1 Higher Biology Regulation of Populations

2 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 slowly 2. The population grows more quickly 3. 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-dependent factors Density-independent factors

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 going to 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 going to be more severely affected than a less densely packed one.

10 Predation/Parasitism A more densely packed population is going to 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 population increases Prey population decreases Predator population increases More foodLess food Predator population decreases More food Predator population increases Less food Prey population decreases Predator population decreases

13 Competition  As a population’s density increases, environmental resources eventually become limiting. A more densely packed population is going to 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 of temperature Excessive rainfall Drought Forest Fires Floods

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 population size Increased effect of density dependent factors Population increase Decreased effect of density dependent factors Population decrease Population increase Population decrease

18 Practice Questions  Torrance 1. TYK pages 297 Q1&3 2. AYK pages 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.