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Populations and the Environment
1.1 Populations and Ecosystems
Ecology is the study of the interrelationships between organisms and their
The environment includes abiotic components, such as temperature and
rainfall, and biotic components, such as competition and predation
Ecology is a complex area of study which includes most aspects of…

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A community is defined as all the populations of different organisms living and
interacting in a particular place at the same time
Within an oak woodland, a community may include a large range of organisms,
such as oak trees, hazel shrubs, bluebells, sparrowhawks, ladybirds, fungi and


A habitat…

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1.2 Investigating Populations
To study a habitat, you need to count the number of individuals of a species in
a given area
This is known as abundance
It is virtually impossible to identify and count every organism ­ it would be too
time consuming and cause damage to the habitat…

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Random Sampling

It's important sampling is random to avoid any bias in collecting data
Avoiding bias ensures data obtained is valid

E.g. investigating the effects of grazing animals on the species of plant
growing in a field
o Choose two fields as close together as possible ­ minimise soil, climatic…

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It can be measured in several ways, depending upon the size of the species
being counted and the habitat
Examples include:
o Frequency, which is the likelihood of a particular species occurring in a
quadrat. If a species occur in 15 out of 30 quadrats, the frequency of
its occurrence…

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o The marked individuals released from the first sample distribute
themselves evenly among the remainder of the population and have
sufficient time to do so
o The population has a definite boundary so that there is no immigration
into or emigration out of the population
o There are few, if…

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The numbers of 2 species in a population may correlate very well but it is
possible that both of them are affected by the same environmental factor,

e.g. temperature

1.3 Variation in Population Size
Population Growth Curves

The usual pattern of growth for a natural population has 3 phases

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each interval of time and the gradient of the curve becomes
3) A period when the population growth declines until its size remains
more or less stable. The decline may be due to the food supply
limiting numbers or to increased predation. The graph therefore
levels out with only…

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Abiotic Factors

o Each species has a different optimum temperature at which it is best
able to survive
o The further away from the optimum, the smaller the population that
can be supported
o In plants and coldblooded animals, as temperatures fall below the
optimum, the enzymes work more…

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1.4 Competition
Competition results where 2 or more individuals share any resource that is
insufficient to satisfy all their requirements fully
Intraspecific competition is where competition arises between members
of the same species
Interspecific competition is where competition arises between members
of different species

Intraspecific Competition

This occurs when members…


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