1

Higher Biology

Regulation of Populations

2

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.

3

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.

4

Population Size

The carrying capacity is the maximum size of population that the environmental resources can maintain.

Dynamic Equilibrium

5

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.

6

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

7

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

8

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.

9

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.

10

Predation/Parasitism

A more densely packed population is goingto be more severely affected than

a less densely packed one.

11

Predator Prey Interactions

12

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

13

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.

14

Density-independent factors These affect the population

regardless of it’s density (or size).

Examples:

Extremes oftemperature

Excessiverainfall

Drought

Forest FiresFloods

15

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.

16

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.

17

Homeostatic control of population size

Optimum populationsize

Increased effectof density

dependent factorsPopulation

increase

Decreased effectof density

dependent factors

Population

decrease

Population

increase

Population

decrease

18

Practice Questions

Torrance1. TYK pages 297 Q1&32. AYK pages 298-291 Q2, 3a+b, 5 and 7

19

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.