Pyramid of Numbers Why So Many?

An Interesting Fact On an average day, one dragonfly can eat up to two times its weight in mosquitoes! This fearsome looking insect scours forests and fields looking for mosquitoes. Once found, the dragonfly traps a mosquito in its front wings, then uses its forelimbs to pull the mosquito toward its mouth. All the while, its back wings keep it aloft.

Ever Wonder… If dragonfly’s eat so many mosquitoes per day, why does there never seem to be a shortage of the pesky little insects? Why are there always more mosquitoes than dragonfly’s?

Follow the Trail of Blood After a mosquito has bitten its victim it still has a wide range of activities to complete before its day ends. It must fly, find shelter, mate and remove wastes, just to name a few. In doing all of this, the mosquito uses up a large portion of the energy it received from the blood.

Therefore… When the dragonfly eats the mosquito, it only receives a small portion of the energy that the mosquito originally received from the blood. 10% to be exact! Thus, in order for the dragonfly to receive enough energy to complete its own daily tasks, it must eat a larger number of mosquitoes.

As a Result… Because a dragonfly must eat a larger number of mosquitoes to survive, the ecosystem has to be designed to support its eating habits. This means, in order for the dragonfly to eat the number of mosquitoes it needs to survive, there must be more mosquitoes than dragonfly’s. Otherwise the whole ecosystem would collapse!

Logic It is logical then, that if only 10% of the energy from one organism is passed along to the next organism in the food chain, there must be more organisms at the bottom of the chain, than at the top. i.e.: more grass (producers) than bears (tertiary consumers) It also makes sense that the consumers at the top (bears), must therefore eat more than those at the bottom (rabbits), just to get enough energy to survive.

Pyramid of Numbers This model of having more organisms at the bottom of the food chain than at the top (due to the fact that energy decreases as it is passed from one organism to another in the food chain) is scientifically known as the Pyramid of Numbers.

Pyramid of Biomass Pyramid of Biomass – It is a graphical representation that shows the relationship between biomass and trophic level. It states an amount of biomass present at each trophic level of an ecological community at a particular time. Biomass is the total amount of living or organic matter in an ecosystem present in unit area in different tropic levels. In order for this community to sustain itself, there needs to be more biomass at the bottom than at the top.

Pyramid Showing of Biomass Each step of the pyramid is a trophic level At each level the amount of biomass is stated

Pyramid of Energy The energy pyramid shows how the amount of energy entering each level  varies across trophic levels.  In general, only about 10% of the energy entering a trophic level is transferred to the trophic level above it, so the energy pyramid always has a distinct step-like pattern with less energy entering each trophic level up the food chain.   The shape of the energy pyramid affects the length of food chains because eventually the amount of energy entering the highest trophic level is not large enough to support a higher trophic level.

Pyramid of Energy Each step is a different trophic level The percentage of energy present at each trophic level is shown in purple

Pyramid of Energy Shows how much energy is available at each trophic level – J or joule is the measurement for energy