Impact of changes to trophic pyramids | High school biology | Khan Academy

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 is that it tells us who is eating whom and who is producing energy, and then who is able to leverage that energy.

At the base of a trophic pyramid, you have your 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. 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.

Let's just say if you were to take a square meter and 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 is that it helps describe, "Well, okay, that energy 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—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 percent that makes it from one level of our trophic pyramid to the next.

So, at this level, on average per square meter, instead of 20,000 kilocalories being stored as biomass per year, you'd only have 10 percent of that, so it might only be 2,000 kilocalories per square meter per year. Notice you have that drop-off. Then you 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 90 percent drop-off; only 10 percent gets transferred, so about 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.

This doesn't mean that every square meter will have exactly 20 kilocalories of biomass of, let's say, in this example, a snake; it just means that if you look at the biomass of snakes and you were to average them across this ecosystem, the surface area that you might average about 20 kilocalories per square meter per year. Then you get to the top of this pyramid, and this you could use 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 at every stage here, you only are able to transfer ten percent of the biomass. So here you have 2 kilocalories per square meter per year. What's useful about this is that 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.

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, 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—yes, even plants—generate heat.

So all of these characters, there’s some energy that’s being released as heat. Also, when these players die, they’re decomposed by other organisms. 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 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? That volcano—let me draw that; it's a fun thing to draw—erupts, and ash goes into the sky, and all year it’s just really gray. What would 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 dropped 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 2; instead of 2, this would be 0.2. So, something like the loss of light energy—even though this apex predator doesn’t directly photosynthesize—many fewer of them are going to be able to be supported in that ecosystem if you have much less energy stored at biomass at the primary production level.

Because there's just less of the sun's energy to be stored as biomass. There could be other things that can happen in the ecosystem. Let’s say that some pesticides get introduced, and some of these level 1 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.

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 is 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.