Worked example: analyzing an ocean food web | Middle school biology | Khan Academy

So this diagram right over here describes a food web, and a food web models how energy and matter move in an ecosystem. We're going to use this food web to answer some questions to make sure we understand food webs.

So the first thing I'm going to ask you is: what are the producers in this food web? Pause this video and see if you can figure this out.

All right, now let's work through this together. The producers are the ones that can take matter in the ecosystem, in the environment, and energy from an outside source, usually the sun, in order to construct themselves and then also, in order to store that energy. When you look at a food web, producers are going to have arrows pointed away from them.

So they're getting their energy and their matter from the environment, and then other organisms are going to consume them. We’re going to talk about them in a little bit, but we can see here that the kelp is a producer. This is an aquatic ecosystem that we're looking at, but the kelp is a producer. There are no arrows going into it, so it's using the sun and elements and atoms and molecules that it's finding in its environment in order to grow.

We can also see that’s the case with the phytoplankton. It's also able to do photosynthesis, like the kelp, and capture that energy from the sun. Now the next question I have is: where are the decomposers? Pause the video and try to answer that one.

All right, now you might remember decomposers. You can think about that as where a lot of the energy and the matter ends up to be decomposed and then recycled again, and reused again by the producers. If you're looking at a food web, these would be organisms where all the arrows point to them, but you don't have any arrows coming from them. It’s clear that the bacteria, and there are many different species of bacteria or many different types of bacteria, are the decomposers.

Next, I am going to ask you about the primary consumers. Pause the video and see if you can identify them. Well, primary consumers are consumers that directly eat the producers. So let's see who is eating the kelp. You have the sea urchin eating the kelp, and you have the fish eating the kelp. Who's eating the phytoplankton? You have the zooplankton eating the phytoplankton, so these are primary consumers.

Next, let's think about secondary consumers. These are those that eat primary consumers. Pause the video and think about that. Well, let's think about who's eating these primary consumers. You have the otter. What's interesting is the fish is a primary consumer because it consumes kelp, which is a producer, but it also can consume another primary consumer.

So, a fish or the fish here are both primary and secondary consumers, and then I think we are done.

So you can see the general notions here: the producers have arrows going away from them but no arrows coming into them because they're getting their energy from the sun, and they're using the matter in the ecosystem. Primary consumers are eating those producers, and you can see they have arrows going into them, and then they're eaten by other organisms. Secondary consumers are those that eat the primary consumers, and then last but not least, you have the bacteria, which are decomposers, where the arrows all point to them.

They continue to use that energy and then they take those nutrients, those molecules, and they put them back into the ecosystem so that it can be recycled again.