Community structure & diversity
Ecosystems and ecological networks
- [Narrator] If we think about our biodiversity tutorials as an archipelago, then today we visit this island to discover why biodiversity is so important. In terms of biodiversity, we mean a lot of different things. In this case, we're gonna talk about something that's known as species richness because that's something that we can measure. Because counting up the number of species, going out and finding out how many species there are in a given environment, is something that we can actually do. There are different species of plants, there are many different species of animals, many different species of microbes, and many different species of fungi. And they all interact in their environment to create what we would call an ecosystem. Eco is an interesting word. It's an ancient word that means house. So it's a system of what goes on in your house, that is, where we all live. All of these different organisms are interacting. They're behaving together. They're interacting with one another, some of them eat each other, some of them eat what others decompose into, and they, plus the physical environment, or the house, form the ecosystem. Why would the number of species, in other words, species richness, be crucial to the way an ecosystem functions? What is it about the number of species that makes the ecosystem work better and contribute to the resiliency or the stability of the ecosystem? Scientists are really beginning to study this, and the emerging field is referred to as BEF, biodiversity-ecosystem function. We can think of any ecosystem as species that network one to another. They have interactions with each other, they can be a lot of different interactions. They can live on top of one another like certain birds nesting in trees. Or even more crucially, these things might eat one another. This is a diagram, a network diagram, of, believe it or not, a relatively simple ecosystem in which the organisms are interacting one with the other. We can draw lines between the species to indicate those interactions. And, we can make the lines directional to show that material, or matter, and energy are moving from one species to another. When one organism takes a bite of another, it not only gets a mouthful of matter, or food, but that food contains energy. So these arrows show the direction that energy is flowing from one species to another. The most important thing about these webs is that the strength of these interactions can vary. That is, the interdependence of the organisms in the web can vary. And here's something that might be counterintuitive. These interactions become less important the more species you have within a network. You can think of it as the interactions being spread among more players. But what happens when there are fewer players? Let's say we've got a really super simple set of interactions, where this owl is eating that mouse, but can also eat this squirrel. This gives the owl options. If we remove one of these, take that guy right out of there, then we only have two species that are interacting with each other. And it's really easy to disturb this system. This guy would just eat all of these guys, and boom. You can cause total ecological collapse. So the higher the biodiversity, or species richness in a system, or in a network, the stronger it is. The more stable it will be because of all the additional options open to the organisms in it. Think of that old saying about putting all your eggs in one basket. Not all interactions within a given ecosystem or network are exactly the same strength because some species have stronger interactions with each other than they do with some other species. For example, let's imagine that somewhere in here, you've got one of these guys. And he's eating plankton, the microscopic organisms living in the water. Plankton are goin' in. These guys are the producer end of things, the ones who can photosynthesize and make chemical energy from sunlight. And remember, food equals energy. So what we're talking about is a lot of energy flowing from the producers to the consumer, represented by Mister Whale over here. That's what these lines represent. They represent the flow of energy through the ecosystem. Eventually, the whale succumbs to life and dies. And, poor Mister Whale ends up on the sea floor. Are things over at that point? Definitely not, because at that point, he's gonna give up stuff that ends up as part of the producers' food web. It's shown, actually we're starting to get some really great new evidence that these events, where whales fall onto the sea bottom start an ecosystem of their own in ways. This is known as a whale fall. Don't get under one, it's always best not to be under one. But when the whale does hit the bottom, all kinds of interesting stuff happens. Lots of organisms comes and feed on the whale. There's a succession of organisms, that is, organismal communities that change over time as the condition of the whale itself changes, that turn this whale from a fleshy organism to bones. And eventually even the bones are eaten. So this huge influx of energy that the whale's been accumulating from these producers, the plankton that it's been feeding on, is returned to the environment cycled back through the ecosystem. You might imagine that that complex set of events that I just diagrammed here are part of this little bit of web. It turns out, that where you get concentrated clusters of things happening in these webs, it's usually because energy's being run through the larger organisms in the system. And when you disturb a bit of that, where you've got large amounts of energy flowing through the system, you can get a real drop in ecosystem function. If you take the whales out of the equation by over-hunting them, you get this drop in ecosystem function, 'cause there's this removal of an entire major energy flow system from this network. Think of it a little bit like this tremendous thing that we have now, this internet. If you draw a diagram of the interactions amongst all of the servers and all of the things that push messages through the internet, there are places where that network's going to show a lot of stuff going on. A major hub in the internet. A lot of messages are flowing through. Like Google's servers, for example. Google is going to be at the center of one of these big clusters of a lot of messages going through. Other servers may not be so important. Those servers are gonna be places like my desktop, where I'm just messaging my kid to tell him to come home for dinner. If you take my server out of the system, it's not going to disturb the entire web. But you can see what happens if you suddenly take Google out of the system. You're gonna greatly perturb that particular ecosystem. So these webs of interaction throughout an ecosystem are really, really important. If the system is bigger, with more species and more interactions, you're gonna reduce the chance that a perturbation or a disturbance is gonna have a really negative impact. Because you're just reducing the chance that you're gonna take out something that's really crucial. The other thing about this is that all of these things are doing something different. They're all doing different things in the ecosystem. So if you imagine one of my favorite things; I love planes. Here's a complicated plane with four engines, lots of moving parts, piston engines roaring away, you've got a rudder here, you've got ailerons. Imagine this aircraft flying along. If you take out, say, the curtains that are in the window, you're probably not gonna bring the plane down. But if you do something like remove a couple of the crucial bolts at the base of the wing here where it connects to the fuselage, then the wing's gonna fall off, and we're gonna have a problem. 'Cause before you know it, the airplane's going to crash. Same thing with the ecosystem. Everything's doing something different. Some things will matter more than others if you remove them. The more bolts, the better.
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