Energy flow through ecosystems
Impact of changes to trophic pyramids
- [Instructor] What we see here is known as a trophic pyramid, and the word trophic, in a biology context, is referring to food relationships. So one way to think about this, it tells us who is eating whom, and who is producing energy, and then who is able to leverage that energy. So the base of a trophic pyramid, you have your primary producers, primary producers. These are often known as autotrophs, because they're able to take energy from the sun, so sun's energy, and nutrients that are available to them, and store some of that energy as biomass. And biomass is just a fancy word for the actual substance of the organism that is inherently storing energy. When someone tells you that that piece of food has a certain number of calories, that's because there's energy stored in it. Calories are a unit of energy. And so, let's just say, if you were to take a square meter, if you were to average on a per square meter basis, these primary producers in this environment, let's say they're able to store 20,000 kilocalories per square meter per year. What's interesting about a trophic pyramid, it helps describe, well, okay, that energy's stored as biomass, what happens then? Well, then you could go to the next level, and you could view these as your primary or first-level consumers. So first-level, well, I could call them level one or primary consumers, these are the organisms that would eat the primary producers. But not all of that energy gets restored as biomass in these organisms. In fact, there is a lot of loss. On average, when we look at ecosystems, it tends to be only about 10% makes it from one level of our trophic pyramid to the next. So at this level, on a per, on average, per square meter, instead of 20,000 kilocalories being stored as biomass per year, you'd only have 10% of that. So it might only be 2,000. 2,000 kilocalories per square meter per year. So notice, you have that drop-off. And then you could go to the next level after that, you could view these as the secondary consumers. These are the folks who might eat the primary consumers, the first-level consumers. You get another 10% drop-off, so you would, or I should say, a 90% drop-off, only 10% gets transferred. So about 200, 200 kilocalories per square meter per year. And it keeps happening, the folks who eat those folks, well then, you've dropped off at this level right over here. You could call these the third-level consumers, or sometimes viewed as tertiary consumers. This would be about 20 kilocalories per square meter per year. And this doesn't mean that every square meter will have exactly 20 kilocalories of biomass of, let's say in this example, snake. It just means that if you were to look at the biomass of snakes, and you were to average them across this ecosystem, the surface area, then you might average about 20 kilocalories per square meter per year. And then you get to the top of this pyramid. And this, you could view as your level four consumer. You could view this as, since they're at the top of the pyramid, this is sometimes known as the apex predator. But every stage here, you only are able to transfer 10% of the biomass. So here, you have two kilocalories per square meter per year. And what's useful about this, it helps us understand what an ecosystem can support. It can support a lot of biomass of our primary producers, but it can support very little biomass of our apex predators, and that's why, if you were to go into the forest, you would see very few apex predators. If you were to look at the apex predators and you were to think about their biomass in terms of kilocalories, and spread it over their region where they have to find food, it would be much lower than the average biomass per square meter of, say, the grass and the trees. Now, an interesting thing is, well, where is a lot of that energy getting lost to? Well, in a lot of cases, these organisms are moving around. They have to do things, they have processes in their own body, and those things all generate heat. So even plants, even plants generate heat. So all of these characters are, there's some energy that's being released as heat. Also, when these players die, they are decomposed by other organisms. So all this biomass, it's either just going to die, or it's going to get eaten, but even when it gets eaten, all of the energy doesn't get transferred. Some of it stays in the undigested material, which we refer to as poop, and so, those dead bodies, that dead biomass, or this poop that still contains energy, is going to be fed on by what we call decomposers, decomposers. And they really break things down into nutrients, which then can be consumed by the primary producers as they utilize the sun's energy and keep the cycle going after that. Now, one interesting thing to think about is, what if there are changes to the ecosystem? What if, for example, a nearby volcano erupts? And that volcano, let me draw that, it's a fun thing to draw, that volcano erupts and ash goes into the sky, and all year it's just really gray. What do you think would happen? Well, the sun's energy that's able to hit the surface of the earth in this area would go down by a good bit. And so, if that went down by a good bit, then the primary producers might not be able to store 20,000 kilocalories per square meter per year. It might go down to 2,000 kilocalories per square meter per year, and in which case, you'd drop to a zero off of all of these. So instead of 20,000, it might go to 2,000. Instead of 2,000, this would be 200. Instead of 200, this would be 20. Instead of 20, this would be two. Instead of two, this would 0.2. And so, something like the loss of light energy, even though this apex predator doesn't directly photosynthesize, a lot fewer of them are going to be able to be supported in that ecosystem if you have a lot less energy stored at biomass, at the primary production level, because there's just less of sun's energy to be able to be stored as biomass. There could be other things that happen in the ecosystem. Let's say that some pesticides get introduced, and some of these level one consumers start dying out. Well then, you might have less of the transfer of biomass from the primary producer level to the primary consumer level, which, once again, might affect these other levels of the trophic pyramid. So this is an interesting thing to think about. Ecosystems are these really complex, intertwined things, and one impact at one area could have far-reaching consequences throughout the entire ecosystem. It's also interesting to think of ecosystems as energy transfer. Energy's coming from the sun, and it's being cycled through this ecosystem, through this pyramid. Some of it gets stored as biomass and gets transferred to other forms of biomass, but a lot of it gets lost as heat as these organisms' cells go through different processes. The organisms are running around, living their life, doing whatever.
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