Abi Starr
Unit 4- Chapter 1: Populations
1.1: Populations and ecosystems
Ecology is the study of the relationships that exist between organisms and their environment. The
environment includes both non-living (abiotic) components such as rainfall and temperature, as well
as biotic (living) components such as competition and predation. Ecology is a complex area of study
which involves most aspects of biology. Essentially it is the study of the layer of land, air and water
that surrounds the earth, this layer is known as the biosphere.
Ecosystems
An ecosystem consists of all the interactive biotic and abiotic features in a specific area. Ecosystems
are more or less self-contained functional units. Within any ecosystem there are two important
processes to consider:
The flow of energy through the system
The cycling of the elements within the system
An example of an ecosystem is a freshwater pond or lake. It has its own community of plants to
absorb the light to supply the other organisms within the pond. Within a pond or a lake, nutrients
such as nitrates and phosphates are recycled. This means there is little or no loss or gain between
one ecosystem and another. Another example of an ecosystem is an oak woodland. Within each
ecosystem there are a number of species. Each species is made up of many groups of individuals,
which together make up the population.
Populations
A population is a group of interbreeding individuals of one species in a particular habitat. In the
various habitats of an oak woodland, there are populations of many species woodpeckers, worms,
beetles etc. It is often hard to define the boundaries of a population. In the woodland, for example,
all of the mature woodpeckers are able to breed with each other, thus making a single population.
However, the woodlice living in a decaying log at one end of the forest, can in theory breed with
woodlice living a kilometre away at the opposite end of the forest. In practice, the sheer distance
between the two makes interbreeding unlikely and therefore the woodlice living at each end of the
forest can be deemed as separate populations. Where exactly the boundary lies between the two
populations of woodlice, however, is unclear. Populations of different species form a community.
Communities
A community is defined as the populations of all species interacting in a particular place at the same
time. For example, within the oak woodland, a community may include a large range of organisms,
such as oak trees, hazel shrubs, bluebells, sparrowhawks, blue tits, ladybirds, aphids, woodlice, fungi
and bacteria.
Habitat
A habitat is the place in which a community of organisms lives. Within one ecosystem there are
many habitats. In the oak woodland, the leaf canopy may be the habitat for the blue tits whilst a
decaying log is the ideal habitat for the wildlife. A stream flowing through the forest provides a
different habitat altogether, in which aquatic plants and water beetles can live. For a water vole, the
stream and the surrounding banks are its habitat. Within each habitat there are smaller units, each
with their own microclimate. These are called microhabitats. The mud on the bed of the stream may
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, Abi Starr
be the microhabitat for a bloodworm, while a crevice in the bark of one of the oak trees could be the
microhabitat for a lichen.
Ecological niche
A niche describes how one organism or species fits into the environment. A niche refers to not only
where an organism lives, but also what it does there. It includes all the biotic and abiotic conditions
required for an organism to survive, reproduce and maintain a viable population. Some species might
appear very similar, but their nesting habits, or other components of their behaviour will be
different, or they may have difference levels of tolerance to environmental factors, such as a
pollutant or a shortage of oxygen or nitrates. No two species occupy exactly the same niche.
1.2: Investigating populations
To study a habitat, often it is necessary to count the number of individuals of a particular species in a
given space. This is known as abundance. It is virtually impossible to count and identify every
organism. Doing this would be incredibly time consuming and would almost always inflict damage on
the habitat being studied. For this reason, usually only small samples of the habitat are studied in
detail. As long as the samples used are representative of the habitat as a whole, any conclusions
drawn from the findings will be valid. There are a number of sampling techniques that could be used
when studying habitats. These include:
Random sampling using frame quadrats or point quadrats
Systematic sampling along transects
Quadrats
There are three factors to consider when using quadrats:
The size of the quadrat being used – This will depend of the size of the plants or animals
being counted and how they are distributed within the area. Larger species will need larger
quadrats. Where a species occurs in groups rather than being equally spread throughout the
area, a large number of smaller quadrats will give more representative results than just a few
large quadrats.
The number of sample quadrats to record within the area – The larger the number of
sample quadrats used, the more reliable the results will be. As it is fairly time consuming
recording the species within a quadrat, a balance must be struck between the validity of the
results and the time available to collect them in. The more different species present in the
study area, the greater the number of quadrats required to make the results valid.
The position of each quadrat within the study area- To produce statistically significant
results, a technique known as random sampling must be used
Random sampling
It is important that any sampling done is random to avoid any bias in collecting the data. Avoiding
bias ensures that the data obtained are valid. How do we make sampling random?
Imagine we want to investigate the effects of grazing animals on the species of plants growing in a
field. We start by selecting two fields as close to each other as possible in order to minimise changes
in soil, climatic differences and other abiotic factors. One field is regularly grazed by sheep, whereas
the other has not been grazed for some years. We then take random samples at many sites in each
field by placing the quadrat on the ground and recording the names and numbers of every species
within the area of the quadrat.
Page 2 of 11
Unit 4- Chapter 1: Populations
1.1: Populations and ecosystems
Ecology is the study of the relationships that exist between organisms and their environment. The
environment includes both non-living (abiotic) components such as rainfall and temperature, as well
as biotic (living) components such as competition and predation. Ecology is a complex area of study
which involves most aspects of biology. Essentially it is the study of the layer of land, air and water
that surrounds the earth, this layer is known as the biosphere.
Ecosystems
An ecosystem consists of all the interactive biotic and abiotic features in a specific area. Ecosystems
are more or less self-contained functional units. Within any ecosystem there are two important
processes to consider:
The flow of energy through the system
The cycling of the elements within the system
An example of an ecosystem is a freshwater pond or lake. It has its own community of plants to
absorb the light to supply the other organisms within the pond. Within a pond or a lake, nutrients
such as nitrates and phosphates are recycled. This means there is little or no loss or gain between
one ecosystem and another. Another example of an ecosystem is an oak woodland. Within each
ecosystem there are a number of species. Each species is made up of many groups of individuals,
which together make up the population.
Populations
A population is a group of interbreeding individuals of one species in a particular habitat. In the
various habitats of an oak woodland, there are populations of many species woodpeckers, worms,
beetles etc. It is often hard to define the boundaries of a population. In the woodland, for example,
all of the mature woodpeckers are able to breed with each other, thus making a single population.
However, the woodlice living in a decaying log at one end of the forest, can in theory breed with
woodlice living a kilometre away at the opposite end of the forest. In practice, the sheer distance
between the two makes interbreeding unlikely and therefore the woodlice living at each end of the
forest can be deemed as separate populations. Where exactly the boundary lies between the two
populations of woodlice, however, is unclear. Populations of different species form a community.
Communities
A community is defined as the populations of all species interacting in a particular place at the same
time. For example, within the oak woodland, a community may include a large range of organisms,
such as oak trees, hazel shrubs, bluebells, sparrowhawks, blue tits, ladybirds, aphids, woodlice, fungi
and bacteria.
Habitat
A habitat is the place in which a community of organisms lives. Within one ecosystem there are
many habitats. In the oak woodland, the leaf canopy may be the habitat for the blue tits whilst a
decaying log is the ideal habitat for the wildlife. A stream flowing through the forest provides a
different habitat altogether, in which aquatic plants and water beetles can live. For a water vole, the
stream and the surrounding banks are its habitat. Within each habitat there are smaller units, each
with their own microclimate. These are called microhabitats. The mud on the bed of the stream may
Page 1 of 11
, Abi Starr
be the microhabitat for a bloodworm, while a crevice in the bark of one of the oak trees could be the
microhabitat for a lichen.
Ecological niche
A niche describes how one organism or species fits into the environment. A niche refers to not only
where an organism lives, but also what it does there. It includes all the biotic and abiotic conditions
required for an organism to survive, reproduce and maintain a viable population. Some species might
appear very similar, but their nesting habits, or other components of their behaviour will be
different, or they may have difference levels of tolerance to environmental factors, such as a
pollutant or a shortage of oxygen or nitrates. No two species occupy exactly the same niche.
1.2: Investigating populations
To study a habitat, often it is necessary to count the number of individuals of a particular species in a
given space. This is known as abundance. It is virtually impossible to count and identify every
organism. Doing this would be incredibly time consuming and would almost always inflict damage on
the habitat being studied. For this reason, usually only small samples of the habitat are studied in
detail. As long as the samples used are representative of the habitat as a whole, any conclusions
drawn from the findings will be valid. There are a number of sampling techniques that could be used
when studying habitats. These include:
Random sampling using frame quadrats or point quadrats
Systematic sampling along transects
Quadrats
There are three factors to consider when using quadrats:
The size of the quadrat being used – This will depend of the size of the plants or animals
being counted and how they are distributed within the area. Larger species will need larger
quadrats. Where a species occurs in groups rather than being equally spread throughout the
area, a large number of smaller quadrats will give more representative results than just a few
large quadrats.
The number of sample quadrats to record within the area – The larger the number of
sample quadrats used, the more reliable the results will be. As it is fairly time consuming
recording the species within a quadrat, a balance must be struck between the validity of the
results and the time available to collect them in. The more different species present in the
study area, the greater the number of quadrats required to make the results valid.
The position of each quadrat within the study area- To produce statistically significant
results, a technique known as random sampling must be used
Random sampling
It is important that any sampling done is random to avoid any bias in collecting the data. Avoiding
bias ensures that the data obtained are valid. How do we make sampling random?
Imagine we want to investigate the effects of grazing animals on the species of plants growing in a
field. We start by selecting two fields as close to each other as possible in order to minimise changes
in soil, climatic differences and other abiotic factors. One field is regularly grazed by sheep, whereas
the other has not been grazed for some years. We then take random samples at many sites in each
field by placing the quadrat on the ground and recording the names and numbers of every species
within the area of the quadrat.
Page 2 of 11
