Often when looking at a community, we want to know about its trophic structure, which is the transfer of energy through trophic levels. Now, trophic levels are often expressed in a food chain, which is basically a network of organisms that shows who eats whom. So let's take a look at this food chain right here. It's going to start with this algae at the bottom, and this algae is a producer, all the rest of these guys are consumers. So this algae is going to be eaten by this organism, and this organism is going to eat that little shrimp thing, and this fish is going to eat that fish, and so on and so forth. So each one of these levels is a trophic level. And basically, what trophic structure is concerned with is how much energy gets transferred from one level to the next. Now, we're going to look at that in more detail later on.

Now, when looking at the food web we see that it's linear, but in actuality in communities these networks are really more of a web than a chain, and you can see how this food chain we had over here on the left would actually sort of merge into a larger food web where there are more interconnections between the organisms. I mean, for example, what's to stop this bird from, I don't know, say eating this fish. That's why a food web is, you know, generally a better overall view of what's going on in the community.

And the main thing to realize about these trophic levels and in the food web or in the food chain is that the energy that gets transferred between levels is transferred very inefficiently. So the length of the food chain is actually going to be limited by that inefficient energy transfer, and we're going to, again, we're going to look at this in more detail, put some numbers to it, make it extra clear.

Now, one thing I want to point out while we're looking at these food webs and food chains is the idea of bottom-up and top-down models. Bottom-up models look at food webs and look at the influence of lower trophic levels as determining what happens at higher levels. Top-down models show community structures where usually a predator controls community structures by limiting herbivores. So the predator at the top of the food chain will control the elements of the food chain below it, so top-down.

Now, with that, let's go ahead and flip the page.