AQA Biology Unit 4 Populations

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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 a habitat.
Populations of different species form a community.
Community: All the populations of different organisms living and interacting in a particular
place at the same time.
Habitat: The place where a community of organisms lives.
Niche: The role of an organism within its ecosystem that allows that ecosystem to survive
and function efficiently.
No two species occupy the same niche.
Investigating populations
There are three factors to consider when using quadrats.
The size of the quadrat: large number of small quadrats more representative of the
The number of sample quadrats to use: the larger the number of quadrats the more
reliable the results.
The position of each quadrat: random sampling.
Random sampling
Split the area into a grid with co-ordinates.
Use a random number generator to produce co-ordinates
Place the quadrat at the intersection of each co-ordinate and record the species
within it.
Random sampling with quadrats is used to obtain measurements of abundance.
Abundance: Number of individuals of a species within a given space.
Mark ­release-recapture
It is difficult to use quadrats to measure the population size of moving animals.
Catch a known number of animals, mark them in some way and then release them
back into the community.
After a suitable amount of time, a known number of the animal are caught and the
number of marked individuals is recorded.

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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
Population growth curves
The usual pattern of growth for a natural population has three phases
A period of slow growth: Initially small numbers of individuals reproduce to build up
numbers.…read more

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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
Predators eat their prey, reducing the prey population
Fewer prey so greater competition between predators.
Predator population reduced as some individuals cannot obtain enough prey for their
Fewer predators so fewer prey are eaten.…read more

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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 on the future trends of populations.
Stable population: birth rate and death rate are in balance and so there is no increase
or decrease in the population size.…read more


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