Ecological Pyramids: Pyramid of Number, Biomass, Energy

An ecological pyramid is a diagram that shows how number, biomass or energy changes at each trophic level in an ecosystem. The base is wide because producers are many, and each higher level becomes smaller.

These pyramids help us see why top carnivores are rare and why eating plants directly is more energy efficient than eating meat. They also reveal unhealthy ecosystems where the pyramid shape is inverted.

Key Concepts

Pyramid of number counts individual organisms at each level. Pyramid of biomass measures total dry mass of living tissue, usually in grams per square metre (g m-2). Pyramid of energy measures energy flow in kilojoules per square metre per year (kJ m-2 yr-1). Trophic level is the feeding step. Biomass is mass without water because water content varies. Energy pyramid is always upright because energy cannot be created and is lost as heat.

Formulas

Energy transfer efficiency is calculated as: Efficiency = (Energy at higher level / Energy at lower level) x 100. Energy is in kJ m-2 yr-1. The rule of 10 percent is an average and real values range from 5 to 20 percent depending on species and ecosystem. Species-area relationship is not used here, but pyramid values must be measured over the same area and time to be comparable.

Assumption: sampling represents the whole ecosystem and seasonal changes are averaged over a year.

Worked Example

In a grassland, producers store 20,000 kJ m-2 yr-1.

Step 1: Primary consumers receive about 10 percent = 20,000 x 0.10 = 2,000 kJ.

Step 2: Secondary consumers receive 10 percent of 2,000 = 200 kJ.

Step 3: Tertiary consumers receive 10 percent of 200 = 20 kJ.

So the energy pyramid values are 20,000, 2,000, 200, 20. You can see each level is ten times smaller, forming a clear upright pyramid.

Common Misconceptions

• Pyramid of number is always upright. One large tree can support thousands of insects, so the number pyramid can be partly inverted.
• Biomass pyramid is always upright. In oceans, phytoplankton reproduce fast and have low standing biomass but support larger zooplankton biomass at a moment, giving an inverted shape.
• Energy can increase at higher levels. This is impossible due to heat loss and the laws of thermodynamics.

Real-World Applications

Pyramids guide sustainable harvesting, showing how many kilograms of fish can be taken without collapsing the base. They help explain why land used for beef needs more area than land for grains. Pollution studies use pyramids to track biomagnification, where toxins like DDT concentrate at higher levels.

To sum up, number, biomass and energy pyramids compare trophic levels, energy pyramids are always upright and smallest at the top, and the shape warns us about ecosystem health.