John Baez

April 8, 2006

Species are going extinct, and it's bad... but just how bad is it? This book puts the current crisis in an interesting perspective: It's about the Permian extinction 250 million years ago, when about 90% of all species died out. This was much more serious extinction than the "end of the age of the dinosaurs", in which about half of all species died out. But it's also much more mysterious!

Nowadays many scientists blame the death of the dinosaurs on an asteroid impact about 65 million years ago. This asteroid was about 10 kilometers in diameter, and it slammed into shallow waters covering what is now the Yucatan peninsula. The resulting crater, called Chicxulub or "Tail of the Devil", is about 150 kilometers across! The resulting tsunami would have hit Texas with a wave 50 to 100 meters high. Millions of tons of material were hurled into the atmosphere, causing wildfires across the world as they landed. Rocks called "tektites" formed from droplets of molten quartz can be found as far as Wyoming. A layer of dust from the impact can be found in rocks world-wide, marking the boundary between the Cretaceous and Tertiary. Scientists guess that this dust made it too dark to see for 1 to 6 months, and too dark for photosynthesis for sometime between 2 months and a year. Carbon dioxide released from heated limestone would have also had effects on the climate.

But this disaster at the end of the Cretaceous was only one of the 5 big extinctions life on Earth has suffered throughout the Phanerozoic eon, which began with the Cambrian period 540 million years ago. You can see the "Big Five" as sudden declines in this graph of biodiversity:

This graph shows how many "families" of marine animals there have been as a function of time. A "family" is a grouping of organisms that's bigger than a genus but smaller than an order. For example, if you're reading this, your genus is probably Homo and your species sapiens - but your family is Hominidae, which also contains gorillas, chimpanzees, bonobos and orangutans.

In the above graph, created by an expert on the statistics of mass extinctions named John J. Sepkoski Jr., families have been divided into Cambrian (Cm), Paleozoic (Pz), and Modern (Md) fauna. What really matters for us, though, are the five biggest dips! These are the "Big Five":

Perhaps the main lesson to take away from all this is that with the possible exception of the KT extinction, the causes of mass extinctions remain highly mysterious. Yet they are undeniably real events! You can also see them as peaks in this graph by Sepkoski and Raup, which plots the number of extinctions of marine families per million years (or "Myr"):

The advantage of working with marine life, by the way, is that it's much more likely to get fossilized than life on land.

Another fascinating feature of the above graph is the general downwards trend in the rate of extinction: the middle line is the average rate, while the two outlying lines represent a 95% confidence interval. So, perhaps life is getting better.


We may now be in the middle of yet another mass extinction! The Pleistocene began around 1.8 million years ago, bringing with it an erratic fluctuation between ice ages and warmer periods. The latest of these ice ages ended around 8000 years ago, right around when Homo sapiens was starting to really take over the planet. Starting around 11,000 years ago, most of the large mammals went extinct: mammoths, saber-toothed tigers, dire wolves, elephant-sized ground sloths, and so on. Though there is much debate about the causes, it seems that human hunting contributed to their demise. This is called the "Pleistocene overkill hypothesis".

We're now seeing an intensification of the rate of extinctions as wilderness areas are obliterated throughout the planet. Nobody knows what the extinction rate is: since most species haven't even been catalogued yet, all we have are lower bounds. These are only close to being accurate for the biggest and most charismatic species (e.g. mammals, birds and trees), but these represent a tiny fraction of all the species that are out there. So, any reasonable guess of the extinction rate requires extrapolation. If we keep track only of recorded extinctions, the story looks like this:

But what about all the species we haven't even catalogued yet?

Phillip and Donald Levin estimate that right now one species is going extinct every 20 minutes, and that half of bird and mammal species will be gone in 200 to 300 years. Richard Leakey estimates a loss of between 50,000 and 100,000 species a year, and says that only during the Big Five mass extinctions was the rate comparably high. E. O. Wilson gives a similar estimate. In his book, Michael Benton reviews the sources of uncertainty and makes an estimate of his own: given that there are probably somewhere between 20 and 100 million species in total, he estimates an extinction rate of between 5,000 and 25,000 species per year. This means between 14 and 70 species wiped out per day.

Skeptics find these numbers alarmist. For example, in Chapter 23 of this book:

the author does his best to tear apart Leakey and Wilson's estimates. Wilson has issued a convincing rebuttal. However, the really interesting thing is that Lomborg's own estimates also point to a high extinction rate! He estimates that over the next 50 years, about 0.7 percent of all species will go extinct. This may not sound like much until you realize how short 50 years is on a geological time scale. To put things in perspective, note that given Lomborg's estimate that there are between 10 and 80 million species total, a loss of .7 percent of all species would mean between 70,000 and 560,000 extinctions in the next 50 years. This amounts to 1,200 and 10,000 per year, or between 4 and 30 a day - the same order of magnitude as what Benton suggests! Perhaps more to the point, Lomborg says the current extinction rate is about 1500 times the natural background rate.

In short, despite plenty of bickering, there seems to be agreement that humans are causing a vastly elevated extinction rate.

And there's also lots of other data pointing to a massive human-caused disruption of the biosphere. One in eight plant species are in danger of extinction within the next 30 years, according to the IUCN Red List of threatened species, along with one in eight bird species and a quarter of all mammals. The Audubon Society reports that 30% of North American songbird species are in significant decline. Worldwide populations of frogs and other amphibians have been declining drastically, and a recent detailed study shows that of 5743 known species of amphibians recorded in the last couple of centuries, 34 are now extinct, while another 122 are probably extinct: they can no longer be found. Even worse, of these 122 missing species, 113 have disappeared since 1980!

In the oceans, 90% of all large fish have disappeared in the last half century, thanks to overfishing. We see the spread of dead zones near the mouths of rivers, where nutrients from fertilizer create blooms of plankton leading to low-oxygen water where few organisms can survive. Coral reefs are becoming unhealthy around the world, with a strong upswing in the bleaching of reefs since the 1970s. "Bleaching" is the loss of algae called zooxanthellae which live in coral and give it its color. It seems to be caused by higher water temperatures due to global warming.

And so on, and on, and on....

So, lots of evidence suggests that are in the midst of a mass extinction. It's very different than all previous ones. I don't think we can halt it; it's governed by seemingly unstoppable demographic and economic forces. Until the configuration of these forces shifts, at best we can only ameliorate their effects. We can't save every species, but maybe we can save one from every genus, or one from every family. Every little bit helps! The Permian extinction offers a lesson of hope: from the feeblest glow life can eventually rekindle.

That's the one great thing about this crisis: even a small nudge can make the difference between extinct   and not extinct.   I think it's worthwhile contributing to these organizations:

so I do, but there are many other ways to help - and if you have a fondness for a particular kind of animal or plant, there's probably an organization out there working to save it. Tigers? Gorillas? Birds? Take your pick... they're going fast.


Here are my favorite books on this stuff: I copied the graph of families of marine life from Erwin's book, but it originally came from this paper: I also copied the chart of geologic eras showing the big five extinctions from Erwin's book.

The figures on families and genera that went extinct in the Big Five extinctions came from these papers:

If the titles of these articles sound interesting, I should explain. In 1986 Raup and Sepkoski proposed a 26-million-year periodicity for mass extinctions. Possible causes could include the solar system's motion through the galaxy or an unseen companion of the Sun, dubbed "Nemesis" - either of these could in principle shake loose comets from the Oort cloud, or asteroids from the asteroid belt. However, this periodicity is no longer widely accepted: the supposed periodicity doesn't seem to hold up, the period of the solar system's motion through the galaxy is not 26 million years, and despite searches by astronomers no Nemesis has been found. Furthermore, only the Cretaceous-Tertiary extinction is unequivocally linked with an impact!

I copied the graph of the mass extinction rate from Benton's book, but it originally came from this paper:

I also copied the before-and-after pictures of Permian and Triassic seabeds from Benton's book, as well as the graph of extinctions after 1600.

I am here for a purpose... to explain to you, and in reality to others and not least to myself, what has happened to the world we both have loved... - E. O. Wilson

© 2006 John Baez