1. Why do populations vary in size?
Population growth
Limiting factors
Interspecific competition
Intraspecific competition
Predation

2. Population growth curves
Slow growth Rapid Stable state – no growth
Small fluctuations due to changes in factors such as food supply.

3. Limiting factors
The growth of a population is slowed by limiting factors.
Imagine a single celled Algae living in a new pond in summer, the algae divides rapidly at first as there are no limiting factors. What will slow the algal growth?
Lack of mineral ions
Algae at the surface prevent light reaching algae further down.
Other species – competition or predators
Winter – lower temperatures or light intensity

4. Limiting factors Abiotic
Temperature: each species has an optimum temperature.
Below, the metabolic rate drops.
Above, enzymes denature.
Even animals that can maintain a relatively constant temperature will be affected by temperature as they will expend more energy trying to maintain their normal body temperature. So they mature and reproduce slower.

5. Limiting factors Abiotic Light
The rate of photosynthesis increases as the light intensity does. Therefore more plants grow and more animals can be fed.
Water and humidity
Lack of water results in a small population of animals adapted to dry conditions.
Humidity affects plant transpiration rates and evaporation of water from animals bodies.

6. Questions Why do populations never grow indefinitely?
What is the difference between biotic and abiotic factors?
Suggest the level and type of abiotic factor that is most likely to limit the population size of the organisms:
Ground plants on a forest floor
Hares in a sandy desert.
Bacteria on a high mountain summit.

7. Application Where would only the blue species be found?
Where would moisture and temperature allow both species to live?
The temperature too high for the blue’s?
There is competition between the 2? 2 1 3 5 4

9. Intraspecific competition
Competition between members of the same species.
Competing for food, water, breeding sites etc.
Availability of resources determines the size of the population.
Examples?

10. Interspecific competition
Competition between members of different species.
Where 2 species occupy the same niche one may have a competitive advantage and will increase in size while the other diminishes. This can lead to the complete removal of a species = the competitive exclusion principle. No two species can occupy the same niche indefinitely when resources are limiting.

11. When cultured in isolation, the population density of each species of Paramecium increases rapidly and then levels off
In mixed culture, competition for food, space and other minerals results in the elimination of Paramecium caudatum after about 20 days; the more rapidly reproducing Paramecium aurelia outcompetes its rival

12. Predator and Prey
An organism’s niche includes how it avoids being eaten and how it finds or captures its food. Predators are consumers that capture and eat other consumers. The prey is the organism that is captured by the predator.

13. Niches and barnacles
Balanus balanoides
chthamalus stellatus

15. Predator and Prey The python has captured and is eating the deer.
The deer is being eaten by the python. It is prey.
The python has captured and is eating the deer.

16. Predator and Prey
Predator
Prey

17. Predator and Prey
Predator
Prey

18. Predator and Prey
Predator
Prey

19. Predator and Prey This alligator is the predator. It eats the python.
This python is the prey. It is eaten by the alligator.

20. Lynx vs. Hare

21. Predator/Prey Relationship

22. Predator/Prey Relationship

23. Predator/Prey Relationship
The presence of predators usually increases the number of different species that can live in an ecosystem. Predators limit the size of prey populations. As a result, food and other resources are less likely to become scarce, and competition between species is reduced.