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Populations and
Investigating the concept of a niche
In this worksheet, you will investigate the idea of a niche and look at the concepts of a
fundamental niche and a realised niche. You will also be asked to use information on a
particular species’ niche, to predict where we may find this organism, and how its population
may change.
Fundamental and realised niches
The concept of a niche is difficult but very important. It plays a large role in our
understanding of why a particular population lives in a particular habitat. It can also help to
explain why some species thrive in the presence of others species. Finally, it is also of
fundamental importance when studying interspecific competition (competition between
organisms of different species).
No two organisms can share an identical niche. This has been described by the Competitive
exclusion principle. It states:
 If two competing species coexist they have differentiated niches.
 If there is no difference between the niches of competing organisms then one species will be
eliminated or excluded.
The Competitive exclusion principle often leads to assumptions being made about observed
communities. When species with similar niches are observed coexisting, it is often assumed
that the species are competing but are able to coexist due to niche differentiation. As all
species must have unique niches, this is not necessarily correct. Often species may have
competed in the past, but again this is not a certainty.
Ecologists often talk about two different types of niche – a fundamental niche and a realised
niche. Both types of niche are referring to how an organism fits into the environment, where
it lives and the abiotic and biotic factors that may affect it. However, a fundamental niche is
normally much wider than a realised niche. An organism has the potential to survive
anywhere within the limits of its niche. In reality, this is not often the case due to factors such
as competition. The whole of the potential niche is called the fundamental niche and the
actual niche occupied by the organism is the realised niche.
All environments contain a set of resources that are available to the communities that live
within it, for example, nutrients, water, etc. The range of resources that are used represents
the niche breadths of the species. The more of the resources that are used up, the more
saturated the environment is said to be. The number of species that the environment can
support can be affected by the following issues – a species niche breadth and the amount of
overlap between niches. Any overlap could cause competition and affect an organism’s
realised niche.
During the study of environmental quality, scientists may use the concept of a fundamental
niche. This allows the comparison of an expected value for a population, if the area was
unpolluted, with the value that is actually observed. The expected value is generated by
describing abiotic factors, such as air temperature or soil pH etc., that make up an organism’s
niche and predicting the size of population the area in question can contain for an organism
found there. The environmental quality index (EQI) is worked out in the following way:
observed population
predicted population
AQA Biology A2 Stretch and challenge © Nelson Thornes Ltd 2009
Populations and
1 What is the difference between a habitat and a niche?
2 Explain the difference between a fundamental niche and a realised niche.
3 Explain why a realised niche may have much stricter criteria than a realised niche.
4 Describe and explain (using an example if possible) how two similar organisms can live in the same
habitat but occupy different niches.
5 Each species will have its own specific niche and no two organisms within a habitat will have
identical niches. Suggest why.
6 Demonstrate how organisms with similar niches can avoid conflict by altering their own niches.
7 The equation for working out the environmental quality index is likely to be inaccurate. Explain
your answer.
8 Where two species occupy a similar niche, to what extent is there competition between them?
9 Design a simple experiment that could show whether two organisms with similar niches compete
with each other.
10 Suggest why some habitats contain more species than others. Use your knowledge of the
requirements for a specific species’ niche, and the issues caused by niche overlap and competition,
to help with your answer.
AQA Biology A2 Stretch and challenge © Nelson Thornes Ltd 2009