GCSE Biology revision

Trophic levels

Trophic levels in an ecosystem (biology only)

AQA 4.1.1.1

Your specification

AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Identify and describe What Is a Food Chain?, Producers and Biomass and Consumers in a Food Chain in the context of trophic levels.

MOST 🎯🎯

Explain the relationships between What Is a Food Chain?, Producers and Biomass and Consumers in a Food Chain and how they contribute to trophic levels.

SOME 🎯🎯🎯

Apply and analyse knowledge of What Is a Food Chain?, Producers and Biomass and Consumers in a Food Chain to interpret unfamiliar information about trophic levels.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What Is a Food Chain?

A food chain shows what gets eaten by what in an ecosystem, representing a simplified version of a food web by displaying just one chain of interactions
Arrows in a food chain represent the flow of energy between levels, pointing from the organism being eaten towards the organism eating it
All food chains begin with a producer, which is a photosynthetic organism such as a green plant or alga that uses the sun's energy to make biological molecules

1. What Is a Food Chain?AQA

Notes

A food chain shows what gets eaten by what in an ecosystem, representing a simplified version of a food web by displaying just one chain of interactions
Arrows in a food chain represent the flow of energy between levels, pointing from the organism being eaten towards the organism eating it
All food chains begin with a producer, which is a photosynthetic organism such as a green plant or alga that uses the sun's energy to make biological molecules

Producers and Biomass

Producers are photosynthetic organisms, meaning they produce glucose using the sun's energy through the process of photosynthesis
Once the biological molecules made during photosynthesis are incorporated into the plant, the stored energy is referred to as biomass

Consumers in a Food Chain

Primary consumers are the first level of consumers in a food chain and feed directly on producers, for example mice eating grass
Secondary consumers are predators that feed on primary consumers, for example an owl feeding on mice
Tertiary consumers feed on secondary consumers and represent a third level of consumption in the food chain
Not all ecosystems have the same number of levels — some may have more trophic levels than others, whilst some may have fewer

3. Consumers in a Food ChainAQA

Notes

Primary consumers are the first level of consumers in a food chain and feed directly on producers, for example mice eating grass
Secondary consumers are predators that feed on primary consumers, for example an owl feeding on mice
Tertiary consumers feed on secondary consumers and represent a third level of consumption in the food chain
Not all ecosystems have the same number of levels — some may have more trophic levels than others, whilst some may have fewer

Energy Loss Between Trophic Levels

As energy passes up the food chain, most of it is lost at each level — for example, 1,000
J in grass may become only around 100 J in mice and around 20 J in owls
Energy is lost between trophic levels through processes such as heat released during respiration, movement, and waste products like urine and faeces
Because so much energy is lost at each level, food chains rarely have more than four or five trophic levels

4. Energy Loss Between Trophic LevelsAQA

Notes

As energy passes up the food chain, most of it is lost at each level — for example, 1,000
J in grass may become only around 100 J in mice and around 20 J in owls
Energy is lost between trophic levels through processes such as heat released during respiration, movement, and waste products like urine and faeces
Because so much energy is lost at each level, food chains rarely have more than four or five trophic levels

Introduction to Predator-Prey Cycles

A predator-prey cycle is a graph showing how the populations of a predator species and a prey species change together over time
The populations of both species cycle up and down, but the predator population always lags slightly behind the prey population
Because the predator and prey populations do not peak at exactly the same time, we say they are out of phase with each other

5. Introduction to Predator-Prey CyclesAQA

Notes

A predator-prey cycle is a graph showing how the populations of a predator species and a prey species change together over time
The populations of both species cycle up and down, but the predator population always lags slightly behind the prey population
Because the predator and prey populations do not peak at exactly the same time, we say they are out of phase with each other

Explaining the Predator-Prey Cycle

When the predator population is low, the prey population is free to increase as fewer individuals are being eaten
As prey numbers rise, there is more food available for predators, so the predator population begins to increase
When the predator population becomes very high, prey are eaten rapidly, causing the prey population to decline
As prey numbers fall, food becomes scarce for predators, so the predator population also begins to decline, allowing the prey population to recover and the cycle to repeat

6. Explaining the Predator-Prey CycleAQA

Notes

When the predator population is low, the prey population is free to increase as fewer individuals are being eaten
As prey numbers rise, there is more food available for predators, so the predator population begins to increase
When the predator population becomes very high, prey are eaten rapidly, causing the prey population to decline
As prey numbers fall, food becomes scarce for predators, so the predator population also begins to decline, allowing the prey population to recover and the cycle to repeat

Why a Cycle Forms Rather Than Equilibrium

The cycle occurs rather than a steady equilibrium because it takes a long time for entire populations to increase or decrease
Even when food is plentiful, predator populations can only grow gradually over multiple generations of breeding, creating the characteristic lag seen in the cycle

7. Why a Cycle Forms Rather Than EquilibriumAQA

Notes

The cycle occurs rather than a steady equilibrium because it takes a long time for entire populations to increase or decrease
Even when food is plentiful, predator populations can only grow gradually over multiple generations of breeding, creating the characteristic lag seen in the cycle