Energy flow describes how energy moves through an ecosystem from one organism to another. Unlike nutrients, energy flows in one direction only (unidirectional) and cannot be recycled.
The primary source of energy for almost all ecosystems is solar radiation (sunlight). Energy enters the biotic (living) component of an ecosystem through producers (autotrophs), which capture light energy and convert it into chemical energy via photosynthesis.
A small number of ecosystems (e.g., deep-sea hydrothermal vents) rely on chemosynthesis instead of sunlight.
A trophic level is a specific step or position in a food chain occupied by organisms that obtain energy from the same source.
| Trophic Level | Organisms | Example |
|---|---|---|
| 1st | Producers (Autotrophs) | Grass, Algae |
| 2nd | Primary Consumers (Herbivores) | Grasshopper, Rabbit |
| 3rd | Secondary Consumers (Carnivores) | Frog, Fox |
| 4th | Tertiary Consumers | Snake, Eagle |
| — | Decomposers | Bacteria, Fungi |
Decomposers (saprotrophs) break down dead organic matter at every level and return nutrients to the environment.
A food chain is a linear sequence showing the transfer of energy from one organism to the next:
A food web is a complex network of multiple interconnected food chains within an ecosystem. It more accurately represents real feeding relationships because most organisms feed on more than one species.
Key difference: A food chain is linear; a food web is a network of many food chains.
When energy passes from one trophic level to the next, only approximately 10% is transferred; the remaining ~90% is lost as:
If producers contain 10,000 J of energy:
| Trophic Level | Energy Available |
|---|---|
| Producers | 10,000 J |
| Primary Consumers | 1,000 J (10%) |
| Secondary Consumers | 100 J (10%) |
| Tertiary Consumers | 10 J (10%) |
This explains why food chains rarely exceed 4–5 trophic levels — too little energy remains to support a viable population at higher levels.
Energy flow is:
This contrasts with nutrients, which cycle continuously through biotic and abiotic components (biogeochemical cycles).
An ecological pyramid is a graphical representation of the quantitative relationship between trophic levels. There are three types:
The Pyramid of Energy is the only ecological pyramid that is always upright and can never be inverted.
| Concept | Key Point |
|---|---|
| Energy source | Sunlight (solar radiation) |
| Entry point | Producers via photosynthesis |
| Energy transfer | ~10% per trophic level |
| Energy flow direction | Unidirectional, non-cyclic |
| Food chain limit | 4–5 levels due to energy loss |
| Always upright pyramid | Pyramid of Energy |