1 higher biology regulation of populations. 2 by the end of this lesson you should be able to: ...
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Higher Biology
Regulation of Populations
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Regulation of Populations
By the end of this lesson you should be able to:
Explain the term population fluctuations. Understand why populations remain relatively stable
in size. Explain what is meant by density-dependent factors. Give examples of density-dependent factors. Explain what is meant by density-independent factors. Give examples of density-independent factors.
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A bit of revision:
1. What is a population?2. Name 4 factors that can affect the
size of a population.3. For a population to be stable, what
would be the connection between the birth rate and death rate?
4. Draw a sketch graph to show the growth of a small population introduced into a new favourable area.
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Population Size
The carrying capacity is the maximum size of population that the environmental resources can maintain.
Dynamic Equilibrium
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Population Fluctuations
If a population is introduced into a new area:
1. The population grows slowly2. The population grows more quickly3. The population levels off.4. Then the population remains fairly
steady although the numbers fluctuate both above and slightly below that value.
i.e. In a state of dynamic equilibrium.
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Factors influencing population change
Population density is the number of individuals of the same species in a given area.
There are two types of factors that can maintain a population’s size:
Density-dependentfactors
Density-independentfactors
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Density-dependent factors These only affect the
population when it reaches a certain density (or size).
Then their effect increases as the population density increases.
Examples:Disease
Predation
Food supply
Competition
Toxic waste
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Disease
Diseases are caused by pathogenic micro-organisms (e.g. bacteria, viruses, fungi)
Diseases spread by air, water or contact.
A more densely packed population is goingto be more severely affected than
a less densely packed one.
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Food supply In the absence of disease or
predators, a population will continue to grow until the food availability becomes a limiting factor.
As food becomes more scarce the individual’s rate of reproduction decreases, and the death rate increases (from starvation).A more densely packed population is goingto be more severely affected than
a less densely packed one.
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Predation/Parasitism
A more densely packed population is goingto be more severely affected than
a less densely packed one.
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Predator Prey Interactions
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Predator/Prey interactions
There is a delicate balance between populations of predators and prey.
Put the following stages into the correct order, starting with:
Prey populationincreases Prey population
decreases
Predator populationincreasesMore foodLess food
Predator populationdecreases
More food Predator populationincreases
Less food
Prey populationdecreases
Predator populationdecreases
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Competition As a population’s density increases,
environmental resources eventually become limiting.
A more densely packed population is goingto be more severely affected than
a less densely packed one.
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Density-independent factors These affect the population
regardless of it’s density (or size).
Examples:
Extremes oftemperature
Excessiverainfall
Drought
Forest FiresFloods
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Density-independent factors Tend to cause a sudden drastic
reduction in the population size. However, given enough time, the
population normally returns to it’s original size.
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Homeostatic control of population size The size of a population is regulated by
homeostasis in the external environment and involves negative feedback control.
This means that the population is maintained at a relatively stable equilibrium, enabling it to make maximum use of available resources.
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Homeostatic control of population size
Optimum populationsize
Increased effectof density
dependent factorsPopulation
increase
Decreased effectof density
dependent factors
Population
decrease
Population
increase
Population
decrease
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Practice Questions
Torrance1. TYK pages 297 Q1&32. AYK pages 298-291 Q2, 3a+b, 5 and 7
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Regulation of Populations
Can you do it?
Explain the term population fluctuations. Understand why populations remain relatively stable
in size. Explain what is meant by density-independent factors. Give examples of density-independent factors. Explain what is meant by density-dependent factors. Give examples of density-dependent factors.