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Video transcript

- [Voiceover] Sharks are one of the scariest animals in the ocean and almost nothing else dares to eat them. But if nothing's eating sharks, then why isn't the ocean just overrun with sharks? We can answer the question with "food chains." Now to understand food chains, we first have to talk about food and energy. We eat food during breakfast, lunch, and dinner to give us energy. What do we do with that energy? Well first, we use that energy to do work, like moving boxes. Second, we use some of the energy to grow. Third, feel your skin. Is it a little warm? We also produce heat when we break down food, so some of the energy in the food is lost as heat. And finally, some of that energy is lost as waste. So remember, we use the energy in food to grow, do work, and produce heat and waste. Animals and plants also get energy from the food they eat, and we can classify them based on the source of their food. Producers are living things that make their own food like little factories. For example, plants like seaweed and microscopic plantlike creatures called phytoplankton in the oceans make their food by using energy from the sunlight and nutrients in the environment in a process called "photosynthesis." Consumers are living things that can't make their own food, and so instead they eat other living creatures. Humans are consumers, and so are sea urchins, sharks, fish, seals, and sea otters. Some living things only eat the waste or dead bodies of other creatures, and these are called "decomposers." Shrimp and bacteria are examples of decomposers. These different groups are connected by food chains. A food chain shows who eats whom in an ecosystem. It always starts with a producer because they can make their own food. Then this producer gets eaten by the first consumer, who gets eaten by the second consumer, who sometimes gets eaten by a third and a fourth consumer. For example, seaweed's eaten by the sea urchin, who's eaten by the sea otter, who's then eaten by the shark. Or the phytoplankton's eaten by the fish, who's eaten by the seal, who's also eaten by the shark. Notice that even though the animals might be different, the idea stays the same. The chain always starts with a producer and ends with a consumer. Why is that? Well the food chain doesn't just show who eats whom. Because we get our energy from our food, a food chain also shows the flow of energy from one creature in the chain to another. So a food chain always has to start with a producer because they can harness the energy in sunlight to make food. The energy in this food is then the source of all the rest of the energy that enters the food chain. But remember that living things don't just use the energy in food to grow and get bigger. We also use the energy to do work and produce heat and waste. But only the energy that's used for growth can be transferred through the food chain, since that's what's being eaten by the consumers. So in fact, only about 10% of the energy is transferred from one member of the food chain to the next. That continuous loss of energy means that if you look at the food chain in terms of how much energy gets to each member of the chain, you actually get a pyramid. The producers form the base of the pyramid because they make the food that's the source of the energy. Then with each member of the food chain, energy is lost as work, heat, or waste until very little remains for the top consumer. So remember that our model food chain was composed of the producer, first, second, and third consumer? Now you know why there isn't always a fourth consumer, and only very rarely a fifth consumer. There wouldn't be a lot of energy left for that animal. Now you also know why the oceans aren't overrun by sharks, even if there isn't a fourth consumer eating sharks and controlling their population. There just isn't enough energy getting to the third consumer level to support such a large population of sharks. Cool! Now you noticed that the shark was a top consumer in both the example food chains, and this is because in the real world, just like how you eat carrots, beef, and all kinds of food, other animals also eat many different things. For example, seals also eat squid and penguins, and seaweed's also eaten by fish and snails. So we have many different food chains all in the same ecosystem. We can then combine all these food chains to make a food web. Since a food web is the combination of the many food chains that exist in the ecosystem, it's a much more realistic picture of what goes on in the environment. Unfortunately, human beings have had huge and terrible impacts on the food webs in the ocean. One way we've done this is by destroying shark populations. Sharks are hunted either for their fins, which are eaten in shark fin soup, or killed accidentally as bycatch when trying to catch fish like tuna. Up to 100 million sharks are killed every year. That's a number bigger than all the people who live in Switzerland, Canada, Cambodia, Denmark, Australia, and Zimbabwe combined. This is bad because as we saw in our food chains, sharks are the top consumers. This means they keep the populations of other animals in check. So for example, killing all the sharks would mean more seals, who would need so much fish to support themselves that they would destroy the fish populations. The whole ecosystem would break down. So what can you do to save sharks? First of all, don't eat shark fin soup. And second, try to buy and eat fish that's been caught without bycatch. Remember, we've said that sharks are the top consumers, but in many ways, it's actually human beings, and we need to eat responsibly if we don't want to destroy the food chain.