Learning Objectives

After studying this topic, you should be able to:

Define Ecological Pyramids.
Explain the three types of ecological pyramids.
Differentiate between Energy, Biomass, and Number Pyramids.
Understand why the Pyramid of Energy is always upright.
Explain why the Pyramid of Biomass is inverted in aquatic ecosystems.

Ecological Pyramid

Definition

An Ecological Pyramid is a graphical representation of the trophic structure of an ecosystem, showing the quantitative relationship between different trophic levels based on:

Energy
Biomass
Number of organisms

General Structure of an Ecological Pyramid

          Apex Predators
                ▲
      Tertiary Consumers
                ▲
     Secondary Consumers
                ▲
      Primary Consumers
                ▲
          Producers
             (Base)

Key Points

Base → Producers (First trophic level)
Middle → Consumers
Top (Apex) → Top predators

Types of Ecological Pyramids

Ecological pyramids are classified into three types:

Pyramid of Energy
Pyramid of Biomass
Pyramid of Numbers

1. Pyramid of Energy

Definition

Represents the amount of energy available at each trophic level.

Characteristics

Always Upright
Follows Lindeman's 10% Law
Energy decreases at every trophic level.
Energy flow is unidirectional.

Why is it Always Upright?

According to the 10% Law:

Only 10% of energy is transferred to the next trophic level.
Around 90% is lost as:
- Heat
- Respiration
- Metabolic activities

Therefore, energy continuously decreases from producers to top consumers, making an inverted energy pyramid impossible.

Example

Trophic Level	Energy (kcal)
Producers	10,000
Primary Consumers	1,000
Secondary Consumers	100
Tertiary Consumers	10

2. Pyramid of Biomass

Definition

Represents the total dry weight (biomass) of living organisms present at each trophic level.

Biomass

Biomass = Dry Weight × Number of Organisms

A. Terrestrial Ecosystem

Shape

Upright

Reason

Trees and plants possess enormous biomass.
Herbivores have less biomass.
Carnivores possess the least biomass.

Example:

Trees
   ▲
Deer
   ▲
Tiger

B. Aquatic Ecosystem

Shape

Inverted

Reason

Producers are microscopic phytoplankton.
They possess very low standing biomass.
They reproduce extremely fast (high turnover rate).
Continuous reproduction supports larger consumer biomass.

Example

Large Fish
     ▲
Zooplankton
     ▲
Phytoplankton

3. Pyramid of Numbers

Definition

Represents the number of individual organisms present at each trophic level.

A. Grassland Ecosystem

Shape

Upright

Example

Snakes
   ▲
Frogs
   ▲
Grasshoppers
   ▲
Grass

Reason:

Many grasses support fewer herbivores.
Herbivores support fewer carnivores.

B. Tree Ecosystem

Shape

Inverted or Spindle-shaped

Reason:

One tree supports many birds.
Birds support numerous parasites.
Higher predators are fewer.

Example

Few Hawks
     ▲
Thousands of Parasites
     ▲
Hundreds of Birds
     ▲
One Tree

Shapes of Ecological Pyramids

Pyramid	Shape
Energy	Always Upright
Biomass (Terrestrial)	Upright
Biomass (Aquatic)	Inverted
Numbers (Grassland)	Upright
Numbers (Tree Ecosystem)	Inverted / Spindle-shaped

Comparison Tables

Energy Pyramid vs Biomass Pyramid

Energy Pyramid	Biomass Pyramid
Measures energy	Measures dry weight
Always upright	Upright or inverted
Follows 10% Law	Depends on ecosystem
Never inverted	May be inverted

Biomass Pyramid: Terrestrial vs Aquatic

Terrestrial	Aquatic
Upright	Inverted
Trees have maximum biomass	Phytoplankton have very low standing biomass
Large producers	Microscopic producers
Low turnover	High turnover

Pyramid of Numbers: Grassland vs Tree Ecosystem

Grassland	Tree Ecosystem
Upright	Inverted / Spindle
Many grasses	One tree
Fewer herbivores	Many birds
Few predators	Numerous parasites

Important Definitions