One of the topics that drew me in to the study of evolution was, oddly enough, extinction. While this might seem to be counter-intuitive, evolution being the origins of species and extinction being their end, if we cannot understand or recognize extinction we’re going to have a pretty poor idea of evolution (in fact, the establishment of extinction of fact was one of the key antecedents to the establishment of evolution as fact by Charles Darwin). Despite the important of understanding how extinction happens, it’s a matter that doesn’t seem to get as much attention as it probably should, and I’ve decided to summarize a few recent papers/articles on the subject here for anyone interested in the “life and death” of species.
[Note: By “‘life and death’ of species” I do not mean the antiquated and incorrect belief that species go through stages of birth, growth, and death just like an individual organism does, owing it’s premise to a vitalistic approach to evolution. If you want to read a good refutation of such ideas, see G.G. Simpson’s The Meaning of Evolution.]
First up, a recent Nature article by Nick Lane dubbed “Reading the Book of Death,” that focuses on the Permian extinction event. While the late-Cretaceous extinction that wiped out the non-avian dinosaurs is more widely known (thanks in part to the controversy about an impact at the K/T boundary during the 1980’s and 1990’s), the end-Permian extinction was much more severe, and Michael Benton aptly called his book on the subject When Life Nearly Died (although Peter Ward is perhaps a better known “spokesman” for the Permian, especially because of books like Rivers in Time and Gorgon). . While I have yet to read Benton’s book, I am familiar with Ward’s work and the general global-warming hypothesis that was brought to life in the BBC documentary Walking With Monsters (and, in fact, Ward has a new book out that I assume attempts to tie the Permian to the present in terms of climate change, Under a Green Sky); due to volcanism or other changes, massive amounts of carbon dioxide were dumped into the atmosphere, the temperature rising by about 43 degrees Fahrenheit. Such changes, if they occurred quickly, would have dire implications for life on earth.
There is certainly more to the story, however; volcanoes don’t simply spew out CO2 and little else, and the sheer amount of volcanic activity would have had potentially larger implications than a rise in temperature. While the volcanic activity in the Siberian Traps at the end of the Permian is well known, just exactly what effects they had is still being debated. We know that there was massive amounts of volcanic activity, we know the temperature shot up, we know that a large amount of volcanism can put chemicals into the air that damage the ozone layer (hence letting more UV radiation in), that oxygen levels likely fell at a gradual rate between the Carboniferous and the end-Permian (from 30-13% oxygen in the atmosphere, the article states), and that there were carbon spikes (probably from released methane) prior to the Permian extinctions. How do all these factors come together to explain which groups survived and which did not?
Lane sums up the situation at the end Permian as a hellish nightmare of a world gone awry, the pressures on organisms at that time being so great that many would not survive;
So with dangerously low atmospheric oxygen levels, ecosystems were compressed and fragmented. The deep oceans were largely uninhabitable. Land plants were dying back in the arid greenhouse climate, making food hard to come by. And then came the hammer blows of fate, the greatest volcanic outpourings in the history of our planet, releasing vast quantities of methane and carbon dioxide, raising global temperatures by 6 °C, and lifting the Strangelove conditions to the very surface.
Head for the hills and there’s no oxygen; stay on the shores and you risk breathing hydrogen sulphide. High carbon dioxide levels sabotage your respiratory pigments and choke you from within. Even if death doesn’t get you right away, you’re unlikely to have much spare energy for sex. Population sizes fall; so do body sizes. Even for those that survive the immediate toxicity, slow extinction was likely over a few generations — the blink of an eye in geological terms.
While the importance of each of these factors may vary globally or locally, it is important to realize that extinction events are ecological catastrophes; if we try to find just one smoking gun we’ll likely be mistaken. In fact, extinction is more life a firing squad than a single bullet that knocks the whole system off-kilter. Likewise, Lane also correctly notes that the creatures that survived largely did not do so because they were able to react more quickly to a changing world. Instead, they were already pre-adapted, the precursors of dinosaurs and mammals (and even later lines that would go extinct in the Triassic) having unique systems of air-sacs, palates, or turbinate nasal bones (see the opening of Schmidt-Neilsen’s How Animals Work) that may have helped them cope with an arid world in which water was the most precious commodity of all. While the mystery of the end-Permian catastrophe is still far-from-solved, the more integrated approach of scientists working on the problem may give us a more accurate look at why some groups died while some continued to great success in later periods.
Extinction is not something that has only occurred in the past, however; it is still occurring today. In a new PLoS Biology paper “Projected Impacts of Climate and Land-Use Change on the Global Diversity of Birds,” Jetz, Wilcove, and Dobson determine that global climate change in high latitudes and destruction (and restriction) of habitat in tropical regions will probably significantly reduce a number of bird species. Part of the modern extinction problem, as outline in David Raup’s book Extinction: Bad Genes or Bad Luck?, narrowing the range of a species can set them up for extinction, even when they seem to be doing well or even recovering. The classic case study is of heath hens in New England, where despite the fact that the population was recovering, the species died out in the blink of an eye because enough adverse factors accumulated to wipe all the birds out almost overnight (see an earlier post here).
Indeed, small geographic range or isolation are strongly correlated with extinction, both past and present. Even amongst animals that are not highly specialized, the inability to spread beyond a small area makes them much more vulnerable to extinction if they cannot withstand local pressures. Conversely, wide geographic range can be a good indicator of survivorship amongst species, and this was recently detailed amongst benthic marine invertebrates by Payne and Finnegan in their paper “The effect of geographic range on extinction risk during background and mass extinction” published in PNAS. The findings of the researchers make sense; widely distributed genera fare much better in the long run than those that have more narrow environmental needs that keep them from spreading. This spread also can help spur evolution (given that ecologies are constantly changing, to a greater or lesser degree), and so a greater diversity can arise from genera that can be found over a wider range for a longer amount of time. This does not exempt such groups from extinction, the researchers warn, as more catastrophic events that effect ecology on a global scale can wipe out groups that are widely distributed just as they can specialists. Hence, wide ranges for a group is important for survivorship during times of relatively slow global ecological change, but this does not necessarily exempt then from devastating extinction events like those seen during the end-Permian (as mentioned above).
In any case, I would highly recommend that anyone interested in extinction or evolution check out the above papers and articles, as the constant overturn of life is something vital to our understanding of the history of life on this planet. In case you’re in the mood for some further reading, check out Jeremy Bruno’s post on how mass extinctions affected chelicerates, or check out any of the books mentioned above.