SPI Biology · Ecology and Environment

Ecosystems and Food Chains

An ecosystem is the living things of a place together with its non-living surroundings, and a food chain is the order in which food, and the energy in it, passes from plants to animals.

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Why this matters

Three questions in the bank: one EASY (what an ecosystem includes), one MODERATE (the links of a food chain) and one HARD (a pyramid of biomass for a pond chain). The MODERATE one is lost by putting decomposers into the chain; the HARD one by forgetting that a pond's tiny plants can weigh less than the animals eating them.

Concept 1 of 3: The parts of an ecosystem

A pond is an ecosystem: the fish, plants and microbes in it, and also the water, mud, sunlight and air they depend on. Neither half works without the other.

Definition

Biotic (living) components: producers (green plants), consumers (animals) and decomposers (bacteria and fungi).
Abiotic (non-living) components: air, water, soil, sunlight, temperature, minerals.
An ecosystem always has both, interacting with each other.
Examples: natural (pond, forest, grassland, desert) and man-made (garden, crop field, aquarium).
Decomposers break down dead plants and animals and return minerals to the soil.

Component	Living or not	Examples
Producers	Living	Grass, trees, algae
Consumers	Living	Deer, frog, lion
Decomposers	Living	Bacteria, fungi
Abiotic factors	Non-living	Air, water, soil, sunlight An ecosystem includes the non-living part too, not living things alone.

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The same idea in a practice question:

Practice questionEasy

Example 1 · Ecology and Environment · Ecosystems and Food Chains

An ecosystem includes

An ecosystem as living things only

A community is the living things alone. An ecosystem adds the non-living part (air, water, soil, light) that they live in and use.

Concept 2 of 3: Food chains, trophic levels and pyramids

A food chain answers 'who eats whom', in order: a plant, then the animal that eats the plant, then the animal that eats that animal. Each step is a trophic level. Draw a bar for each level and you get an ecological pyramid.

Definition

Food chain links: producer (plant), herbivore (primary consumer), carnivore (secondary consumer), and sometimes a top carnivore. Example: grass, deer, lion.
Decomposers act on every level but are not a link in the chain.
A food web is many food chains joined together.
Pyramid of numbers and pyramid of energy: usually widest at the producers. The pyramid of energy is always upright.
Pyramid of biomass (living weight): upright on land. In a pond or sea, tiny producers (phytoplankton) are eaten as fast as they grow, so their biomass at any moment can be less than the herbivores': the bottom bar is the narrow one.

Trophic level	Name	Example (grassland)	Example (pond)
First	Producer	Grass	Phytoplankton
Second	Herbivore (primary consumer)	Grasshopper	Small crustaceans
Third	Carnivore (secondary consumer)	Frog	Small fish
Fourth	Top carnivore	Snake	Large fish Decomposers are not a level in the chain.

The pyramid of energy is always upright; a pond's pyramid of biomass can have a narrow base.

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The same idea in a practice question:

Practice questionModerate

Example 2 · Ecology and Environment · Ecosystems and Food Chains

A food chain consists of:

Putting decomposers in the chain

Producers, consumers and decomposers are the parts of an ecosystem. A food chain is producers, herbivores and carnivores; decomposers work on every level from the side.

Every pyramid is upright

The pyramid of energy is always upright, but a pond's pyramid of biomass can be narrow at the bottom, because tiny producers are eaten as fast as they grow.

Drill 1 more on food chains, trophic levels and pyramids

Concept 3 of 3: The ten percent law of energy flow

At each step of a food chain, most of the energy is used up in living and lost as heat. Only about a tenth of it is stored in the body of the next animal.

Definition

Only about 10% of the energy at one trophic level reaches the next.
So the energy falls by ten times at each step, and food chains are usually only 3 or 4 steps long.
Green plants store only about 1% of the sunlight that falls on them: a different figure from the 10%.

Energy at the next level

E_{\text{next}} = \frac{10}{100} \times E_{\text{this}}

Worked example

Plants in a field store 20,000 J of energy. How much reaches a snake in the chain plants, grasshopper, frog, snake?

Practice this conceptself-check · 3 quick reps

Mixing up the 1% and the 10%

Plants store about 1% of sunlight; about 10% of energy passes from one animal level to the next. A question about sunlight wants 1%.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

The ten percent law of energy flow
Energy at the next level
$E_{\text{next}} = \frac{10}{100} \times E_{\text{this}}$

Reference tables (2)

The parts of an ecosystem4 rows

Component	Living or not	Examples
Producers	Living	Grass, trees, algae
Consumers	Living	Deer, frog, lion
Decomposers	Living	Bacteria, fungi
Abiotic factors	Non-living	Air, water, soil, sunlight An ecosystem includes the non-living part too, not living things alone.

Food chains, trophic levels and pyramids4 rows

Trophic level	Name	Example (grassland)	Example (pond)
First	Producer	Grass	Phytoplankton
Second	Herbivore (primary consumer)	Grasshopper	Small crustaceans
Third	Carnivore (secondary consumer)	Frog	Small fish
Fourth	Top carnivore	Snake	Large fish Decomposers are not a level in the chain.

The pyramid of energy is always upright; a pond's pyramid of biomass can have a narrow base.

Watch out for (4)

An ecosystem as living things only
→ The parts of an ecosystem
Putting decomposers in the chain
→ Food chains, trophic levels and pyramids
Every pyramid is upright
→ Food chains, trophic levels and pyramids
Mixing up the 1% and the 10%
→ The ten percent law of energy flow

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