Ecosystems, populations and nichesAQA A-Level Biology: Revision notes
Section 1
Populations, communities and ecosystems
A population is all the organisms of one species living in the same place at the same time. Populations of different species living together form a community. A community and the non-living (abiotic) components of its environment together form an ecosystem.
Ecosystems range from very small, such as a rock pool or a rotting log, to very large, such as an ocean or a rainforest.
A community is only the living organisms. An ecosystem also includes the abiotic components.
Section 2
Habitat and niche
The habitat is the place where an organism lives. Within a habitat, each species occupies a niche: its role in the ecosystem, including what it eats, what eats it, where and when it is active and how it responds to conditions.
A niche is governed by the species' adaptation to both abiotic and biotic conditions. A species can only occupy a niche for which it is adapted, and two species cannot occupy exactly the same niche in the same habitat without competing.
When you define a niche, mention both abiotic and biotic conditions and adaptation. 'Where it lives' is the habitat.
Section 3
Abiotic and biotic factors
Abiotic (physicochemical) factors are the non-living features: temperature, light intensity, pH, water availability, oxygen concentration, salinity, wind and the flow of water.
Biotic factors are caused by living organisms: competition (within and between species), predation, parasitism, disease and the availability of food.
Population sizes are determined by both kinds of factor.
Section 4
Populations affect their environment
Populations are affected by biotic and abiotic factors, and in turn affect them.
- Predator and prey populations affect each other: more prey supports more predators, which then reduce the prey.
- Plants change light levels, soil pH and moisture; beavers change water depth, flow and temperature by building dams.
- Changes in an abiotic factor can change which species survive, which alters the biotic factors (for example the food available).
The effect goes both ways, so changing one part of an ecosystem has knock-on effects.
In explanation questions, say which factor is abiotic and which is biotic, and then link the change to survival and population size.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Ecosystems, populations and niches
- A woodland contains oak trees, caterpillars, blackbirds, mites that live on the blackbirds, and fungi and bacteria in the soil. The woodland also has non-living components such as soil pH, light intensity and temperature.Explain the difference between a community and an ecosystem.2 marks
- Two species of barnacle live on a rocky shore. Species X is found on the upper shore, where it is exposed to air and drying out for long periods. Species Y is found on the lower shore, which is covered by the sea for most of the time. When species Y was removed from an area of the lower shore, species X spread into that area and survived there. When species X was removed from the upper shore, species Y still did not survive there.Explain why species X is found only on the upper shore.2 marks
- In a northern forest, the population of snowshoe hares rises and falls in a cycle. The population of lynx, which prey on the hares, follows the same cycle about a year later. The hares feed on young plant shoots and twigs.Explain how an increase in the hare population leads to changes in the lynx population and then in the hare population.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).