1. The Biology of Populations

2. Population Characteristics
Population Density:
The number of organisms per unit area of land or volume of water
Spatial Distribution:
Dispersion: The pattern of spacing of a population within an area
3 main types of dispersion:
Clumped
Uniform
Random
The primary cause of dispersion is resource availability

3. Population Characteristics
Population growth rate
How fast a given population grows
Factors that determine this are:
Natality (birth rate)
Mortality (death rate)
Emigration (the number of individuals moving away from a population)
Immigration (the number of individuals moving into a population)

4. Population Characteristics
Actual population growth is
a simple formula:
NATALITY + IMMIGRATION = MORTALITY + EMIGRATION

5. Understanding Exponentials
Put your pens down for a minute and think about this:
An employer offers you two equal jobs for one hour each day for fourteen days.
The first pays $10 an hour.
The second pays only 1 cent the first day, but the rate doubles each day.
Which job will you accept?

6. Understanding Exponentials 1->2->4->8->16
Now, how much would your employer owe you if you stayed at this job for another 2 weeks?
Job 2 lags for a long time before exponential growth kicks in!
What would happen if this type of growth took place within a population?

7. Population Growth Population growth models Exponential growth model
First growth phase is slow and called the lag phase
Second growth phase is rapid and called the exponential growth phase
Unchecked, this produces a J curve
Bacteria can grow at this rate, so why aren’t we up to our ears in bacterial cells?

8. Population Growth
What population do
you think this is?

9. Population Growth Why do you think this is happening? Be a
Elephants in Kruger National Park, SA
Why do you think this
is happening? Be a
conservation manager!

10. Population Growth with Limiting Factors
Population growth models
But usually there are limits to exponential growth
Population Density – as the absolute number of individuals increases, so does the number of individuals per unit area
Competition follows as nutrients and resources are used up, so population growth slows – this is the third or transitional phase
Eventually the population size stabilises – the plateau phase
This produces a sigmoid (S) curve for population growth
The limit to population size that a particular environment can support is called carrying capacity (k)

11. Population Growth with Limiting Factors
Population growth models
Logistic Growth Model
Often called the S-shaped or SIGMOID growth curve
Occurs when a population’s growth slows or stops following exponential growth.
Growth stops at the population’s carrying capacity
Populations stop increasing when:
Birth rate is less than death rate (Birth rate < Death rate)
Emigration exceeds Immigration (Emigration > Immigration)

12. Population Growth with Limiting Factors
The Sigmoid (S) Curve of Population growth

13. Population Growth Limiting Factors
Density-independent factors
Factors that limit population size, regardless of population density
These are usually abiotic factors
They include natural phenomena, such as weather events
Drought, floods, extreme heat or cold, tornadoes, hurricanes, fires, etc.

14. Population Growth Limiting Factors
Density-dependent factors
Any factor in the environment that depends on the size of a population per unit area
Usually biotic factors
These include:
Predation
Disease
Parasites
Competition

15. Population Growth Limiting Factors

16. Population Growth Population growth models
Populations don’t reach k as smoothly as in the logistic graph.
Boom-and-Bust Cycles
Predator-Prey Cycles

17. Population Growth Species that follow this pattern usually
Have a Boom-and-Bust Life Cycle
Have short life spans
Produce many offspring
Smaller organisms
i.e., fruit flies, mice, locusts
Don’t maintain a population near carrying capacity
Controlled by density-independent factors
They are called r-strategists (for rate of increase)

18. Population Growth Species that follow this pattern usually
Have long life spans
Produce few offspring that have a better chance of living to a reproductive age
Are larger organisms
i.e., elephants
Maintain a population at or near k
Controlled by density-dependent factors
They are called k-strategists (for karrying kapacity)

19. Virtual Population Lab
Now try this virtual lab. It adds the idea of competition to the population growth of two species. Virtual Population Lab

20. Reading
And finally get into your text book (Biology for the IB Diploma, C.J. Clegg, Hodder Education, 2007), pages