About this video
Following a kelp forest food chain (kelp, sea urchin, sea otter, orca), the video explains trophic levels and the 10% rule: only about 10% of the energy at one level passes to the next, which is why higher levels have fewer organisms and why we draw an energy pyramid. Energy enters from the sun and leaves as heat, while matter is recycled by decomposers.
You'll be able to
- place organisms in a food chain at the correct trophic level.
- use the 10% rule to calculate energy passed up a food chain.
- explain why an energy pyramid gets narrower at the top.
- explain the difference between how energy flows and how matter cycles.
Key words
- food chain
- A sequence showing how energy moves from one organism to another as they eat each other.
- trophic level
- Each step or position in a food chain.
- producer
- An organism, like kelp, that makes its own food from sunlight; trophic level one.
- 10% rule
- Only about 10% of the energy at one trophic level is passed on to the next.
- energy pyramid
- A diagram showing less and less energy available at each higher trophic level.
- decomposer
- An organism, like bacteria or fungi, that breaks down dead material and returns nutrients to the environment.
Transcript
- [Instructor] You dive into a kelp forest off the coast of California. Giant kelp towers are above you. Sea urchins are crawling along the sea floor. A sea otter is eating an urchin, and occasionally, even an orca might pass through. All these organisms need energy to survive.
But where does all that energy come from? And how does it move through the ecosystem? Let's follow it. It all starts with the sun. Producers use sunlight to make food through photosynthesis. Here, our producers are the giant kelp. It captures energy from the sunlight and stores some of it as chemical energy in glucose and other molecules that make up its body.
But now, a sea urchin comes along and eats that kelp. So some of that stored energy is now transferred from the kelp to the sea urchin. Then a sea otter eats the sea urchin, transferring some of its energy again. And occasionally, an orca can eat some otters, transferring energy once more.
So look, energy has moved from the kelp to the sea urchin, to the sea otter, and finally to the orca. This sequence is called a food chain. It shows how energy moves through an ecosystem as organisms eat each other. And each step in this chain is called a trophic level.
The kelp is the producer, and producers are at trophic level one. Then the sea urchins eat the producers. So it's called a primary consumer at trophic level two. The sea otter eats the primary consumer, so it's a secondary consumer at trophic level three. And the orca can eat the secondary consumer, making it a tertiary consumer, at trophic level four.
So far, things look pretty straightforward, right? But here's where things get really interesting. Think about the energy. Does all the energy from the kelp get transferred to the sea urchin that eats it? No. I mean, the kelp has already used much of the energy it captured from the sun to stay alive, to grow, to reproduce and carry out all its life processes.
Some energy is also eventually released as heat. So only a fraction of the energy captured at one trophic level is available to the next trophic level. On average, that fraction turns out to be about 10%. So whatever energy the kelp has captured, only about 10% of it can pass on to the next level.
And the 10% of that gets passed on to the level above that. And again, a 10% of that gets passed on to the level above that. So this pattern is called the 10% rule for obvious reasons. And this helps us understand something. We see that the energy availability reduces dramatically as we go up the trophic level, and that explains why there are a lot of sea urchins feeding on the kelp, but there are only few otters around because you need a lot of sea urchins to support a much smaller number of sea otters.
And you need a lot of sea otters to support a much smaller number of orcas, for example. So the 10% rule means, as we move up the trophic level, less energy is available, which means the ecosystem can support fewer organisms. And scientists have a very useful way of visualizing this, okay?
So the way we do this is, you take all the energy available in the producer and draw it as a wide block. Above that, draw the energy available to the primary consumers. Now, because only 10% gets transferred there, this block is much smaller. And do the same for the secondary consumers and eventually for the top predators.
And the shape we end up with is a pyramid. That's why we call this the energy pyramid. The wide base represents the producers where most energy is available. And as we move up through the successive trophic levels, less and less energy gets available, so each level gets narrower.
Okay, eventually, where does all the energy go? Well, most of the energy eventually goes out as heat. So that's how it exits the ecosystem. Okay, but where does the energy enter the ecosystem from? Well, it comes from the sun. And so we now see the energy continuously enters the ecosystem from the sun and exits as heat.
Okay, that's how the energy moves through the ecosystem. But what about matter? Does it also follow something similar? No, it doesn't move like energy because matter gets recycled. How? Well, imagine a sea otter dies. What happens? Well, bacteria and fungi, which are called decomposers, they start breaking down its body.
And in doing so, they return the nutrients from this otter's body back into the environment. Those nutrients can now be absorbed by the kelp and used to grow. And then as the sea urchin eats them, that nutrient gets transferred to them. And then some other otter comes and eats them and the nutrients get transferred to them.
And so notice, we have a cycle. So unlike the energy, the same matter can cycle through the ecosystem over and over again. And by the way, this was just an example. Decomposers act at every trophic level. So matter from all trophic levels get recycled back into the ecosystem.
So we see that decomposers are essential at keeping that cycle going. Imagine what would happen if we didn't have them. Without them, the dead material would accumulate and nutrients would just remain locked. And eventually producers would run short of nutrients they need and the whole thing would collapse.
That's why they are essential in recycling the matter through the ecosystem. And so now we understand the full picture. Energy flows in and continuously flows out, but matter gets recycled continuously. Together, these two processes sustain the ecosystem. So to summarize, a food chain shows how energy moves between organisms, and each position along the chain is called a trophic level.
And the 10% rule says that only about 10% of energy available at one trophic level is typically transferred to the next. And we visualize that using the energy pyramid. And while energy continuously flows into the ecosystem from the sun and continuously keeps flowing out as heat, matter is recycled, and decomposers play a crucial role in returning the nutrients from the dead stuff back into the ecosystem.
Energy flows, matter cycles, and together, they keep the ecosystem going.
Video © Khan Academy, used under CC BY-NC-SA. Watch on Khan Academy. Puzzicle is not affiliated with Khan Academy.