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7.1 An introduction to ecosystems
Ecosystems, communities, populations and habitats
An ecosystem is a biological environment comprising all of the living and non-living things which exist
there, and the interrelationships between them. Ecosystems come in a variety of shapes and sizes: from
the large savannah grassland, to smaller ecosystems such as a pond or lake.
There are a number of components to one ecosystem,
the habitat ­ a place where organisms live
populations ­ all of the organisms from within the
same species who live within the same habitat at
the same time, and can interbreed
communities ­ all of the populations of different
species which interact with each other
Each species in an ecosystem is considered to play a role within that ecosystem, and this is called its niche. Because each
species interacts with both many living (biotic) and non-living (abiotic) things, it is very difficult to identify exactly what
the niche of a species is on many occasions. A description of the niche of a species might include information about what
it eats and how it feeds, what it excretes and how it reproduces. It is impossible for two different species to occupy the
exact same niche within an ecosystem.
The living organisms in an ecosystem can affect each other: with biotic factors such as food supply, predation and disease.
There are also the abiotic factors, which are the effects of non-living components within the ecosystem: such as pH,
temperature and soil type.
Ecosystems do not have clear edges ­ it is impossible to draw a clear line around a group of living things and say that they
interact only with each other, rather than other organisms outside that ecosystem. However, thinking of ecosystems in a
`closed' sense makes them easier to comprehend.
Dynamic ecosystems
In most ecosystems, population sizes rise and fall, either very slightly or very noticeably. This is because the community of
living things in an ecosystem interact with each other and with their physical environment. Any small changes in one can
affect the other. Such relationships are discussed in more detail in the chapter 7.5 Populations.
Energy and ecosystems
Matter is constantly recycled within an ecosystem ­ nutrient cycles, such as the nitrogen cycle and carbon cycle, are good
examples. Energy is not recycled ­ it flows through the ecosystem:
light energy biotic component heat energy
photosynthesis respiration
abiotic component
All living organisms need energy. Via respiration, they release energy from organic molecules, such as glucose, in their
food. This energy originally came from sunlight. At the start of nearly all food chains is a plant, which captures light energy
through photosynthesis and converts it to chemical energy stored in molecules like glucose.
Because plants and other photosynthetic organisms, such as algae and some bacteria, supply chemical energy to all
other organisms, they are known as producers
Other organisms, like animals and fungi are called consumers
Some living things called decomposers (bacteria, fungi and some animals) feed on waste material or dead organisms


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