Life has existed on Earth for more than 3.5 billion years with species extinctions a normal part of the evolutionary process. However, when the loss of species rapidly exceeds the emergence of new species we enter a ‘mass extinction’ event generally defined as a loss of about 75% of all species over a short geological time period (<2.8 million years).
Around the Cambrian period about 540 million years ago there was a sudden proliferation of life forms and since that time there have been five extinction events that meet this criterion and which help determine whether human beings have today created the conditions for a sixth mass extinction.
Five mass extinctions
The world’s five mass extinctions have occurred on average every 100 million years since the Cambrian (although there is no clear pattern to their timing) and each extinction event lasted between 50,000 and 2.76 million years.
First mass extinction
At the end of the Ordovician period c. 443 million years ago obliterating 85 per cent of all species. Probably a consequence of two climatic events: a planetary-scale period of glaciation (a global-scale “ice age”); followed by a rapid warming period.
Second mass extinction
In the Late Devonian period c, 374 million years ago the second mass extinction killed around 75 per cent of all species, most being bottom-dwelling invertebrates in tropical seas. This period had high variation in sea levels and rapid global cooling and warming when plants (Pteridophytes) were starting to cover dry land with a drop in global CO2 concentration with soil transformation and periods of low oxygen.
Third mass extinction
The most destructive of all extinction events this occurred at the end of the Permian period around 250 million years ago, wiping out more than 95 per cent of all species. Probably the result of an asteroid impact that filled the air with toxic particulate matter and altering the climate, concealing the sun and generating acid rain. Other theories suggest volcanic activity in today’s Siberia, increasing ocean toxicity with an increase in atmospheric CO2 – or the spread of oxygen-poor water in the deep ocean.
Fourth mass extinction
Fifty million years after the great Permian extinction, about 80 per cent of the world’s species were again obliterated in a Triassic event, probably the result of geological activity in today’s Atlantic Ocean, raising atmospheric CO2 levels, increasing global temperatures, and acidifying the oceans.
Fifth mass extinction
In the Cretaceous about 145 million years ago dinosaurs were struggling until wiped out by an asteroid impact when about 76% of all species became extinct. This was an impact in the Yucatán of modern-day Mexico, a massive volcanic eruption in the Deccan Province of modern-day west-central India, or both in combination. This gave mammals an opportunity to diversify and occupy new habitats, from which human beings eventually evolved.
The most likely cause of the Cretaceous mass extinction was an extraterrestrial
Sixth extinction event
The Earth is currently experiencing an extinction crisis resulting from planetary exploitation of resources by human beings although whether this constitutes a sixth mass extinction depends on whether today’s extinction rate is greater than the ‘background’ rate that occurs between mass extinctions which indicates how rapidly species would die out in the absence of human activity using mostly fossil evidence. The generally accepted figure suggests an average lifespan of about 1 million years for a species, or one species extinction per million species-years. But this estimated rate is highly uncertain, ranging between 0.1 and 2.0 extinctions per million species-years and depends an accurate figure.
Extinction has many direct and indirect human causes: the destruction and fragmentation of habitats; exploitation by fishing and hunting; chemical pollution; swamping by invasive species; global warming and more. The rate of extinction appears to be between 10 and 10,000 times higher than the background rate.
Among land vertebrates (species with an internal skeleton), 322 species have been recorded going extinct since the year 1500, or about 1.2 species going extinction every two years.
If this doesn’t sound like much, it’s important to remember extinction is always preceded by a loss in population abundance and shrinking distributions.
Based on the number of decreasing vertebrate species listed in the International Union for Conservation of Nature’s Red List of Threatened Species, 32 per cent of all known species across all ecosystems and groups are decreasing in abundance and range. In fact, the Earth has lost about 60 per cent of all vertebrate individuals since 1970.
How does Australia fit into the pattern?
Australia has one of the worst recent extinction records of any continent, with more than 100 species of vertebrates going extinct since the first people arrived over 50,000 years ago. And more than 300 animal and 1,000 plant species are now considered threatened with imminent extinction.
Although biologists are still debating how much the current extinction rate exceeds the background rate, even the most conservative estimates reveal an exceptionally rapid loss of biodiversity typical of a mass extinction event.
In fact, some studies show that the interacting conditions experienced today, such as accelerated climate change, changing atmospheric composition caused by human industry, and abnormal ecological stresses arising from human consumption of resources, define a perfect storm for extinctions.
All these conditions together indicate that a sixth mass extinction is already well underway.
Adapted from an article in The Conversation by Frederik Saltre and Corey J A Bradshaw.