Worked example: analyzing a generic food web | Middle school biology | Khan Academy

What we have here is a diagram of a food web that shows us how matter and energy are transferred between organisms in an ecosystem, but it's a little bit abstract. They don't tell us what these organisms are; they just say organism one, organism two, organism three, and so forth. But even this gives us some information about which organisms are the producers, the primary consumers, the secondary consumers, and the decomposers.

So the first thing I want you to think about are is which arrows show us going from producers? I'll write that like that to primary consumers. Pause the video and think about that. Which arrows show producers to primary consumers?

All right, now in a food web, the giveaway that something is a producer is that it doesn't have arrows coming into it. It only has arrows coming out of it. And so organism one is a producer; organism five is also a producer. Where are they getting their matter and energy from? Well, they're getting their matter from the environment, and the energy in most cases is coming from the Sun. The things that eat the producers, those are primary consumers.

So this is an arrow that goes from a producer to a primary consumer. This is an arrow that goes from a producer to a primary consumer. And this is an arrow that goes from a producer to a primary consumer.

Now with that out of the way, which arrows show us going from a primary consumer to a secondary consumer? Pause the video and think about that. Well, I just said that these things that are consuming the producers, these are primary—I'll just say P for primary. Primary consumer; this is a primary consumer; this is a primary consumer as well.

Now secondary consumers are, by definition, things that aren't decomposers, but they're eating primary consumers. So they're going to have arrows coming in from primary consumers and out to either other secondary consumers or to decomposers. So organism three right over here is a secondary consumer. So we're going from a primary consumer to a secondary consumer. This arrow is from a primary consumer to a secondary consumer.

This arrow over here is interesting; it's a secondary consumer being consumed. But you can see that organism four only has arrows going into it, which is a pretty good clue that this is a decomposer. So I'm not going to call organism four a secondary consumer.

So let's keep going. What about the arrow that goes from organism six to organism seven? That looks like primary consumer to primary consumer. Well, it turns out that something can be both a primary consumer and a secondary consumer. So this is also a secondary consumer. Why? Because it's eating a primary consumer and it's not a decomposer. So this is also another example of an arrow from a primary consumer to a secondary consumer.

Last but not least, what are the arrows that go from a consumer to a decomposer? And we already broke this down a little bit, no pun intended. Pause the video and try to think about that.

Well, this is the only decomposer that's depicted in this diagram. And so this is going from a consumer to a decomposer right over here. This is going from a consumer to a decomposer, and that's all we have here.

To be clear, you don't have to go from only a consumer to a decomposer; you could have gone from a producer straight to a decomposer. You could have arrows that look like that as well because if a plant dies and it gets decomposed, well, that matter and energy is going to be used by that decomposer.