Ideally, your expenses are offset by your paycheck, so as you spend money for say, rent and food, you have cash coming in. On the surface at least, this is similar to the dovetailing of extinction and speciation. The vast majority (~99%) of all the species ever to have existed on Earth are extinct, never to be seen alive again. However, this process is balanced by new species evolving, a process known as speciation. So what happens when species extinctions far outpace species creation? Mass extinctions, timeframes during which 75% of species are lost in a relatively short period (usually less than two million years and sometimes significantly less), have occurred five times (the Big Five) in the past 540 million years. They are unique, singular events that stand out above the background level of extinction that is constantly ongoing. However, more and more modern extinctions are being observed, amidst the myriad of human-derived disturbances, such as rapid climate change, invasive species introductions, habitat fragmentation, directly killing species, among others.
It is not a straightforward task to ascertain whether or not we are on track to another mass extinction—data from the fossil record must be comparable to historic and modern species assessments. In a paper recently published in Nature, Dr Anthony Barnosky and colleagues point out why data comparisons of this type are difficult and proceed to broadly get around them by looking at the global picture.
The fossil record is not evenly distributed across taxa or geographies. Fossils are particularly meager in some broad swathes of Earth, such as the tropics. On the other hand, distributions are known for many modern species. In terms of taxa, or groups of species, usually only animals with hard bits fossilize well. Studying modern species is easier, because they’re still around, but less than 2.7% of known species have been assessed for risks (e.g. endangered, extinct in the wild, etc.) by the International Union for the Conservation of Nature (IUCN). There’s also trouble with the species concept. Fossils are usually identified at the genus, rather than species, level; modern work frequently uses genetic approaches to identify individuals to species. Fossils are also not distributed evenly through time. Fossil extinctions are recorded when a certain group of animals vanishes from the fossil record, the extinctions known are likely underestimates since most species have no fossil record.
In spite of these caveats, the researchers evaluated the existing data and show that it is possible to circumvent these various data comparison issues by taking a ‘big-picture’ global approach. Conservatively, mass extinctions occur when the extinction to speciation ratio becomes so unbalanced that three quarters of species disappear, usually within less than two million years. If two million years sounds like a leisurely long time, bear in mind that the Earth is ~4.54 billion years old. Most living things forever blinking of out existence in roughly 0.04% of that time is a colossally unique situation indeed.
Using a rate-based method, the researchers compared extinctions per million species-years (E/MSY)1 from throughout the fossil record and modern time. By using various paleontology databases and accounting for data biases, they were able to establish a background rate of extinction. Through this approach, it is clear that the maximum extinction rates since about 1,000 years ago are much higher than the average fossil rate and the recent average extinction rates are also significantly higher when compared to pre-anthropogenic (that’s pre-us, mind you) averages.
Another way to consider this is by splitting up the fossil record into 500-year intervals and calculating the likelihood that extinction rates were as high in many of these 500-year intervals as they were in the most recent 500 years. In the case of mammals, which have an average of 1.8 extinctions per million species-years, only 6.3% of these 500-year segments could have extinction rates comparable to the current 500-year interval in order to preserve the background E/MSY. So no, it is supremely unlikely that many of these past 500-year bins had extinction rates that were as high as they are today.
But would these current rates produce a large magnitude extinction event? By using modern species assessments of very well-surveyed groups coupled with fossil data, Dr Barnoksy and his team calculate that extinction rates for mammals, birds, amphibians, and reptiles are as quick or quicker than all rates that would have been responsible for the previous five mass extinctions. If all threatened species (defined by IUCN criteria) are lost within a century, and the current extinction rate continue, land-based amphibians, birds, and mammals would reach mass extinction thresholds in ~240 -540 years. This slows down a bit if ‘only’ critically endangered species disappear within the next 100 years to ~890 – 2,265 years for those same groups of animals. Current extinction rates are higher or as high as those that preceded and caused the previous mass extinctions. The researchers point out that while a pressing need for new research exists, the 75% species loss threshold could occur within the next three centuries.
Modern species losses are serious but does not pass the threshold for a mass extinction event yet. Relatively small numbers of species surveyed historically have been lost, although scores of species have yet to be discovered and/or evaluated. However, the researchers point out that losing critically endangered species would put us on the fast track to mass extinction, and losing endangered and vulnerable species would achieve the sixth extinction even faster, within a few hundred years. Sobering commonalities between the present-day and the past mass extinctions exist:
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. [Barnosky et al.]
The diversity of life should be preserved while it’s still here.
1 Think of this as man-hours (or really person-hours). On a purely mathematical basis, if you had one million species and an extinction rate of 1 per million species-years, one species would go extinct a year.
Barnosky AD, Matzke N, Tomiya S, Wogan GO, Swartz B, Quental TB, Marshall C, McGuire JL, Lindsey EL, Maguire KC, Mersey B, & Ferrer EA (2011). Has the Earth’s sixth mass extinction already arrived? Nature, 471 (7336), 51-7 PMID: 21368823
Image: Rae Allen on Flickr (cc).
This article is also posted at The Urban Times.