1. Topic 5: Ecology & Evolution Miss Friedman
5.3: Populations
Topic 5: Ecology & Evolution
Miss Friedman

2. 5.3.1: Factors affecting population size
Four factors exist which influence the size of a population:
Natality (birth rate)
Mortality (death rate)
Immigration (moving in)
Emigration (moving out)

4. 5.3.2: Population growth curve
This is a graph known as a sigmoid (s-shaped) growth curve
It shows changes in population size against time
In this model, the population is expanding into a habitat offering initial low resistance or low limiting factors

5. 5.3.3: Phases of the population growth curve
Exponential Phase
Rapid population growth rate
b) Transition stage
Slowing yet still high growth rate
Plateau phase
Population remains stable with time

6. Exponential Phase Unlimited growth due to absence of limiting factors
Population may be increasing at 2n where n=number of generations
Rate of natality + immigration is greater than mortality + emigration. (Does NOT mean that death/emigration = 0)
This would be typical of a population of germinating annual plants in a new season or bacterial population during initial phases of infection

7. Transitional Phase
Limiting factors start influencing the rate of increase of the population
As population grows there will be increased competition between individuals of population for the same resources
Competition means that some individuals will obtain resources and survive and others will not
Overall the effect is to produce a lower rate of population increase

8. Notice the link between ecology and evolutionary theory!
Natural selection (survival of the fittest) is taking place!
There will not just be a reduced rate of population growth but also a selection (survival and reproduction) of these individuals within the population best suited to using and accessing the resources

9. Population Plateau Population remains constant over time/generations
Number of individuals of the population no longer increases, due to limiting factors
Natality + immigration = mortality + emigration
Population has reached carry capacity of the environment

10. Carry capacity
Refers to the maximum number of individuals of a species that can be sustainability supported by the environment

11. 5.3.4: Factors limiting population growth
Limiting factors
Those factors that control a process such as population growth

12. Examples of limiting factors for ANIMAL populations
Amount of food available
Presence of parasites and/or disease
Amount of predation
Available nesting sites
Examples of limiting factors for PLANT populations
Amount of light available
Temperature
Amount of carbon dioxide
Amount of water available