Scientists detect mass extinctions using carbon dating of ancient rock layers. It's only happened five times in the earth's history. In May 2019, the United Nations reported that 1 million species face extinction, many within decades. Most scientists agree that the earth is in the process of the sixth mass extinction.
The common culprit in all the past five mass extinctions was a change in the level of greenhouse gases. Rising levels caused global warming while falling levels cooled the planet.
The Ordovician extinction occurred 440 million years ago ending the Age of Invertebrates.
The Devonian extinction occurred 365 million years ago, ending the Age of Fishes.
The Permian extinction was the largest extinction event in history. It occurred 250 million years ago and lasted only 200,000 years. It ended the Age of Amphibians.
The Triassic extinction occurred 200 million years ago. The landmass Pangea broke apart. The resultant widespread volcanic eruptions lasted for 40,000 years. They spewed greenhouse gases that caused global warming and ocean acidification. Over 75% of species went extinct. The extinction of other vertebrate species on land allowed dinosaurs to flourish.
The Cretaceous extinction occurred 65.5 million years ago. A nine-mile wide asteroid hit the Gulf of Mexico. The heat wave burned most of the forests and created a dust cover that blocked the sun. It ended the Age of Dinosaurs. Only animals smaller than a dog survived. Ground-dwelling dinosaurs survived the deforestation to evolve into modern birds. It ushered in the Age of Mammals.
Over the past 100 years, species have been going extinct 100 times faster than the natural rate. The usual rate of extinction is a healthy result of evolution by natural selection.
The U.N. report said that 500,000 species no longer have enough land area to support their survival. More than 85% of wetland areas are gone. More than 79 million acres of forest disappeared between 2010 and 2015 alone.
[Four species of great ape, three species of rhino, three species of big cat, two species of porpoise, the Sumatran elephant, and several other species of large mammals are expected to go extinct in a few years. Other animals at very high risk include Chimpanzee, the blue whale, and two species of tuna.]
.... The biggest cause of insect decline is habitat destruction due to farming and deforestation. Contributing factors also include pesticide pollution, invasive species, and climate change.
Amphibians. At least one-third of the 6,300 known species of frogs, toads, and salamanders are at risk of extinction.... The current extinction rate [of amphibians] is at least 25,000 times the background rate. The chytrid fungus is decimating those that have survived habitat destruction, pollution, and commercial exploitation. [and is said to be] s "the most destructive pathogen ever described by science."
... BirdLife International estimates that 12% of the world's 9,865 bird species are now considered threatened.... Globally, one out of five species [of fish] faces extinction. This includes more than a third of sharks and rays. Also at risk are bluefin tuna, the Atlantic white marlin, and wild Atlantic salmon.
The six major causes of this catastrophe are loss of habitat, the introduction of foreign species, pandemic diseases, hunting and fishing, pollution, and climate change. All of these are man-made. This impact is so prevalent that some scientists are calling this the Anthropocene extinction.
A 2004 study found that human population density was the biggest cause of local higher extinction rates. When people moved into an area, animal species died off. They were hunted, their habitat was cleared for farming, and they were polluted by waste. Humans also brought along foreign species, such as rats, and pandemic diseases that killed off other species.
According to a 2019 United Nations study, the increase in the extinction rate has hurt agriculture. Since 2000, 20% of the earth's vegetated surface has become less productive. In the oceans, a third of fishing areas are being overharvested. Birds that eat crop pests are down by 11%.
Bats and birds that pollinate plants are down 17%....
Farming practices are themselves to blame. Most farmland is used for one of only nine crops: sugar cane, corn, rice, wheat, potatoes, soybeans, palm oil, sugar beet, and cassava. These crops rely on pesticides that also kill useful insects. Although organic farming is on the rise, it only accounts for 1% of farmland.
"Around the world, the library of life that has evolved over billions of years our biodiversity is being destroyed, poisoned, polluted, invaded, fragmented, plundered, drained, and burned at a rate not seen in human history," Ireland's president, Michael Higgins, said at a biodiversity conference in Dublin on Thursday. "If we were coal miners we'd be up to our waists in dead canaries."
ee colony collapse disorder has reduced the U.S. honeybee population by over 40%. This affects the 100 crop species that make up one-third of the average diet....
As coral reefs die, flood damage from storms will double to $4 billion a year. These reefs protect the shoreline from hurricanes by slowing them down.
Other animals play an important role in keeping the earth's ecosystems functioning. If apes go extinct, the jungles they lived in could disappear. Many plants depend on them to propagate their larger seeds. Whales play a similar role in the ocean by recycling nutrients from the bottom to the top layers.