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Biodiversity and extinction, then and now
(gentle music) - [Narrator] Biodiversity is in crisis today. This much we know. However, extinction, the human-caused basis of the crisis, can also be a natural phenomenon. Species go extinct for a variety of natural reasons, like meteor strikes, volcanic eruptions, natural climate shifts, movements of continents over geological eons, things that occur all the time without the involvement of humans. Therefore, scientists know there's a natural background rate of extinction that's estimated to be somewhere in the neighborhood of one to five extinctions a year, averaged over geologic time. This background rate exists because organisms are constantly being confronted by environmental changes. Some can deal with it, others can't. A major change has the potential result of an extinction. Even without a direct, natural phenomenon that could kill individuals directly, a species could go extinct due to its inability to keep up with the competitors, predators, or parasites that are always influencing its local environment or changing the way that it interacts with its ecosystem. It's now estimated that the present extinction rate is somewhere between 1000 to 10,000 times the background rate, and we also know the main reason for this. The reason is us. What I'd like to do here is frame this within nothing less than the entire history of life on earth. Have extinctions of this magnitude happened before? And what can we learn from that? The answer is yes, there have been massive extinctions in the past, and we're now experiencing the highest rate of extinction since the last major mass extinction that occurred at the end of the Cretaceous, some 65 million years ago. This is the one that a lot of people, especially seven year olds, are familiar with because it ended the age of dinosaurs. This extinction was likely caused by effects that occurred after the impact of a large asteroid, which scientists like to call a bolide. So, when you see the term bolide impact, you can tell your friends that that means, basically, kablooey. Some suggest that the smoking gun of this bolide impact is a huge crater 15 to 20 kilometers deep, and over 170 kilometers across, buried deeply in sediments along the Yucatan Peninsula. This crater was made by a bolide about 10 kilometers across. Events like this leave a physical, detectable mark in Earth's crust. Geologists use markers like this, along with other changes in the rocks, to give special names to specific periods, intervals of geologic time and history bounded by these markers, names like Paleozoic, Cambrian, Carboniferous, Cenozoic. These are names of epochs, eras, periods, et cetera, in the history of the planet. Scientists ask if we are, right now, making an event that will be detectable millions of years from now in the geologic record. Some claim that we are, and that we should be calling this present epoch the Anthropocene, anthro for human, with cene being the suffix that we use to designate these epochs. So, these scientists suggest that we signify a special epoch in the history of the Earth, an epoch marked by a new type of fossil evidence, such as plastics or some crusty carbon layer that indicates that we have left an indelible mark in the geology that will be preserved for eons. Right now, what can we learn from the past by looking at this big picture history of extinction on Earth? We know that biodiversity is governed by a balancing act between speciation and extinction. Does biodiversity increase steadily with time? Has it reached an equilibrium limit? Speciation is caused by one species splitting to give rise to two new species, each of those giving rise to two more, and so on. You'd expect the number of species to increase over time. However, that multiplication is offset by extinction. If you look at a graph of biodiversity over the history of our planet, you get something that looks like this. You can see that there is indeed an overall trend towards an increase in biodiversity, but you can also mark certain spots on this curve, here, here, here, here, and here, that indicate significant drops in biodiversity. These are known as major mass extinction events, times during which extraordinarily high numbers of species disappeared, times when the extinction rate was well above the background rate. It's worth looking at these because they suggest some interesting and, I think, relevant reasons for why these extinctions happened, so I want to go into these in a little bit of detail. The first one, here, occurs at the Ordovician-Silurian boundary, 440 to 450 million years ago. What on earth can we learn from something that happened so long ago? Well, for one thing, during this time, 60% of marine invertebrate species died out. These extinctions seemed to be the result of effects caused by movement of the huge supercontinent that existed at that time, Gondwana. Sounds like a pretty interesting place, except that it moved slowly into the south polar region, with a consequent sea level fall as significant amounts of the planet's precipitation became tied up in snow and ice on Gondwana, rather than flowing back into the sea, and this change disrupted marine ecosystems along the continental shelf, resulting in extinctions, extinctions recorded as another marker in the fossil record. The second major extinction, here, occurred during the late Devonian period, about 374 million years ago. I'm amazed that scientists can get that precise, but they can. We now know that 50% of all the genera on Earth went extinct at this time. Think about that for a moment. We're not talking about species. We're talking about entire genera, and that means a lot of species. There's some thought that this was due to a bolide impact that occurred long before the one that wiped out the dinosaurs, but maybe not. Others think maybe it was oceanic volcanism or some global cooling mechanism that we don't fully understand. Some suggest that it was due to a drop in speciation rate, rather than an increase in extinction rate, and clearly, more research is needed here. The third of the big five extinction events, here, is something that occurred at the end of the Permian, between the Permian and Triassic periods, about 252 million years ago. This is sometimes known as The Great Dying, the biggest known extinction event, during which 96% of all marine and 70% of all terrestrial vertebrates died out. It also appears that it's one of the few mass extinctions in which insects took a big hit. I guess it takes a lot to kill all those cockroaches. In total, 83% of all the genera on earth disappeared. The suggested reasons behind this mass extinction range all over the map. Some people think it might have been one or more bolide impacts, or volcanism on a huge scale, possibly from a place called the Siberian Traps, where there were huge outpourings of lava, big fires going on, and all kinds of horrible things happening to the environment because of that volcanism. It could also have been a runaway greenhouse effect triggered by the sudden release of methane from the ocean floor. Methane can be stored by the activities of bacteria as something called methane clathrates in sediments of the ocean floor. Geologic disturbance might be able to release these stores of clathrates, releasing huge amounts of the potent greenhouse gas, methane. Other people think that there might have been major sea level changes at the time, due to overall climate changes. Increasing anoxic conditions or lack of oxygen in the deep sea, or perhaps shifts in oceanic circulation patterns could be results of those climate changes, or maybe a hellish convergence of all these things. I can easily imagine the volcanism causing earthquakes that would release methane clathrates, and cause oceanic circulation changes, and so on and so forth, a cascade of disasters. The fourth major extinction occurred at the Triassic-Jurassic boundary, 201 million years ago. 34% of all marine genera died out, along with a whole bunch of terrestrial vertebrate groups, clearing the stage for the rise of the dinosaurs. Hooray. Where would all of our five to 10 year olds be if it weren't for the Triassic-Jurassic extinction? Statistical analysis suggests some complications in figuring out what the story is here, but a drop in speciation, rather than an increase in extinction, might have caused this drop in biodiversity, too. The fifth and possibly most famous of all the big five extinctions is the Cretaceous-Paleogene event, 65 to 66 million years ago, and we know the story. A kablooey, a bolide impact with dinosaurs dying out in the aftermath, along with lots of other groups. About 75% of all known species went extinct, but this was followed by rapid recovery, giving way to what many scientists refer to as the modern biota, including the rise of mammals, and I think we can learn something from that, too. Life is amazingly tenacious. For all the delicacy of an individual or a species, life recovered in impressive ways at least five different times, which leads to a fascinating question. What happens after a mass extinction? What are the contingencies, as evolutionists call them, that happen when ecosystems are suddenly opened up by the removal of many species. Who comes next? Who can take advantage of the changes and the openings, and how fast can all that happen? Those are difficult but very interesting and very relevant scientific questions, so there are lots of people working on them because, as I like to say, the past is where you came from. Without knowing where you came from, sometimes you can't know where you're going, and lots of the extinction causes I just described should sound a bit familiar, because they bring us back full circle to the beginning of this video in thinking about a sixth mass extinction during the Anthropocene. For the first time in the entire history of life on earth, there's a mass extinction happening due to a single species: us. What's gonna happen while this is going on and after it runs it present course? I hate to imagine a world without tigers or blue whales or Sequoias or salmon, giant clams, coral reefs, but without them and the ecosystems that depend on them, our time is limited. We'll not only be the cause of a mass extinction, we'll be part of it.
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