Competition and population sizeIB MYP Biology: Revision notes
Section 1
What is competition?
Competition happens when organisms need the same resource and the supply is not enough for all of them. Resources are things an organism needs to live, grow and reproduce.
- Intraspecific competition is between members of the same species, e.g. two oak trees. It is usually the strongest, because they need exactly the same things.
- Interspecific competition is between members of different species, e.g. lions and hyenas competing for the same prey.
Competition means individuals that get more of the resource survive and breed better.
Section 2
What do organisms compete for?
Plants compete for:
- light for photosynthesis
- water
- space for roots and leaves
- mineral ions (e.g. nitrates) from the soil
Animals compete for:
- food
- water
- space (territory, shelter, nesting sites)
- mates to breed with
Plants do not compete for mates or food, as they make their own food by photosynthesis.
Name the resource in your answer. 'They compete' scores nothing, but 'they compete for light' does.
Section 3
Limiting factors and carrying capacity
A limiting factor is anything that stops a population from growing any larger. Examples are food supply, water, space, light, disease, predators and the build-up of waste.
Every habitat has only a limited supply of resources. The largest population that the environment can support for a long time is the carrying capacity.
- Below the carrying capacity, there is enough for everyone and the population can grow.
- Above it, resources run short, so more individuals die or leave than are born, and the population falls back.
Do not say the population 'stops growing because it is full'. Name the limiting factor, such as food running out.
Section 4
The population growth curve
When a few organisms (e.g. bacteria or yeast) are put into a new habitat with plenty of food, the population follows an S-shaped curve with four phases.
- Lag phase: slow growth. Few organisms are present and they are adjusting to the new conditions.
- Exponential (log) phase: rapid growth. Food is plentiful, so births are far higher than deaths and the population doubles at regular intervals.
- Stationary phase: the population levels off at about the carrying capacity. Births equal deaths because a limiting factor, such as food or space, now restricts growth.
- Decline phase: deaths exceed births because food has run out or toxic waste has built up, so the population falls.
Must know
- Competition is for limited resources; intraspecific is within a species, interspecific between species.
- Plants compete for light, water, space and mineral ions; animals for food, water, space and mates.
- A limiting factor stops population growth; the carrying capacity is the largest population the environment can support.
- The growth curve has four phases: lag, exponential, stationary and decline.
- In the stationary phase births equal deaths; in decline deaths exceed births.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Competition and population size
- A forestry team in Poland planted young oak seedlings in two plots of the same woodland clearing. In plot 1 the seedlings were planted 10 cm apart. In plot 2 they were planted 1 m apart. Both plots have the same soil and receive the same rainfall.Explain why the seedlings in plot 1 are likely to grow more slowly than those in plot 2.2 marks
- On a beach in Namibia, a colony of Cape fur seals gathers every year to breed. Each adult male defends a small patch of beach and tries to keep other males away from the females on it. The colony grows larger each year, but the amount of fish in the nearby sea stays about the same.Explain why competition for food between the seals is likely to increase as the colony grows.2 marks
- A student grew yeast in a sealed flask containing a fixed volume of sugar solution kept at 30 °C. Every three hours she counted the yeast cells in a small sample. The count was 2000 cells per mm³ at 0 hours and 2400 at 3 hours. It rose to 38 000 at 9 hours and 52 000 at 15 hours, stayed at about 52 000 until 30 hours, and then fell to 20 000 by 48 hours.Name the phase of population growth shown between 0 and 3 hours, between 3 and 9 hours, and between 15 and 30 hours.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).