Population size and competitionAQA A-Level Biology: Revision notes

An ecosystem can only supply a limited amount of resources such as food, water, light, shelter and nesting sites. The carrying capacity is the maximum population size of a species that an ecosystem can support indefinitely.

Population size is not fixed. It usually fluctuates around the carrying capacity, because it takes time for birth and death rates to respond to changes. If the population rises above the carrying capacity, resources run short, deaths rise and births fall, so numbers fall back. If it falls below, resources are plentiful and numbers rise. This is negative feedback.

The carrying capacity itself changes when the environment changes, for example in a drought or when a new competitor arrives.

Abiotic factors are the non-living features of the environment, such as temperature, light intensity, water availability, pH, salinity and mineral ion concentration. They change the population size by affecting survival and reproduction.

• Temperature: extreme cold or heat slows enzyme-controlled reactions or denatures enzymes, so more organisms die and fewer breed.
• Light: for plants, low light limits photosynthesis, so growth and seed production fall.
• Water and pH: organisms with narrow tolerance die or move away outside their range.

The effect of an abiotic factor is usually independent of population density: a hard frost kills a similar proportion of a population whether it is large or small.

Intraspecific competition is competition between members of the same species for the same limited resources, such as food, water, light, mates, territory or nest sites. All members have identical requirements, so competition is most intense between them.

As a population grows, the resources available per individual fall. Fewer individuals get enough to survive and breed, so the death rate rises and the birth rate falls. This stops the population growing without limit and is the main reason it settles around the carrying capacity. Because its effect depends on how many individuals there are, it is density-dependent.

The individuals best adapted to obtain resources survive and reproduce, which is a source of selection pressure.

Interspecific competition is competition between individuals of different species that use the same resources, because their niches overlap.

The species that is better adapted to get the resource, for example by growing faster or digesting food more efficiently, survives and reproduces better. Its population increases while the other species' population decreases because it has less of the resource. If the overlap is complete, one species may be eliminated from the area (competitive exclusion). Species can coexist if their niches differ enough, for example by feeding on different foods or living in different zones.

The outcome depends on abiotic conditions too. Barnacle species on a rocky shore show this: one grows faster and smothers the other in the lower zone, but cannot survive drying out in the upper zone, where the other persists.

Predation is when one organism, the predator, kills and eats another, the prey. Predator and prey populations often show linked cycles.

1. Prey numbers rise, so there is more food for predators.
2. Predators survive and breed more, so predator numbers rise after a time delay.
3. More prey are eaten, so prey numbers fall.
4. Less food means predator numbers fall.
5. With fewer predators, prey numbers recover and the cycle repeats.

The peak in the predator population lags behind the prey peak. Predation is not the only influence: prey numbers are also limited by their own food supply, disease and abiotic factors, and predators by competition among themselves. Correlation between the two cycles does not prove that predation is the only cause.

Population size is controlled by a combination of abiotic factors (independent of density) and biotic interactions: intraspecific competition, interspecific competition and predation (usually density-dependent).

To answer an application question:

• Name the resource that is limited.
• Say whether it is intra- or interspecific.
• Link it to birth rate and death rate, then to population size relative to the carrying capacity.
• Use named figures from the data, with units, when given.