Definition
Ecological pyramids
Ecological pyramids are graphical representations used to show the relative numbers, biomass, or energy at different trophic levels in an ecosystem.
- Ecological pyramids are graphical representations that show the quantitative differences between trophic levels in an ecosystem.
- They help visualize how energy, biomass, or number of organisms change as energy flows through an ecosystem’s food chain.
- 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.
- These pyramids are typically measured for a given area and time to represent ecosystem structure accurately.
Types of Ecological Pyramids
1. Pyramid of Numbers
- A pyramid of numbers shows the number of individual organisms at each trophic level within a defined area.
- The width of each bar corresponds to the number of organisms present in that level.
Note
It provides a snapshot of the population size at a given time.
Typical Shape and Variations
- 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.
- This shape follows the energy loss rule.
- Less energy supports fewer organisms at higher levels.
Common Mistake
- Don’t assume all pyramids of numbers are triangular.
- Some ecosystems produce inverted or irregular shapes.
Inverted Pyramids of Numbers
- When large producers (like trees) support many smaller herbivores or consumers, the pyramid may appear inverted (upside down).
- This occurs when individuals at the lower trophic levels are fewer in number but larger in size.
Example
- Oak tree → Insects → Woodpeckers
- A single oak tree (producer) supports thousands of insects (primary consumers) and several birds (secondary consumers), resulting in an inverted pyramid.
2. Pyramid of Biomass
- A pyramid of biomass represents the total dry mass of organisms at each trophic level at a given time (standing crop).
- Biomass measures the amount of living organic material, essentially the energy stored in the bodies of organisms.
- Units are typically grams per square meter (g m⁻²) or kilograms per square meter (kg m⁻²).
Typical Shape
- Usually, pyramids of biomass are pyramid-shaped, reflecting decreasing biomass with higher trophic levels.
- Producers have the largest biomass because they form the foundation of energy capture.
- Biomass decreases upward because energy transfer is inefficient, and energy is lost as heat through respiration.
Example
- Clover (80 kg) → Snail (30 kg) → Thrush (10 kg) → Sparrowhawk (2 kg)
- The pyramid narrows toward the top as biomass decreases.
Inverted Pyramids of Biomass
- In some aquatic ecosystems, the pyramid may appear inverted — with lower producer biomass than consumer biomass.
- This occurs when:
- Phytoplankton (producers) have a low standing biomass but reproduce rapidly.
- Zooplankton and fish (consumers) have greater total biomass at any instant.
Example
- In a marine food chain, phytoplankton are small and short-lived but reproduce so fast that they support a larger biomass of fish and whales.
- Despite being inverted, productivity over time still follows the same rule- producers supply the base energy.
Tip
- Biomass levels can fluctuate seasonally, especially in ecosystems dependent on sunlight and temperature.
- For example, in a temperate pond, biomass of phytoplankton peaks during summer due to higher light intensity and drops during winter.
3. Pyramid of Energy
- The pyramid of energy represents the flow of energy through trophic levels over time.
- It measures the rate of energy transfer, not just the amount present at a single moment.
- Units are typically kJ m⁻² year⁻¹ or J m⁻² year⁻¹.
Shape and Characteristics
- Always upright, never inverted.
- The base (producers) is the widest, representing the largest energy input from sunlight.
- Each successive level becomes narrower, as only ~10% of energy is transferred upward.
Example
In a forest ecosystem:
- Producers: 20,000 kJ m⁻² yr⁻¹
- Primary consumers: 2,000 kJ m⁻² yr⁻¹
- Secondary consumers: 200 kJ m⁻² yr⁻¹
- Tertiary consumers: 20 kJ m⁻² yr⁻¹
Note
Because they measure flow over time, energy pyramids cannot be inverted and are the most accurate reflection of ecosystem functioning.
Advantages of Pyramids of Energy
- Show energy transfer efficiency between trophic levels.
- Allow comparison between ecosystems regardless of organism size or number.
- Avoid the limitations of inverted number or biomass pyramids.
- Reflect the Second Law of Thermodynamics: energy degrades as it moves through trophic levels.
Analogy
- You can’t compare two shops by how full their shelves are.
- You need to know how fast goods are sold and restocked.
- Similarly, biomass shows stock, while energy pyramids show flow - how much energy passes through each level.
Constructing Pyramids
Step 1: Collect Data
- Identify the trophic levels (e.g., producers, primary consumers, secondary consumers).
- Gather quantitative data:
- Number of individuals
- Dry biomass (g/m²)
- Energy content (kJ/m²/year)
Step 2: Draw the Axes
- Draw a vertical axis representing trophic levels.
- Plot bars symmetrically around the central axis — the width of each bar represents the relative quantity (number, biomass, or energy).
Step 3: Label and Compare
- Label each trophic level clearly with the organism name.
- Compare shapes across pyramids to analyze efficiency and productivity.
Measuring Biomass and Energy
Procedure to Measure Biomass (Plant Material Only)
- Collect plant samples from a defined area (1 m² quadrat).
- Dry at 80°C in an oven until a constant mass is reached.
- Weigh and record dry mass.
- Extrapolate to the total area to determine biomass per unit area.
Procedure to Measure Energy (Calorimetry Method)
- Burn a known mass of dried sample in a calorimeter.
- Measure temperature change of a known volume of water.
- Calculate energy released using:
Energy (J) = mass of water × ΔT × 4.18 J g⁻¹ °C⁻¹ - Extrapolate to total biomass for total energy per trophic level.
Example
- Burning 2 g of dried leaf tissue raises 200 g of water by 10°C.
- Energy = 200 × 10 × 4.18 = 8,360 J → 4,180 J g⁻¹ of biomass.
Factors Influencing Pyramid Shape
- Energy Transfer Efficiency: Inefficient transfer leads to narrower upper levels.
- Turnover Rates: Rapidly reproducing organisms (phytoplankton) may cause inverted biomass pyramids.
- Body Size and Longevity: Large producers (trees) distort number pyramids.
- Seasonal Changes: Affects productivity and standing biomass.
- Abiotic Factors: Light intensity, temperature, and nutrient levels influence energy flow.
Self review
- Define ecological pyramids and list their three main types.
- Explain how pyramids of number and biomass differ in what they measure.
- Describe why energy pyramids are always upright.
- Discuss situations that can lead to inverted biomass or number pyramids.
