The first recorded mass extinction event took place at the end of the Ordovician Period around 443 million years ago. The second mass extinction occurred in the Late Devonian Period about 360 million years ago. The Permian/Triassic mass extinction event was the third mass extinction event about 250 million years ago. The fourth occurred at the end of the Triassic some 200 million years ago and the fifth at the end of the Cretaceous, around 66 mya.
Causes of all these extinction events involved some combination of climate change (temperature crises, ocean warming, acidification, sulfide poisoning and hypoxia), Earth movement and shifting continents, volcanic activity, and possible asteroid impacts. The situation in the late Permian, of increasing atmospheric greenhouse gases creating warmer temperatures on Earth, is similar to what is happening today.
According to many scientists, we are currently experiencing a sixth extinction event. Also called the Holocene-Anthropocene Extinction, some call what is happening a “biological annihilation.” Scientists agree that the present extinction rate is thousands of times higher than the natural baseline rate. The baseline is about one species per every one million species per year; currently that rate is one in a thousand.
If this isn’t sufficiently alarming, we are not only witnessing an increase in extinction rate but a loss of total population and distribution of common species–which is even more alarming. Scientists analysing both common and rare species have found billions of regional and local populations lost in recent years. In a study by Gerardo Ceballos published in the Proceedings of the National Academy of Sciences, half of the 177 mammal species surveyed lost more than 80% of their distribution between 1900 and 2015. “Scientists found that a third of the thousands of species losing populations are not currently considered endangered and that up to 50% of all individual animals have been lost in recent decades,” reports The Guardian. “Detailed data is available for land mammals, and almost half of these have lost 80% of their range in the last century.”
The Horsetail: Icon of Planetary Evolution and Survivor of Five Extinction Events
Over three hundred million years ago horsetails were large trees, reaching 20 metres (66 ft) tall, higher than today’s oaks and beeches. Calamites was a spore-bearing tree-size sphenopsid that lived in the Carboniferous and early Permian Periods of the late Paleozoic Era (about 360 to 250 million years ago).
These sphenopsids lived in the steamy ‘equatorial’ rainforest swamps that covered the vast continent and coastal shallows of tropical Laurasia (Europe, Asia, and North America) and Cathaysia. Warm and humid coastal shallows accumulated large amounts of peat, rich in stored carbon.
Somewhere during the millions or so years that followed, Calamites’ descendants became the diminutive horsetails we see today. Some scientists believe that the Calamites contributed to their own extinction. Because they were such efficient CO2 sinks, storing great quantities of atmospheric carbon dioxide in the soil, the Earth eventually cooled. With climate change, the hot, humid environment that supported Calamites gave way to the cooler drier environment of the Permian Period, which ushered in the reptiles, and led to what is called Carboniferous rainforest collapse. The Permian Period also ended an entire age with the greatest mass extinction the Earth has ever experienced.
What Contributed to the Mass Extinctions?
Here’s how The Guardian neatly summarizes the five mass extinctions:
Most scientists acknowledge that the sixth mass extinction, which is occurring now, is caused by human activity (such as unsustainable use of land, water and energy) and human-induced climate change. Extreme global biodiversity loss over the past 50 years has been mostly driven by habitat destruction to do with clearing of forests for farmland, the expansion of roads and cities, logging, hunting, overfishing, water pollution and transport of invasive species around the globe.
Nina Munteanu is a Canadian ecologist / limnologist and novelist. She is co-editor of Europa SF and currently teaches writing courses at George Brown College and the University of Toronto. Visit www.ninamunteanu.ca for the latest on her books. Nina’s bilingual “La natura dell’acqua / The Way of Water” was published by Mincione Edizioni in Rome. Her non-fiction book “Water Is…” by Pixl Press(Vancouver) was selected by Margaret Atwood in the New York Times ‘Year in Reading’ and was chosen as the 2017 Summer Read by Water Canada. Her novel “A Diary in the Age of Water” was released by Inanna Publications (Toronto) in June 2020.