In this well-researched book, science writer Elizabeth Kolbert casts a strong light on the damage humans are doing to planet Earth. In one example Kolbert describes declining populations of the golden frog, which is rapidly disappearing from all its native habitats. Turns out humans have inadvertently spread a type of fungus that infects the skin of amphibians and kills them. In another example, almost six million North American bats have (so far) died from a skin infection caused by a different fungus, also accidently spread by people.
Perhaps less ecologically-minded people might think "who cares about frogs and bats?" But all species on Earth are part of an interactive ecosystem, and the disappearance of any one organism might set off a domino effect that has unseen consequences down the line. Moreover, these sad occurrences are just the teeny tip of a humongous iceberg when it comes to changes wrought by human activity.
Species extinction is not a recent phenomenon on Earth. In fact there have been five documented instances of mass extinctions (the disappearance of a large number of species in a short time) in the course of the planet's history. These are:
• The Ordovician-Silurian extinction, about 440 million years ago, thought to be caused by cycles of glaciation and melting.
• The Late Devonian extinction, about 360-375 million years ago. The cause is unknown but some experts suggest periods of global cooling and glaciation.
• The Permian-Triassic extinction, about 250 million years ago, which may have resulted from an asteroid impact or massive volcanic eruptions (or both). This was the largest extinction event in Earth's history, wiping out 95 percent of species living at the time.
• The Triassic-Jurassic extinction, about 200-215 million years ago, apparently caused by colossal lava floods - and perhaps global warming - related to the breakup of Pangaea (a supercontinent made of all Earth's landmasses).
• The Cretaceous-Paleogene extinction, about 66 million years ago, thought to be due to an asteroid impact. Evidence for this is the Chicxulub crater in the Yucutan Peninsula of Mexico. This extinction is well known in popular culture because it wiped out the dinosaurs.
Each extinction event left vacant ecological niches
and - over time - these were filled by the expansion of remaining species and the evolution of new organisms. Taking into account all the cycles of extinction and speciation in the planet's history, scientists speculate that 99.9 percent of species that lived on Earth are gone. Unfortunately, humans - by causing profound changes in Earth's ecosystems - may now be causing the sixth mass extinction. Examples of what humans are doing to Earth include:
• Burning fossil fuels, which adds CO2 to the atmosphere. This has a dual effect. It causes global warming, which affects the distribution (and survival) of plants and animals; and it acidifies the oceans, causing calcite to dissolve. Thus, coral reefs are being destroyed and molluscs are getting holes in their shells.
• Destroying habitats to accomodate expanding human populations. This includes cutting down forests, constructing roads and buildings, and cultivating monoculture farms - all of which demolishes the homes of native organisms.
• Transferring organisms to new habitats. When people started moving from place to place they - purposely or not - took other organisms with them. For instance, brown rats - which seem to be indestructible - rode ships to almost every corner of the world, ravaging native species; rabbits brought to Australia as food animals became one of the biggest pests on the continent; brown snakes, introduced to Guam, wiped out nearly all the native birds; and kudzu vines - introduced to the U.S. from Asia - cover and smother all vegetation in their path. It's estimated that people are moving 10,000 species around the world every day, mostly in supertanker ship ballast. The consequences of this are potentially disastrous for indigenous plants and animals everywhere.
• Overharvesting and hunting animals to extinction. In the North Sea, Grand Banks of Newfoundland, and East China Sea, overfishing has severely depleted fish stocks. In addition, many animals have been completely wiped out by humans, including the dodo, Tasmanian tiger, passenger pigeon, Steller's sea cow, and great auk (a flightless bird). In a sad anecdote Kolbert describes how - on July 3, 1844 - a hunter named Sigurður Ísleifsson strangled the world's last two great auks on Eldey Island, near Iceland.
In "The Sixth Extinction" Kolbert sounds the alarm about humans wreaking changes on Earth in the current era - dubbed the "Anthropocene." With luck, Kolbert's book might help persuade concerned people to stop damaging the environment, curtail global warming, and save threatened species. Some measures are already in place: the U.S. has an "Endangered Species Act" designed to protect imperiled organisms; international agreements have been made to alleviate global warming; and "frozen zoos" store DNA from thousands of plants and animals, in hopes of resurrecting them if they disappear. Still, it may be too little too late.
As far as the Earth is concerned, a "sixth extinction" could be just another cataclysmic event from which the planet will gradually recover. For humans though...well...we might just wipe ourselves out in such a catastrophe. If so, something will inevitably take our place. Elizabeth Kolbert (half jokingly) suggests it might be giant intelligent rats (ha ha ha).
Some people think humans can counteract the harm we've done to the Earth. One "solution" for global warming, for example, involves spraying salt water into low-lying clouds, to enhance their ability to reflect sunlight. Even if this worked, though, it would solve only one problem of many. In the extreme case of irreparable harm to Earth, some optimists(?) believe the human race will survive by colonizing other planets. Only time will tell.
Kolbert's book is well-written, engaging, and personal - with anecdotes based on her own observations as well as interviews with scientists she accompanied on their research trips. I'd recommend this enlightening and interesting book to everyone interested in the Earth's future.
FYI: If you like the 'move to other planets' scenario you might enjoy the novel Seveneves by Neal Stephenson...which has a related theme.