The Sixth Extinction

In the 525 million years or so since creatures with backbones first appeared on our planet, there have been five mass extinctions.  A mass extinction (aka an "extinction event") is a widespread, geologically rapid loss of biodiversity.  In each of the five events classified as mass extinctions, no less than 60% (Ordovician-Silurian event, ~450 million years ago) and as much as 96% (Permian-Triassic event, ~250 million years ago) of all species were lost.  The most famous mass extinction is that which doomed the dinosaurs (and many other species) about 65 million years ago.  An asteroid crashed into the Yucatan, the dinosaurs died off and mammals rose to the top of the food chain.

Elizabeth Kolbert's The Sixth Extinction takes a look at the evidence that we may be in the midst of a sixth mass extinction - what may come to be known as the Anthropocene extinction.  Referring to an article that appeared in the Proceedings of the National Academy of Sciences, Kolbert writes, "If Wake and Vredenburg were correct, then those of us alive today not only are witnessing one of the rarest events in life's history, we are also causing it."

Until the late 18th century the idea that species could go extinct had few, if any, supporters. Evidence from the fossil record gradually proved that extinctions did occur and that there were some extinctions that were so massive that they only could have been caused by a catastrophic event - for example, an asteroid,  a huge volcanic eruption or a severe change in the climate.  Kolbert takes us on a journey through the causes of extinctions in a series of vignettes focusing on one or another creature that has become, or is in the process of becoming, extinct.

Panamanian golden frog (Wikipedia)

She first takes us to Central America where the Panamanian golden frog is disappearing - apparently from a previously unknown, but now ubiquitous, fungus.  There are two theories on how the fungus became so widespread.  The first theory is that it spread with shipments of African clawed frogs used in pregnancy tests in the 1950's and 60's.  The second is that it spread with the export of North American bullfrogs, often for human consumption, to Europe, Asia and South America.  In either case, without being loaded by someone on a boat or a plane, it would have been impossible for a frog carrying the fungus to spread from Africa or North America to other parts of the world.  The spread of the deadly fungus "appears to be, for all intents and purposes, unstoppable." Amphibians are the world's most endangered class.of animals. "But extinction rates among many other groups," Kolbert writes, "are approaching amphibian levels.  It is estimated that one-third of all reef-building corals, a third of all fresh water mollusks, a third of sharks and rays, a quarter of all mammals, a fifth of all reptiles, and a sixth of all birds are headed toward oblivion."

Human hunters played a role, along with climate change, in the extinction of the North American mastodons.  The last of the great auks was killed by hunters on an island near Iceland in 1844.  But besides hunting animals to extinction, we've found less direct ways to endanger species. Since the Industrial Revolution began, the CO2 level in the atmosphere has increased by nearly 50%; atmospheric methane levels have more than doubled.  Our impact on the world around us is so great that some scientists are beginning to refer to the current geologic age as the Anthropocene.  "Roughly one-third of the CO2 that humans have so far pumped into the air has been absorbed by the oceans...a stunning 150 billion tons.  As with most aspects of the Anthropocene, though, it's not only the scale of the transfer but also the speed that's significant."

Coral reef near Fiji
The Guardian article on World Research Institute's 2011 Report: "Reefs at Risk"

Besides the impact on atmospheric temperatures and sea levels, the excess carbon dioxide being absorbed by the sea is acidifying the sea - to the detriment of sea life. Of the myriad possible impacts of this acidification,  Kolbert tells us, "probably the most significant involves the group of creatures known as calcifiers."  This varied group includes starfish, sea urchins, clams, oysters, coral, seaweed, algae, and many other species. Coral reefs, besides being threatened themselves, provide an amazing ecosystem. "Thousands - perhaps millions of species have evolved to rely on coral reefs, either directly for protection or food, or indirectly, to prey on species that come seeking protection or food."  Ocean acidification is just one of the threats facing coral reefs -overfishing and deforestation also endanger them - because of increased algae growth and susceptibility to pathogens,  Climate change also adds to the extinction threat - if water temperatures rise too high, coral reef bleaching occurs and the colony stops growing or, in the worst case, dies.  Quoting British scientists,  Kolbert writes: "It is likely that reefs will be the first major ecosystem in the modern era to go extinct."

Kolbert discusses forests and the impact of climate change on species distribution, invasive species and man's role in transporting them, and the near-extinction of the Sumatran rhino and its relationship to the mega-fauna extinctions that occurred 40,000 and 20,000 years ago.  She notes that people have directly transformed roughly 27 million square miles of the 50 million square miles of land on the planet that are are ice-free and habitat disruption plays a role in the loss of species.

She has an intriguing chapter on the disappearance of our older siblings - the Neanderthals, the Denisovans, and the recently discovered homo floresiensis - and on the threat to our next closest kin - the great apes.  Chimpanzee and highland gorilla populations have dropped 50% over the past 50 years, while the lowland gorilla population has shrunk by 60% over the past two decades and Sumatran orangutans are now classified as "critically endangered".

The human ability for cooperative problem-solving, creativity,our restlessness and inquisitiveness, and, perhaps most of all, the capacity for symbolic representation and language - due to "a tiny set of genetic variations" - divide us from Neanderthals and other near relatives, "but that has made all the difference."

The anthropocene or sixth extinction will most likely not be as massive as any of the "Big Five." Writing about the American Museum of Natural History's Hall of Biodiversity, Kolbert asks "In an extinction event of our own making, what happens to us?"  There are two schools of thought on this. One possibility is that "we, too, will eventually be undone by our transformation of the ecological landscape."  The other, more optimistic, possibility is that "human ingenuity will outrun any disaster human ingenuity sets in motion"  - for example, by means of atmospheric re-engineering or, in the worst case, settling the planets and moons of the solar system. But in the end, perhaps the fate of our own species, Kolbert muses, is not "what's most worth attending to.  Right now, in the amazing moment that to us counts as the present, we are deciding. without quite meaning to, which evolutionary paths will remain open and which will be closed forever.  No other creature has ever managed this, and it will, unfortunately, be our most enduring legacy."

Related
Our Closest Call [The Left Bank Cafe, July 29, 2012]

"Levels of heat-trapping carbon dioxide in the atmosphere rose at a record-shattering pace last year, a new report shows, a surge that surprised scientists and spurred fears of an accelerated warming of the planet in decades to come." [Washington Post, September 9]