1. POPULATION GROWTH
© 2016 Paul Billiet ODWS

2. What is a population? A group of organisms of the same species
living in the same habitat
at the same time
where they can freely interbreed.
© 2016 Paul Billiet ODWS

3. How can populations change?
Natality
Mortality
Immigration
Emigration
© 2016 Paul Billiet ODWS

4. Natality Increases population size
Each species will have its own maximum birth rate
Maximum birth rates are seen when conditions are ideal
This can lead to exponential growth.
© 2016 Paul Billiet ODWS

5. Mortality Mortality reduces population growth
It operates more when conditions are not ideal
Overcrowding leading to competition, spread of infectious disease.
© 2016 Paul Billiet ODWS

6. Immigration It increase population growth
It operates when populations are not completely isolated.
© 2016 Paul Billiet ODWS

7. Emigration It decrease population growth
It operates when populations are not completely isolated.
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8. Interactions
Population growth = (Natality + Immigration) - (Mortality + Emigration)
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9. Population growth
Numbers
Time K 3 2 1
© 2016 Paul Billiet ODWS

10. Phases of population growth
Phase 1: Log or exponential phase
Unlimited population growth
The intrinsic rate of increase (r)
Abundant food, no disease, no predators etc
Phase 2: Decline or transitional phase
Limiting factors slowing population growth.
© 2016 Paul Billiet ODWS

11. Phase 3 Plateau or stationary phase No growth
The limiting factors balance the population’s capacity to increase
The population reaches the Carrying Capacity (K) of the environment
Limiting factors will raise or lower K
Limiting factors can be positive or negative.
© 2016 Paul Billiet ODWS

12. Factors affecting the carrying capacity
Positive
Food supply
Nesting sites
Negative
Infectious disease/parasites
Competition
Predation.
© 2016 Paul Billiet ODWS

13. Modelling population growth, the maths
Population growth follows the numbers of individuals in a population through time. The models try to trace what will happen little by little as time passes by
A small change in time is given by ∆t This is usually reduced to dt
Time may be measured in regular units such as years or even days or it may be measured in units such as generations
A small change in numbers is given by ∆N This is usually reduced to dN
A change in numbers as time passes by is given by: dN/dt
© 2016 Paul Billiet ODWS

14. Exponential growth
Numbers
Time
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15. Exponential growth The J-shaped curve
This is an example of positive feedback
1 pair of elephants could produce 19 million elephants in 700 years.
© 2016 Paul Billiet ODWS

16. Modelling the curve dN/dt= rN r is the intrinsic rate of increase
Example if a population increases by 4% per year
dN/dt= 0.04N
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17. Real examples of exponential growth
Pest species show exponential growth humans provide them with a perfect environment
Alien species When a new species is introduced accidentally or deliberately into a new environment It has no natural predators or diseases to keep it under control.
© 2016 Paul Billiet ODWS

18. The Carrying Capacity
Darwin observed that a population never continues to grow exponentially for ever
There is a resistance from the environment
The food supply nesting sites decrease
Competition increases
Predators and pathogens increase
This resistance results from negative feedback.
© 2016 Paul Billiet ODWS

19. K
Numbers
Time
© 2016 Paul Billiet ODWS

20. The Carrying Capacity This too can be modelled
It needs a component in it that will slow down the population growth as it reaches a certain point, the carrying capacity of the environment (K)
The equation is called the logistic equation
dN/dt = rN[(K-N)/N]
When N<K then dN/dt will be positive the population will increase in size
When N=K then dN/dt will be zero the population growth will stop
Should N>K then dN/dt will become negative the population will decrease.
© 2016 Paul Billiet ODWS