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Ecological Pyramids are diagrams used to display Quantitative data and show the relationship
between organisms in an ecosystem. They are made up of bars stacked on top of
one another. They can be used to show numbers of organisms or biomass of each trophic level
in ecosystem. They visually describe the quantities that
we see in food chains and food webs. They visually describe the quantities of the organisms
that we see in food chains and food webs. So how does that work? Well the bottom of
the pyramid represents the primary producers. That first trophic level. On top of that go
the primary consumers. The second trophic level, and so on up through secondary consumers,
tertiary consumers until you reach the highest trophic level in that eco system.
In the eco system the energy is being transferred up the pyramid, as each organism is consumed
by the organism in the next trophic level up.
The height of the bars should be the same, but the width of the bars should be proportional
to the quantity that’s being displayed. So, for an ecological pyramid of numbers,
the width of the bar shows the number of organisms at that step in the food chain. The unit for
this is number of organisms. In the pyramid of biomass it shows the biomass of organisms,
and a common unit for that is grams per metre squared.
So let’s look at the following pyramid of numbers. Which doesn’t take into account
the size of the organisms. In this ecosystem we have grass, which is eaten by rabbits,
who are then eaten by foxes, who are finally eaten by eagles. As you can see in this example,
1000’s of blades of grass are eaten by lots of rabbits, then that feeds very few foxes.
These foxes will then feed a larger number of eagles. The more organisms there are, the
wider the bar is. Pyramids of numbers can end up looking a bit
wonky. For example if one primary producer can feed lots of primary consumers, like an
oak tree, which feeds lots and lots of little caterpillars.
Pyramids of biomass on the other hand, are almost always largest at the bottom and then
get smaller going up. This is because rather than showing the number of organisms they
show the number of the total mass in that trophic level. Think about it this way. One
caterpillar weighs about 3 grams; one oak tree weighs about 30 tonnes, which is 30 million
grams! So that’s 10,000,000 caterpillars to equal the mass of one oak tree!
Because of this, the bar for the oak tree will be much wider than the bar for the caterpillars,
even though there are far more caterpillars and maybe only one oak tree in the ecosystem.
So to recap, ecological pyramids are diagrams that we use to visually represent the relationship
between organisms in an ecosystem. The bars each represent trophic levels, and
their order represents the flow of energy. The width of the bar in number pyramids represents
the number of organisms in that trophic level. The width of the bar in biomass pyramids represents
the biomass of the organisms in that trophic level.