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Ecological Pyramids

Teacher/Instructor Patrick Roisen
Patrick Roisen

M.Ed., Stanford University
Winner of multiple teaching awards

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Ecological pyramids are diagrams which show the abundance of organisms at each trophic level along the food chain of an ecosystem. The shapes of ecological pyramids are somewhat triangular in shape because there are fewer consumers at each ascending tropic level.

When studying Ecology it's very common to come across ecological pyramids. These are diagrams that help show various relationships between the different trophic levels within a particular community. There's 3 kinds of ecological pyramids you'll typically see, one is called a pyramid of biomass. Biomass is the total weight of organisms at a particular trophic level like if I got in a square meter all of the producers, so grass and bushes or whatever and I put them on a scale how much would they weigh? That's their biomass, and if I got all the mice in a particular, in that same area they're eating the grass. So they'll be the primary consumers if I got them on a scale how much would they weigh and so on and so forth.

A pyramid of numbers is what it sounds like, it's you count the number of producers in a particular area like 1, 2, 3 many. Pyramid of production is sometimes called the energy pyramid or sometimes a pyramid of productivity and what that shows is how much energy is available at each level in their community. Let's take a look at a pyramid of biomass, and here's a typical kind of pyramid you may see. So in this pyramid they'll say 900 grams of producers in every square meter so meter by meter square region of say your backyard. If you counted up all of the primary consumers of things that are feeding on the producers, there're might be 120 grams per square meter.

If you then measured the number of secondary consumers, the things that are eating those guys you might find 10 grams per meter squared. And then at the top of this particular ecological pyramid are tertiary consumers you may only have 1.2 grams per square meter. Now you may see these numbers inverted sometimes especially in a aquatic environments for example if you have near the outlet of the river Tim's if you did a biomass of the phytoplankton the algae that are being, that are producing all the food in that are, will actually have less biomass in that kind of environment as opposed to the zoo plankton the little microscopic creatures that are eating them. Why is that? That's because those phytoplankton especially if all the stuff is coming out of the Tim's they're reproducing so quickly they don't live very long and so there's not a whole lot of them at any one moment. So the phytoplankton however are much slower to reproduce and so if you take a quick snap shot you might find fewer phytoplankton by gram per cubic meter in that case since this is aquatic environment, then you'd have of your primary consumer. So that can create a difference in some cases.

You'll often see weirdnesses in pyramids of numbers, but that's because say in this one, you only see one producer and you may be thinking how is this one producer keeping over 350 other creatures alive? Well what if it's a tree and these are bugs on it and then these are birds eating the bugs on the tree and that's the one snake that it is eating the birds, okay. Whereas if we did say in aquatic environment and we're counting the number of algae in an area, then you're talking about a huge base with much smaller numbers on top. Especially if it's like algae being eaten by homburg whale then you're going to have a huge homburg and maybe one homburg whale.

The pyramid of production, the one that's showing the amount of energy that's at each level there you'll almost invariably have the stereotypical triangle going up like this. And this bottom level here the one million joules that represents the sun. So the sun is producing a bucket load of energy in a particular area but the producers maybe at best trap maybe 1% of the energy that's available. Well why is that? Plants can't take advantage of the most of the energy given off by the sun, the cosmic radiation et cetera. At the next level there's typically on average somewhere between 5 to 20% energy efficiency as you go up the pyramid so a good rule of thumb is to just estimate roughly 10% and so you'll see this 10% drop as you go up in the ecological pyramid. So these are what we're talking about in the ecological pyramids.