A “mass extinction” event is characterised as a period during which at least 75% of the Earth’s species die out in a period of a few million years or less. In the past 540 million years, five such mass extinction events have occurred, but according to a study by UC Berkeley’s Anthony Barnosky and colleagues recently published in the journal Nature, there are signs that we may be entering a sixth such event.
The Earth’s five previous mass extinction events occurred during the following eras:
These previous mass extinction events (also known as the “Big Five”) are hypothesised to have been caused by combinations of key events such as unusual climate change, changes in atmospheric composition, and abnormally high stress on the ecosystem (except in the case of the Cretaceous, which was caused by an asteroid impact and subsequent effects).
Barnosky and colleagues note that scientists are increasingly recognising modern species extinctions due to various human influences, including some of the same effects which caused the Big Five:
through co-opting resources, fragmenting habitats, introducing non-native species, spreading pathogens, killing species directly, and changing global climate.
There are, of course, species extinctions which have nothing to do with human influences. Scientists have identified a “background rate” of species extinctions from the fossil record, which allows for a comparison to the current extinction rate, thus allowing us to assess the human impact on the rate of species loss.
The study’s authors find that over the past 1,000 years, the average extinction rate is more than ten times larger than the natural background extinction rate from the fossil record, and recently has reached levels almost 400 times faster than the average natural extinction rate.
The authors also find that the extinctions over the past 500 years are happening at least as fast as the species extinctions which triggered the Big Five mass extinction events. However, the Big Five occurred over periods of hundreds of thousands to millions of years, so Barnosky and colleagues also attempted to determine how long it might take us to reach mass extinction levels (75% of species extinct).
To answer this question, the scientists examined a hypothetical scenario in which the Big Five extinctions occurred suddenly, such that all of the species went extinct over just 500 years rather than over hundreds of thousands to millions of years. They determined that the current extinction rate isn’t quite this rapid, but that if all species currently listed as “threatened” eventually become extinct, the current extinction rate then becomes almost as fast as the hypothetical 500-year Big Five extinction rates. In other words, if we lose all currently threatened species, we will be on a course for a new mass extinction event in just over 500 years.
In a similar hypothetical scenario, examining how many more years it would take for current extinction rates to produce species losses equivalent to Big Five magnitudes, the authors arrive at a similar conclusion:
if all ‘threatened’ species became extinct within a century, and that rate then continued unabated, terrestrial, amphibian, bird and mammal extinction would reach Big Five magnitudes in ~240 to 540 years….This emphasises that current extinction rates are higher than those that caused Big Five extinctions in geological time; they could be severe enough to carry extinction magnitudes to the Big Five benchmark in as little as three centuries.
The authors draw two main conclusions from these findings. The first is that although we’re clearly in dangerous territory in terms of extinction rates, we still have enough time to reverse course, although doing so will be a very difficult task. The second conclusion is that if we continue on our present course, we could be headed towards a mass extinction event within a timeframe of just a few centuries. Therefore, it’s very urgent that we steer away from our mass extinction course immediately.
The huge difference between where we are now, and where we could easily be within a few generations, reveals the urgency of relieving the pressures that are pushing today’s species towards extinction.
It’s also important to bear in mind that it takes a very long time to recover the biodiversity loss from a significant extinction event:
recovery of biodiversity will not occur on any timeframe meaningful to people: evolution of new species typically takes at least hundreds of thousands of years, and recovery from mass extinction episodes probably occurs on timescales encompassing millions of years.
In short, human influences, including our impacts on climate change, are causing extinctions at a rate faster than the average during a mass extinction event. If we continue down our current path, we may face a sixth mass extinction event within the next few centuries. However, we’re still relatively early along in the process, so although it will be a difficult task, there is still time to change course and prevent a huge loss in biodiversity. If we fail to do so, it may take millions of years to recover from the human-caused extinction event, and we’re quickly running out of time to avoid this fate.
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