AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Populations in ecosystems (A-level only): Ecosystems are dynamic systems.
Dynamic means constantly changing rather than fixed.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 7 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Dynamic means constantly changing rather than fixed. Populations within an ecosystem rise and fall around their carrying capacities as abiotic factors change with the season and as competition and predation vary. Energy enters continuously as light, is fixed by producers, is transferred along food chains and is lost as heat at every transfer, so the system can never be static and always needs a fresh input. Nutrients such as carbon and nitrogen are cycled repeatedly between the abiotic reservoir, producers, consumers and saprobionts. Over longer timescales the community itself changes through succession, so even the species list is not permanent. Most of these changes are held within limits by negative feedback: a rise in prey supports more predators, and the predators push the prey back down.
What earns the marks
- Dynamic meaning constantly changing, with populations fluctuating rather than remaining constant.
- Energy flowing through the ecosystem and being lost as heat, so a continuous input of light energy is needed.
- Nutrients being cycled between organisms and the abiotic environment.
- A named mechanism of change, such as seasonal variation in an abiotic factor, predator-prey cycling or succession.
- Negative feedback holding a population close to its carrying capacity.
Common mistakes and how to put them right
Reading dynamic as unstable, and so claiming that populations in a stable ecosystem do not change at all.
Stating that energy is recycled, when energy is transferred and lost as heat while nutrients are recycled.
Describing a fall in one population without following the effect through to the species that depend on it.
Treating succession as the only way an ecosystem can change.
Describing an equilibrium as a fixed population size rather than a mean about which numbers fluctuate.
Exam tips
- Use fluctuates around the carrying capacity rather than stays the same when describing a stable population.
- When a question introduces a change, such as adding a predator or warming a river, trace it through at least two further populations.
- Quote figures from the graph when describing the size of a fluctuation.
Try the lesson
The start of the Populations in ecosystems (A-level only): Ecosystems are dynamic systems. lesson, made from checked teaching notes for this specification point.
Have a go first
What is meant by the statement that an ecosystem is a dynamic system?
Listen
Dynamic means constantly changing rather than fixed. Populations within an ecosystem rise and fall around their carrying capacities as abiotic factors change with the season and as competition and predation vary. Energy enters continuously as light, is fixed by producers, is transferred along food chains and is lost as heat at every transfer, so the system can never be static and always needs a fresh input. Nutrients such as carbon and nitrogen are cycled repeatedly between the abiotic reservoir, producers, consumers and saprobionts. Over longer timescales the community itself changes through succession, so even the species list is not permanent. Most of these changes are held within limits by negative feedback: a rise in prey supports more predators, and the predators push the prey back down. A strong answer earns marks for these points. dynamic meaning constantly changing, with populations fluctuating rather than remaining constant. energy flowing through the ecosystem and being lost as heat, so a continuous input of light energy is needed. nutrients being cycled between organisms and the abiotic environment. a named mechanism of change, such as seasonal variation in an abiotic factor, predator-prey cycling or succession. Exam tip: Use fluctuates around the carrying capacity rather than stays the same when describing a stable population.
Key words
Say what each word means before you turn the card.
dynamic equilibrium
Try answering aloud or in your head before turning the card.
energy flow
Try answering aloud or in your head before turning the card.
nutrient cycling
Try answering aloud or in your head before turning the card.
negative feedback
Try answering aloud or in your head before turning the card.
See every definition
- dynamic equilibrium
- A state in which populations fluctuate around their carrying capacities rather than remaining fixed, held within limits by negative feedback.
- energy flow
- Energy enters continuously as light, is fixed by producers, is transferred along food chains and is lost as heat at every transfer.
- nutrient cycling
- Nutrients such as carbon and nitrogen are cycled repeatedly between the abiotic reservoir, producers, consumers and saprobionts.
- negative feedback
- A mechanism that holds a population close to its carrying capacity, as when a rise in prey supports more predators that push the prey back down.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Populations in ecosystems (A-level only): Ecosystems are dynamic systems.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge