A2 AQA Unit 4 Populations and the Environment

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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 biology
It is, in effect, the study of the lifesupporting layer of land, air and water
that surrounds the Earth which is called the biosphere
An ecosystem is made up of all the interacting biotic and abiotic features in a
specific area
Ecosystems are more or less selfcontained functional units with 2 major
processes to consider
o The flow of energy through the system
o The cycling of elements within the system
An example of an ecosystem is a freshwater pond or lake
It has its own community of plants to collect the necessary sunlight energy to
supply the organisms within it
Nutrients such as nitrates and phosphates are recycled within the pond or lake
There is little or no loss or gain between it and other ecosystems
Within each ecosystem, there are a number of species
Each species is made up of many groups of individuals that together make up a
A population is a group of interbreeding organisms of one species in a habitat
In the different habitats of an oak woodland there are populations of nettles,
worms, woodpeckers etc.
The boundaries of a population are often difficult to define
In the woodland, all mature woodpeckers can breed with one another and so
frm one population.
However, the woodlice on a decaying log at one side of the wood can, in
theory, breed with those on a log a kilometre away or so
In practice, the distance makes interbreeding unlikely and therefore they can
be considered as separate populations
Where the boundary lies between these populations is unclear
Populations of different species form a community

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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 is the place where a community of organisms lives
Within an ecosystem there are many habitats
E.g.…read more

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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 being studied
Therefore, only small samples of the habitat are studied in detail ­ as long as
they are representative, the results and conclusions drawn should be valid
There are a number of sampling techniques used…read more

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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.…read more

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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 is 50%. This method is useful where a species, such as
grass, is hard to count. It gives a quick idea of the species present and
their general distribution within an area.…read more

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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 any deaths and births within the population
o The method of marking is not toxic to the individual nor does it make
the individual more conspicuous and therefore more liable to predation
o The mark or…read more

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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 cyclic fluctuations due to variations in factors
such as food supply or the population size of predators
Population Size
E.g. a single algal cell, capable of asexual reproduction, is placed in a newly
created pond.…read more

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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 slowly and so their metabolic rate is
reduced and populations therefore grow more slowly
o At temperatures above the optimum, the enzymes work less efficiently
because they gradually undergo denaturation and again the population
grows…read more

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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 of the same species compete for resources
(food, water)
It is the availability of such resources that determines the size of a population
The greater the availability, the larger the population
Examples of intraspecific competition include…read more


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