VMBO Year 3 · Ecology · Section 6.2

Ecological pyramids

Pyramids in a food chain

Pyramids of numbers and pyramids of biomass represent feeding levels in a food chain. A pyramid of numbers shows the number of individuals at each level. Often there are many producers, fewer herbivores and still fewer predators. Exceptions occur: one large tree can support many insects, so numbers do not always form an upright pyramid.

Pyramids of biomass

Biomass is the mass of living biological material, often measured as dry mass for fair comparisons. It contains energy-rich substances. A pyramid of biomass shows the total biomass at each feeding level for a given area and time. On land it is usually largest among producers and decreases at higher levels. However, some aquatic systems can have an inverted biomass pyramid at a particular moment because producers such as phytoplankton reproduce and are consumed rapidly.

Why is less available at higher levels?

Only part of the material and energy at one feeding level becomes new biomass at the next.

• Not every organism or body part is eaten.
• Some food is not digested and leaves as faeces.
• Much absorbed food is used in respiration to support activity and maintenance, releasing heat.

Dead material and wastes can enter decomposer food chains.

Energy through the food chain

Producers capture light energy through photosynthesis. Chemical energy passes to herbivores and then to predators. At each transfer, less energy remains available to form biomass at the next level. A pyramid of energy, showing energy flow over a period of time, is therefore always upright. Limited energy helps explain why food chains cannot support unlimited higher feeding levels.