There have been five major episodes of massive extinctions in the 4.5 billion years that our planet has existed. All occurred within the past 500 million years because there was little known as “life” prior to that time. We are now experiencing the sixth massive extinction episode which began approximately 50,000 years ago with the dispersal of humans around the world. The causes of prehistoric extinctions are not fully known, unlike the current episode. We know that we are the cause of the sixth extinction, but we seem to be incapable of preventing it.
The fifth and most recent massive extinction event occurred about 65 million years ago. It brought the age of dinosaurs to an abrupt end. There were no humans or even our primate ancestors at that time. The cause of that extinction was only recently discovered in the 1980s and even more recently accepted by most scientists. There is now general agreement that the entire environment of the planet was radically and suddenly altered by the impact of a huge asteroid that landed on what is now the Yucatan peninsula in Mexico. The impact raised a huge dust cloud that engulfed the earth and precipitated the equivalent of a nuclear winter, killing most vegetation and the animals adapted to a much warmer climate. As with all massive extinctions, it took many millions of years for the environment to recover from that event and for plants and animals to slowly evolve adaptations to the new environment.
Update: There is an alternate theory about the cause of the fifth extinction. Huge volcanic eruptions in India may have been the cause, or perhaps a contributing factor. Explained HERE.
The third and biggest extinction event occurred about 250 million years ago at the end of the Permian geologic period. Paleontologists tell us that about 90% of all living plant and animal species died as a result of that extinction event. Like the fifth extinction, the End-Permian extinction was precipitated by a sudden and radical alteration in the climate. However, less is known about what caused that change in the climate. Like our current round of climate change, there was a massive release of carbon into the atmosphere with a related drop in oxygen. These changes caused temperatures to soar and the chemistry of the oceans to acidify. Although there is not yet consensus amongst scientists, current speculation in the scientific community is that the changes in atmospheric conditions were the result of huge volcanic eruptions in what is now Siberia that emitted carbon dioxide into the atmosphere. (2)
The first massive extinction occurred about 450 million years ago just 50 million years after the first land plants began to emerge on the planet. In fact, the plants may have been a factor in the climate change that caused the extinction at the end of the Ordovician geologic period. The cooling of the climate that caused the extinction was associated with a sharp drop in carbon dioxide levels which may have been partially the result of plants that convert CO₂ to oxygen. The movement of the continents is also thought to have been a factor in the cooling because the breakup of the unified continent, Pangaea, changed the circulation of ocean currents which affect the climate on land.
All of the massive prehistoric extinctions were associated with sudden changes in climate, although human perception of time should not be imposed on the word “sudden.” These events occurred over thousands of years and are only “sudden” when compared to the 4.5 billion years of the existence of our planet.
Extinctions of the Anthropocene
Genus Homo evolved into its only surviving species, Homo sapiens, about 200,000 years ago. That’s us…humans. However, we didn’t begin to extinguish plant and other animal species until our population grew and dispersed throughout the world. And when we did, the first victims of our ability to hunt cooperatively with weapons were the megafauna, now largely gone from the world.
Megafauna are the huge animals now known primarily from their fossil remains that were so large they had no predators until humans brought their intelligence to the task of hunting which was previously limited by size and speed. Megafauna reproduction wasn’t capable of keeping up with the pace of human hunting because they had long gestation periods, many years to sexual maturity, and small numbers of offspring.
Humans reached the Australian continent about 50,000 years ago. When they arrived, Australia had its own megafauna: giant kangaroos and other enormous herbivores. Within 10,000 years the megafauna were gone and the landscape changed as grazing was significantly reduced: “With no more large herbivores around to eat away at the forest, fuel built up, which led to more frequent and more intense fires. This, in turn, pushed the vegetation toward fire-tolerant species.” (1) Conversion to grassland savanna was also accelerated by the frequent fires intentionally set by humans to facilitate their hunting.
The same shift in vegetation occurred in North America when humans arrived about 13,000 years ago and American megafauna such as mastodons and giant sloths were hunted to extinction. Grassland found in North America when Europeans arrived thousands of years later in the 16th century was therefore not adapted to heavy grazing and was largely destroyed by domesticated animals brought by early settlers. Native Americans did not have domesticated animals.
Similar scenarios played out around the world as humans arrived, most recently on the Pacific Islands where Polynesians arrived as recently as 1,500 years ago. Huge flightless birds were found on New Zealand until they were hunted by humans just 500 years ago.
The second wave of extinctions caused by humans occurred during the age of exploration, beginning in the 16th century. Humans wiped out many species of animals all over the world to feed their explorations and early settlements. Huge turtles were brought on board ships to feed the crew on long voyages. Passenger pigeons and American bison were killed by early settlers for food, leather, and sport.
As humans developed agriculture and domesticated animal-herding, hunting wild animals decreased. In developed countries, extinctions today are largely by-products of western civilization, through mechanisms such as climate change and global exchange of diseases and pathogens…all equally deadly to other living things.
There are no longer any physical barriers to the exchange of pathogens and pests. Invasion biology is based on the fiction that such exchanges can be prevented or even reversed. The most deadly invasions prove otherwise:
- Amphibians, especially frogs, are being wiped out all over the world by a fungal disease that is traveling fast. It is now known to exist in Central, South, and North America, as well as Australia. The means of transmission is not yet known.
- Bats are dropping dead by the tens of thousands primarily in New England as they succumb to a different fungal disease. Nothing is known about how this disease is transmitted. We should probably assume that it will also spread beyond its current range.
- Insects, such as the emerald ash borer that is killing millions of ash trees in the United States, have been accidentally introduced as a result of global trade.
We should expect the loss of these species to reverberate throughout the food web, although little is known about the secondary effects of the loss of species. For example, when bats are no longer available to eat insects, what will those insects eat? And what will the animals that ate frogs eat when the frogs are gone? These animals may be playing roles about which we know little and therefore cannot predict the consequences of their loss.
The spread of pathogens and insects that prey on plants could be related to climate change. For example, the pine bark beetle is a native insect that has become a serious problem in the forests of North America because mild winters associated with global warming are not cold enough to cause an annual die-back of the insects. The range of the pine bark beetle has expanded and is killing millions of acres of forests in North America.
Ecosystems are being fragmented by agricultural development. Much of the Amazonian rainforest has been reduced to isolated fragments which are not large enough to support the diverse plants and animals that occupied intact ecosystems.
Climate change…the silent killer
When we look to the distant past, we can see how levels of carbon dioxide in the atmosphere have caused massive extinctions of plant and animal species. Low levels of carbon dioxide have been associated with a cooling phase and high levels of carbon dioxide have caused temperatures to rise. We are now in a period of a huge increase in carbon dioxide levels caused by the activities of humans, particularly emissions associated with the burning of fossil fuels and deforestation. There is scientific consensus that the climate has changed and will continue to change as well as about the causes of those changes. However, we still know little about the long-term consequences of climate change.
One consequence of increased levels of carbon dioxide is well known and that is the acidification of the oceans. The laws of chemistry tell us that when carbon dioxide dissolves in water it forms carbonic acid. Carbonic acid dissolves shells and coral. Aquatic animals such as mussels, clams, oysters, crabs, and lobsters will be incapable of building the shells that protect their bodies when levels of carbonic acid increase. Australian scientists report that coral cover of the Great Barrier Reef has decreased 50% in the past 30 years. A paper published in 2008 predicted the imminent extinction of one-third of 800 reef-building species as a result of increased water temperature and acidity of the oceans. An estimated one-half million to 9 million species “spend at least half their lives on coral reefs.” (1)
So why are we destroying trees?
As disturbing as it is to witness the death of plants and animals which are innocent by-standers to the choices made by humans, we have some sympathy and understanding for why our political system has been incapable of the fundamental changes needed to stop the process. We burn the fossil fuels that emit carbon dioxide and other greenhouse gases to keep us warm in the winter and cool in the summer, to transport us to work and play, to power our industrial processes and many other vital functions.
But, we cannot understand why we continue to destroy millions of healthy trees (that we planted) essentially because they are out of fashion. These trees are storing tons of carbon that will be released into the atmosphere when the trees are destroyed and we will lose their ability to store carbon in the future.
We loved these trees as recently as 50 years ago. Now many people have decided that they “don’t belong” because they aren’t native. Eucalyptus is only one of many targets of this fad. Norway maples are being destroyed in communities in eastern United States for the same reason. And most of the trees being destroyed in the Midwest (because people wish to “restore” the prairie artificially maintained by Native American fires) are even native to the Midwest.
In the case of eucalyptus, the trees are expected to live in California for several hundred more years. How will the climate have changed in 300 years? Will any of the plants presently considered “native” even exist? On our present climate trajectory, the answer to that question is clearly “no.”
Most information in this post is from these two sources:
(1) Elizabeth Kolbert, The Sixth Extinction, An unnatural history, Henry Holt and Company, 2014