1. POPULATION GROWTH

2. What is a population? A group of organism of the same species
living in the same habitat
at the same time
where they can freely interbreed
© 2010 Paul Billiet ODWS

3. How can populations change?
Natality
Mortality
Immigration
Emigration
© 2010 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
© 2010 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
© 2010 Paul Billiet ODWS

6. Immigration It increase population growth
It operates when populations are not completely isolated
© 2010 Paul Billiet ODWS

7. Emigration It decrease population growth
It operates when populations are not completely isolated
© 2010 Paul Billiet ODWS

8. (Natality + Immigration) - (Mortality + Emigration)
Interactions
Population growth =
(Natality + Immigration) - (Mortality + Emigration)
© 2010 Paul Billiet ODWS

9. Population growth
Numbers
Time K 3 2 1
© 2010 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
© 2010 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
Added limiting factors will lower K
Removing a limiting factor will raise K
© 2010 Paul Billiet ODWS

12. Factors affecting the carrying capacity
Food supply
Infectious disease/parasites
Competition
Predation
Nesting sites
© 2010 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
© 2010 Paul Billiet ODWS

14. Exponential growth
Numbers
Time
© 2010 Paul Billiet ODWS

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
© 2010 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
© 2010 Paul Billiet ODWS

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
© 2010 Paul Billiet ODWS

18. © 2010 Paul Billiet ODWS

19. European starling (Sturnus vulgaris)
Between 1890 and 1891, 160 of these birds were released in Central Park New York.
By 1942 they had spread as far as California.
An estimate population of between 140 and 200 million starlings now exist in North America
One of the commonest species of bird on Earth
Image Credit:
© 2010 Paul Billiet ODWS

20. European starling (Sturnus vulgaris)
CJKrebs (1978) Ecology
Current distribution

21. The Colorado Beetle (Leptinotarsa decemlineata)
A potato pest from North America
It spread quickly through Europe
© P Billiet
© 2010 Paul Billiet ODWS

22. The Colorado Beetle (Leptinotarsa decemlineata)
Begon, Townsend & Harper (1990) Ecology

23. r-strategists boom and bust!
Maximum reproductive potential when the opportunity arrives
Periodic population explosions
Pests and pathogens (disease causing organisms) are often r-species
© 2010 Paul Billiet ODWS

24. 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
© 2010 Paul Billiet ODWS

25. K
Numbers
Time
© 2010 Paul Billiet ODWS

26. 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
© 2010 Paul Billiet ODWS

27. K-strategists long term investment
These species are good competitors
They are adapted to environments where all the niches are filled
They have long life spans
Lower reproductive rates but …
High degree of parental care thus …
Low infant mortality
K-strategist flowering plants produce fewer seeds with a large amount of food reserve
© 2010 Paul Billiet ODWS