A2/A-level Biology - Ecosystem and Sustainability (1)

Do you know the limiting factors determining the final size of a population in A2/A-level Biology?

• Population size is a balance between death rate and rate of reproduction.
• A habitat cannot support a population larger than its carrying capacity because of the limiting factors, which place a limit on population size.
• The limiting factors may include food, water, light, oxygen, shelter, predators,parasites, intensity of competition within and between species etc.

• It is the maximum population size that can be maintained over a period of time in a particular habitat.

1.Lag Phase

• Only a few individuals, still acclimatising to their habitat.
• The rate of reproduction is low, and the growth in population size is slow.

2. Log Phase

• Resources are plentiful and conditions are good.
• The rate of reproduction is fast and exceeds mortality.
• The population size increases rapidly.

3. Stationary Phase

• Population has levelled out.
• The birth rate and death rate are equal and the habitat cannot support a larger population.
• The population size therefore stays stable, or fluctuates slightly in response to small variations in environmental conditions.

• Predators are animals that hunt other animals (prey).
• Predation can act as a limiting factor on a prey's population size.
• More predators = more prey eaten.
• Prey population decreases = less food for predators.
• Fewer predators survive = predator population decreases.
• Fewer prey now eaten = prey population increases.
• More prey = predator numbers increase and the cycle begins again.

• It occurs when resources (like food or water) are not present in adequate amounts to satisfy the needs of all the individuals who depend on those resources.
• As the intensity of competition increases, the rate of reproduction decreases (fewer organisms have enough organisms to reproduce), whilst the death rate increases (fewer organisms have enough resources to survive).

There are two types of competition:

(1) Intraspecific competition - competition between individuals of the same species for the same resources.

• E.g. male deer locking horns when competing for mates.

(2) Interspecific competition - competition between individuals of different species for the same resources.

• E.g. cheetahs and lions competing for the same prey.

• In 1934, Georgy Gause grew two species of Paramecium, both separately and together.
• When together, there was competition for food, with Paramecium aurelia obtaining more food effectively than Paramecium caudatum, resulting in Paramecium caudatum dying out and the numbers for Paramecium aurelia increasing, eventually becoming the only species remaining.

• Gause concluded that more overlap between two species’ niches would result in more intense competition.
• If two species have exactly the same niche, one would be out-competed by the other and would die out or become extinct in that habitat – competitive exclusion principle – used to explain why particular species only grow in particular places.

However, other observations and experiments suggest that extinction is not necessarily certainly going to happen.

• Interspecific competition could result in one population being smaller than the other, with both population sizes remaining relatively constant.
• It is important to realise that in the laboratory, it is easy to exclude the effects of other variables, so the habitat of the two species remains very stable.
• In the wild, a wide range of variables may act as limiting factors for the growth of different populations.
• Variables may change on a daily basis or even over the course of a year.
• For example, experiments on competition between flour beetles Triboliumconfusum and Triboliumcastaneum initially confirmed the competitive exclusion principle – the T. castaneum population size increased, whilst T. confusum died out.
• Even a small change in the temperature could change the outcome so that T. confusum survived instead.

In part 2, you will know more about how to maintain biodiversity.

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Drafted by Bonnie (Biology)