% of marine species. Today it’s difficult to find an undamaged reef in Indonesia or the Philippines, for example, where it’s common for the locals to fish using dynamite. “Coral Bleaching,” spurred by higher water temperature, has contributed to a 50% decline in the Great Barrier Reef in the last 30 years, and by 80% for reefs in the Caribbean.

Coral reef and divers
A coral reef in the Caribbean.

Acidification is another peril for reefs and to clams, oysters, starfish, types of seaweed and plankton.  Oceans absorb 30% of the CO2 produced by human activities. This absorption is slowing global warming but it is making seawater 30% more acidic than it was 200 years ago. One can already observe the consequences of acidification not only, as mentioned before, in the ocean off the west coast of Africa but at a natural source near an island in the Mediterranean which emits carbon dioxide into the surrounding sea. An underwater vent turns the water gradually more and more acidic the closer one approaches. When the pH drops to 7.8, a tipping point is reached at which the population of native crustaceans drops by 1/3, and the shells of those that survive are thin and almost transparent. Kolbert states that if the current rate of CO2 absorption continues, all of the world’s reefs will be dead or dying in less than 50 years. Remember, ocean acidification was likely the coup-de-grâce of the end-Permian or “The Great Dying” event, which was the largest mass extinction in Earth’s history; but oceans today are acidifying even faster than they did then.

“Hope is the thing with feathers” – Emily Dickinson

Whooping Cranes and light craft

On one hand, a young man shot the first breeding whooping crane released back into the wild. On the other, volunteers in ultralight aircraft (even Vladimir Putin) help cranes bred in captivity find their way to their breeding sites. What kind of species are we, anyway?

The Holocene extinction, which may expand to include us, is not a natural event. There is no massive outburst of gases from gigantic volcanoes poisoning the air, nor has the Earth’s orbit shifted so that its surface is covered in ice, both of which have happened. Humans are the agents of this event, and we can make it stop. We have the ability to at least mitigate the effects of what we have done. Many people are working to do so, and more people need to be convinced that they must cooperate. We have to change the way we deal with nature, or nature will surely change the way it deals with us.

Appendix – The Magnitude of the Holocene Extinction

The most widely accepted counts of endangered species are produced by the International Union for the Conservation of Nature. The annual IUCN Red List provides seven categories of threat, from recently Extinct down to Least Concern. Species are sorted into eight higher taxonomic groups (classifications) for an overview of the current state of the natural world.

Endangered Species Chart
Data from the IUCN Red List of 2007. Bars indicate the percentage of the total number of species in each group in a category of risk. A conservative approach is taken to listing a species; still, the number of species in each risk category has increased significantly in the years since 2007.

The threat to mammals most captures our attention. There are about 5,500 species of mammals, including ourselves. Recent estimates list 165 as critically endangered and another 275 as endangered. In official terms, “critically endangered” equates to “possibly already extinct in the wild,” and “endangered” means “expected to soon become extinct in the wild.” These two categories contain such familiar animals as two humped-camels, mountain gorillas, giant pandas and chimpanzees, the latter likely to be gone from the wild in 10 years. Add in another 900 or so species listed as vulnerable (“likely to become endangered”), and fully ¼ of the mammals on Earth are threatened.

But what if the Holocene extinction is more limited? At what point for our species does this change from tragedy to peril? It might not take many species lost, compared to the extinctions of the past. 90% of wheat varieties have no resistance to a new type of rust that is now spreading from African tropics into Asia. Wheat supplies 30% of humanity’s calories, a kind of dependence completely unknown to our evolutionary ancestors. Human civilization is more fragile than the human species. Along with crows, cockroaches, and coyotes, humans will likely survive any mass extinction, but, unless many more of us doing something now, they may spend their days picking through rubble.