Ecological Pyramids

The further you go up a food chain, the larger the organisms tend to become and the smaller their population. In order to appreciate this more clearly, ecological pyramids are created in order to quantify food chains.

A pyramid of numbers represents how many organisms there are at each trophic level. The amounts are visually shown by the width of the bars. The area being looked at is usually shown as m² if terrestrial or m³ if marine habitats. A lot of the time the end result is a pyramid however this is not always the case. It all depends on the size of the organism: a lot of terrestrial producers are big, like trees, while a lot of primary consumers are small, like insects and such differences would produce an inverted pyramid.

Pyramids of biomass provide more information in that they look at the total mass of living organisms at each trophic level. The biomass is always dry mass due to the fact that there’s no energy store in water. It can be calculated by drying and weighing an organism or by taking a count and then multiplying by the mean mass of an individual. The measurement used is kg m^-2.

The shape of a pyramid of biomass is always a pyramid (although some can be inverted) because one trophic level takes all its mass from the level preceding it and mass is loss at each stage for a food chain: some biomass is lost in excretion after it’s been used in respiration and some is not eaten or is ingested then egested without being absorbed. This then becomes detritus and moves onto the decomposers.

Pyramids of energy represent how much energy moves from one trophic level to another at a specific time. As with pyramids of biomass, their shape is always that of a pyramid due to the simple fact that energy cannot be created. They’re also very shallow in shape because energy transfer is not efficient. Energy not passed on is lost as heat.