The Earth's Sixth Mass Extinction....

The Earth's Sixth Mass Extinction May Be Underway

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NOTE: A slightly simpler version of this article has also been published on Treehugger
A "mass extinction" event is characterized as a period during which at least 75% of the Earth's species die out in a geologically short interval of time.  In the past 540 million years, only five such mass extinction events have occurred, but according to a review by Barnosky et al. (2011) recently published in the journal Nature, there are signs that we may be entering a sixth such event.

Mass Extinction Events

The Earth's five previous mass extinction events occurred during the:
  • Ordovician (443 million years ago, 86% of species extinct);
  • Devonian (359 million years ago, 75% of species extinct);
  • Permian (251 million years ago, 96% of species extinct);
  • Triassic (200 million years ago, 80% of species extinct); and
  • Cretaceous (65 million years ago, 76% of species extinct)
These previous mass extinction events (also known as the "Big Five") are hypothesized to have been caused by key synergies such as unusual climate dynamics, atmospheric composition, and abnormally high-intensity ecological stressors (or in the case of the Cretaceous, an asteroid impact and subsequent effects).

Barnosky et al. note that scientists are increasingly recognizing modern extinctions of species 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" 
Natural Extinctions
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.  A widely-used metric is extinctions per million species-years (E/MSY), in which background rates are estimated from fossil extinctions that took place in million-year-or-more timeframes.  The authors note that it is difficult to compare the current rate of extinctions, which are occurring over periods of just decades to centuries, to this background rate determined from periods millions of years.

That being said, the average E/MSY over the fossil record is approximately 1.8 (meaning on average, fewer than 2 species go extinct every million species-years), and the most common E/MSY over periods less than 1,000 years is zero.  Bear in mind that these are species-years, and that there are an estimated 20 million species on Earth, so each year constitutes approximately 20 million species-years.
Current Extinction Rate

Branosky et al. find that over the past 1,000 years, the average extinction rate is 24 E/MSY (13x background).  Breaking the data into 1-year bins, the maximum extinction rate over that period is approximatley 693 E/MSY (385x background).  Clearly these values far exceed the background rate.  And the worst case scenario, if all currently threatened species go extinct, results in a clear divergence from the natural extinction rate:
"In the scenario where currently ‘threatened’ species would ultimately go extinct even in as much as a thousand years, the resulting rates would far exceed any reasonable estimation of the upper boundary for variation related to interval length"
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:
"Current extinction rates for mammals, amphibians, birds, and reptiles, if calculated over the last 500 years (a conservatively slow rate) are faster than (birds, mammals, amphibians, which have 100% of species assessed) or as fast as (reptiles, uncertain because only 19% of species are assessed) all rates that would have produced the Big Five extinctions over hundreds of thousands or millions of years" 

Time to Worry?

The study also evaluated whether we are in (relatively) immediate danger of triggering an extinction event by evaluating a hypothetical scenario in which the Big Five extinctions occurred suddenly, over just 500 years rather than hundreds of thousands to millions of years.  In this case, the extinction rates during the Big Five would have had to exceed 1,000 E/MSY; a value which we have not yet reached.  However, Barnosky et al. note that if we consider a scenario where currently threatened species are inevitably extinct, the current extinction rate is 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, 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 emphasizes 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.
"First, the recent loss of species is dramatic and serious but does not yet qualify as a mass extinction in the palaeontological sense of the Big Five. In historic times we have actually lost only a few per cent of assessed species...It is encouraging that there is still much of the world’s biodiversity left to save, but daunting that doing so will require the reversal of many dire and escalating threats"
The authors' 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.
"there are clear indications that losing species now in the ‘critically endangered’ category would propel the world to a state of mass extinction that has previously been seen only five times in about 540 million years. Additional losses of species in the ‘endangered’ and ‘vulnerable’ categories could accomplish the sixth mass extinction in just a few centuries. It may be of particular concern that this extinction trajectory would play out under conditions that resemble the ‘perfect storm’ that coincided with past mass extinctions: multiple, atypical high-intensity ecological stressors, including rapid, unusual climate change and highly elevated atmospheric CO2.  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 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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