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AQA Biology Unit 4

Populations and ecosystems
Ecosystem: The interacting biotic and abiotic features in a specific area.

Within each ecosystem there are a number of species. Each species is made up of individuals
who make up a population.

Population: A group of interbreeding organisms of one species in…

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AQA Biology Unit 4

This technique relies on a number of assumptions:
The marked individuals distribute themselves evenly amongst the population
There is no change in the population size
The marking does not harm the individual
The marking does not rub/fall off during the investigation

Variation in population size

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AQA Biology Unit 4

Where two species occupy the same niche, one will almost always have a competitive
advantage over the other. This competition will lead to the complete removal of one

Predation occurs when one organism is consumed by another.

Effect of predator-prey relationship on population size…

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AQA Biology Unit 4

Natural disasters

Population structure
Demographic transition: a society that has gone from a short life expectancy and high
birth rates to long life expectancy and low birth rate.

This leads to a levelling off of the population.

Population pyramids
Age population pyramids give useful information…


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