Global distribution of biodiversity
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Why biodiversity is distributed unevenly
- [Narrator] Let's talk about some of the factors that contribute to the uneven distributions of life on earth. Why do different organisms have different ranges and different distributions? First, let's imagine the planet without humans. What factors would govern where you find different kinds of life on earth without our influence? In such a world, life would still exhibit distinct distribution patterns, distinct ranges, and different overlapping ranges. The ranges of some species are large, some are small, but the key here is that the distributions would, in fact, clump. There are some places on earth where the distributions overlap in larger numbers than in other places. And this is because every species has its own preferred place to live and sometimes the preferences overlap. More in some places than in others. Now, it doesn't mean the species are thinking about it and saying, "Hawaii's nice this time of year." But it does mean that every species tends to have a characteristic range. There's a set of conditions in that range that allows the organism to get its food, to reproduce, and do all the things that members of a given species have to do to survive in their habitat. Preferences are determined by three main factors: physical, biotic, and evolutionary. Let's have a look at each of these. Physical or environmental factors are things like the geography of the place. Is it fresh water or salt water? Does the organism prefer living up in the air or underwater? Does in prefer living in mountains, in deserts, flatlands, or in caves? Physical factors also include things like climate. Temperature and rainfall are two of the things that drive the distributions of organisms. They can govern where certain lifeforms can live. If we look more closely at temperature, it's not only whether it's hot or cold all the time, like a certain distance from the poles or near the equator, but what the range of the temperatures might be. It can be very cold at night and very warm during the day, or vary by season. Temperature can change with elevation. It's gets colder as you go up mountains. Organisms that prefer those different temperature regimes, as we call them, have specific adaptations for that regime. Another thing about these physical factors is that they can be actual barriers to the movement of organisms. They can physically restrict organisms to a particular place. Mountain ranges, even rivers, can be barriers in a terrestrial environment. Some organisms can't cross them. There're even barriers in the ocean. Fresh water outflow, say, from the Mississippi River, makes a barrier. Different ocean species that like salt water are separated on the east and west sides of the dangerously fresh outflow, at least dangerous to these marine animals, anyway. Or islands. They're isolated by the water around them. A tortoise living in the Galapagos, for example, wouldn't be able to get back to South America or some other land mass without crossing the ocean. And tortoises are pretty good at floating the ocean, but no one would argue that it's their preferred way of life. So the ocean is a barrier to movements of many terrestrial organisms. We're actually talking here about something called island biogeography, which a lot of scientists love to look at because it provides a laboratory in which to study evolution. We could study how the barriers change the properties and ranges and distributions of different organisms that are restricted to living on the islands because the ocean, and even large lakes, are such good barriers to dispersal. Let's talk now very briefly about how biotic factors, living factors, influence the distribution of other life. Here's an example. There are places in the ocean that are essentially deserts where there's low productivity, low biomass, low biodiversity. Without high species richness or large populations of plankton, things like whales are gonna have a hard time. Likewise, on land, no grass, no antelopes. Those are very simple examples of how one type of life can dictate where other kinds of life might have their preferred habitats. Another great example, going back to the whales, this is one I love, is people think of whales as these sleek things that are going through the water at high speeds, but when you look closely at the front end of a humpback whale, there's an entire little ecosystem living there. Everything from barnacles to these, well, you can imagine, whales are big so, their lice are big, too. So they have this type of crustacean, these lice, that live in between the barnacles and all over the whale. If you had lice that big, you'd probably notice. But the whales just seem to deal with it. Basically, the host whale is the preferred habitat for these parasites. The range of the lice is gonna depend on the range of the host. The final factor in the uneven distribution of life is evolution, of course. Evolution is not a ladder, it's not a line of change, it's actually a bush. As life evolves, the pathways of change make a sequence of branches and twigs that tell us that life is constantly diversifying. So you can see how this would very rapidly become a very complicated overlay on top of the fact that life depends on other life to live and the fact that physical factors can govern where living things live. Chance is another element of change that feeds into the complicated branching sequences that lead to the diversity and location of life on earth. If we go back to the Galapagos again, our tortoises floated to the Galapagos, and were presented with a habitat that was a little bit different from the one that they enjoyed on the South American mainland. Like the Rolling Stones said, You can't always get what you want. But the tortoises discovered there was enough. The tortoises were no longer in their preferred habitat, but some adapted to the new circumstances. They survived and reproduced. That is, the population evolved. They arrived at this new habitat by chance and then adapted to a new home that eventually became their preferred habitat. Okay, so we've looked at some natural factors that contribute to the patterns of life on the planet. Now, let's put humans back into the equation. We can't ignore the subject because we humans are clearly an integral part of our environment, whether we wanna be or not. The activities of humans can restrict ranges of naturally occurring species, and in other cases, can actually increase the sizes of ranges. We exploit organisms. We use them for our own purposes. Way back when agriculture was invented, for example, we chose plants that we wanted to grow in order to enhance our ability to get food. We started raising animals for our own use as well. And as humans move from place to place, we purposefully brought these organisms with us and those organisms had their ranges extended. Estuaries were often brought up as examples of invaded environments. San Francisco Bay is one of the most invaded estuaries on the planet, in terms of introduced new species. You're seeing types of organisms that were introduced by human activity by accident. Primarily through the movement of ships that carried those organisms to docks and into our harbors. Now, those organisms find themselves in a place where they can live and they proliferate throughout the estuary. Sometimes a single species can totally dominate the habitat, which brings us to the question of huge biomass, but low species richness. We love to grow crops, but what we like to do with our crops is to make sure it's only that one species that grows in those many thousands and thousands of acres or hectares. To be provocative, I often say that the single greatest environmental change to be perpetrated on the North American continent is corn. Because corn is grown in a monoculture. Nothing else is allowed to grow with the corn. Nothing that eats or competes with the corn is allowed to be there. So the species richness of the millions of acres of cornfield across the U.S. is extremely low, 'bout as low as you can get. There's high biomass, sure, but what was there before we planted all that corn? Much higher species richness. Those prairies were very bio diverse. That richness is preserved in some small pockets here and there, but you can obviously see that the biomass of those wild populations is much lower and their ranges much smaller. Humans can do other things in terms of decreasing the sizes of organisms' ranges. Simply by increasing their own range, humans can change and shrink the ranges of other organisms. We build roads and big highways that can form barriers to natural movements of organisms. We're also changing ranges by changing the water courses of certain rivers. We can change entire habitats by making a lake where a river once existed. Decreasing the number of species and the sizes of the ranges of the organisms that used to live along those river channels. Human activity changes where life on earth finds itself and where it can go. Sometimes more than natural factors or evolutionary chance do. We do it by accident, we do it on purpose, but there's no denying our enormous impact on how biodiversity's distributed on earth.
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