2.2G Ecological Pyramids Notes

1. Ecological pyramids are graphical representations that show the quantitative differences between trophic levels in an ecosystem.
2. They help visualize how energy, biomass, or number of organisms change as energy flows through an ecosystem’s food chain.
3. Each level in the pyramid corresponds to a trophic level, starting with producers at the base and progressing upward to primary, secondary, and tertiary consumers.
4. These pyramids are typically measured for a given area and time to represent ecosystem structure accurately.

Types of Ecological Pyramids

1. Pyramid of Numbers

1. A pyramid of numbers shows the number of individual organisms at each trophic level within a defined area.
2. The width of each bar corresponds to the number of organisms present in that level.

Typical Shape and Variations

1. In most ecosystems, the pyramid of numbers narrows toward the top, reflecting the decrease in the number of individuals as you move up trophic levels.
2. This shape follows the energy loss rule.
3. Less energy supports fewer organisms at higher levels.

Inverted Pyramids of Numbers

1. When large producers (like trees) support many smaller herbivores or consumers, the pyramid may appear inverted (upside down).
2. This occurs when individuals at the lower trophic levels are fewer in number but larger in size.

2. Pyramid of Biomass

1. A pyramid of biomass represents the total dry mass of organisms at each trophic level at a given time (standing crop).
2. Biomass measures the amount of living organic material, essentially the energy stored in the bodies of organisms.
3. Units are typically grams per square meter (g m⁻²) or kilograms per square meter (kg m⁻²).

Typical Shape

1. Usually, pyramids of biomass are pyramid-shaped, reflecting decreasing biomass with higher trophic levels.
2. Producers have the largest biomass because they form the foundation of energy capture.
3. Biomass decreases upward because energy transfer is inefficient, and energy is lost as heat through respiration.

Inverted Pyramids of Biomass

1. In some aquatic ecosystems, the pyramid may appear inverted — with lower producer biomass than consumer biomass.
2. This occurs when:
3. Phytoplankton (producers) have a low standing biomass but reproduce rapidly.
4. Zooplankton and fish (consumers) have greater total biomass at any instant.

3. Pyramid of Energy

1. The pyramid of energy represents the flow of energy through trophic levels over time.
2. It measures the rate of energy transfer, not just the amount present at a single moment.
3. Units are typically kJ m⁻² year⁻¹ or J m⁻² year⁻¹.

Shape and Characteristics

1. Always upright, never inverted.
2. The base (producers) is the widest, representing the largest energy input from sunlight.