Lesson 5: Life's History: The Great Adventure 5.8 Disturbance and Mass Extinction Many mountain slopes in southern California show a strangely patterned patchwork, like a garment made from rags. This is the result of brush fires. Where there has been no fire for some time, there is a dense cover of chaparral. Areas of recent fire show fresh growth of grass and the first sprouts of leaves below the charred skeletons of bushes, next to the ground. Intermediate stages between full chaparral and grass cover have their own communities of plants, with their own specific hues of green, olive and yellow. Taken as a whole, because of the occasional disturbance by fire, the slopes have many more plants than they would without the disturbance. Thus, disturbance helps create and maintain diversity. (Of course, there can be too much of it: where harsh conditions prevail routinely, as on mountain tops in the Sierras, only few species can survive.) There is no question that the same principle "disturbance increases diversity" is also valid through geologic time. When one set of species becomes extinct, through whatever mechanism, physical or biological, others will rise to fill the ecologic space. Seen over the entire interval, then, there are more species than would be otherwise, without disturbance. Extinction as a natural phenomenon was first established by Georges Cuvier in 1796, when he presented his paper "On the species of living and fossil elephants" at a public lecture in Paris. He argued that the mammoth is a new species of elephant and that it is extinct. If it were still alive, we should see it. The fact that extinction is for real changed the level of discussion about the history of life. If things go extinct, they must also be created. If extinction is real, the world is not perfect but in constant flux. Mechanisms capable of causing extinction include climatic change (including glaciation), sea level change, continental drift, changes in the chemistry of ocean (e.g., availability of oxygen) and atmosphere (e.g., ozone layer), and awesome messages from outer space (supernovae, cosmic radiation, impactors). Impact craters on land (such as the ones studied around the world by Gene and Caroline Shoemaker) demonstrate that large rocks can occasionally hit the Earth. If they are large enough, the consequences for living things are dismal, as we now know. See Exhibit on Extinctions at: http://exobio.ucsd.edu/Space_Sciences/extinctions.htm During the Phanerozoic, roughly the last 600 million years, the history of life is punctuated by a number of mass extinctions. The extinction at the end of the Cretaceous (extinction of dinosaurs and ammonites, K-T event) was but one of about half a dozen of such events, several of which were even more severe than the K-T event. Perhaps the largest ever, the end-of-the-Permian extinction, is estimated to have eliminated as many as 95 percent of the marine species of that time. (An estimate made by the American paleontologist David Raup, based on a census of the extinction of families.) The mass extinctions through the Phanerozoic have reset, many times, the Earth's evolutionary systems, like a forest-fire or a hurricane may reset the ecologic system of a given region. Whereas in regional disturbances, re-population proceeds through immigration from nearby areas, global disturbance can only be remedied through evolution of new species. In the race to fill the newly available ecologic space, new players get a chance to show what they can do. Thus, dominance of life forms can move from one group to another (say, from trilobites to ammonites, or from reptiles to birds and mammals). If the environment had not changed since the Devonian, (and there had not been major and multiple catastrophes wiping out major groups of organisms since then,) our seas might still be patrolled by armored fish, chambered cephalopods and scorpion-like arthropods with enormous claws, while the bottom would be crawling with trilobites. None of these creatures would be the same now as then, of course, as they would have adapted to a changing scenery of alliances and enmities between each other, and have become more efficient in maintenance and reproduction. However, they would be a lot more similar, as an entire community, to the life forms of the Devonian period than to what we see today. All this is speculation, obviously. Yet, it is clear that the big changes in the life forms, at least those seen in fossils, have come with catastrophe. One reason the world developed the way it did, for the last 65 million years (and one reason why we are here) is the impact of a large asteroid on the Yucatan peninsula of Mexico, which set off a series of events killing half of the life on Earth, including the dinosaurs and the ammonites. This insight we owe to the work of the father-and-son team Luis Alvarez and Walter Alvarez, and their collaborators [see exhibit]. Luis Alvarez, a physicist, recognized the importance of finding the rare metal iridium at the Cretaceous-Tertiary boundary, sampled in Gubbio, Italy by the geologist Walter Alvarez. Iridium points to an extraterrestrial source. Walter Alvarez picked this particular place for sampling because it had previously been identified as an excellent place to study the transition (by the Italian geologist Isabella Pemoli-Silva and the Swiss geologist Hans Luterbacher).