How are organisms organised into ecosystems, and what makes them depend on each other?
The levels of organisation (organism, population, community, ecosystem), abiotic and biotic factors, interdependence within a community, and competition between organisms for resources.
A focused answer to the OCR Gateway GCSE Biology A topic B4 on ecosystems, covering the levels of organisation, abiotic and biotic factors, interdependence within a community, and competition between organisms for resources.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
OCR wants you to define the levels of organisation, distinguish abiotic and biotic factors, explain interdependence within a community, and explain competition between organisms for resources.
The levels of organisation
Ecologists describe living things at increasing scales:
So a single oak tree is an organism; all the oak trees in a wood are a population; the oaks plus all the other plants, animals and microbes are the community; and the community plus the soil, air, water and climate make the ecosystem.
Abiotic and biotic factors
The conditions in an ecosystem affect which organisms can live there. They are split into two groups:
- Abiotic factors (non-living): light intensity, temperature, water (moisture), soil pH, mineral (nutrient) levels, and oxygen or carbon dioxide concentration.
- Biotic factors (living): the availability of food, the number of predators, competition from other organisms, and disease (pathogens).
A change in any factor can change the populations. For example, a colder winter (abiotic) or the arrival of a new predator (biotic) can both reduce a population.
Interdependence
For example, bees depend on flowers for nectar, and flowers depend on bees for pollination. If the bees disappeared, the flowering plants that need them would also decline, which would affect the animals that feed on those plants. A community where all species are present and the conditions are steady is described as in a state of balance.
Competition
Organisms compete when they need the same limited resource. Competition can be between members of the same species or between different species:
- Plants compete for light, water, mineral ions (nutrients) and space.
- Animals compete for food, water, territory (space) and mates.
The better competitor gets more of the resource, so it survives and reproduces more successfully. A weaker competitor may grow less, reproduce less, and its population may fall, or it could be outcompeted and disappear from that area. This links to natural selection, which you meet in topic B5.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20194 marksDefine the terms population, community and ecosystem, and explain what is meant by interdependence within a community.Show worked answer →
A 4-mark question on ecological vocabulary.
Population: all the organisms of one species living in a habitat at a particular time. Community: all the populations of the different species living together in a habitat. Ecosystem: the community of organisms together with the non-living (abiotic) parts of their environment, interacting together.
Interdependence: the organisms in a community depend on each other for things such as food, shelter, pollination and seed dispersal, so a change in one species can affect others. Markers reward the three correct definitions and the idea that species rely on each other, so a change in one affects the rest.
OCR 20214 marksExplain why two species of plant growing in the same area may compete, naming two resources they compete for, and suggest what could happen to one species if a new, better competitor is introduced.Show worked answer →
A 4-mark question on competition and application (Suggest).
Plants compete because resources are limited and both species need the same things. Two resources (any two): light, water, mineral ions (nutrients) from the soil, and space. They compete for these because there is not enough for all the plants to have as much as they need.
If a new, better competitor is introduced: it may take more of the light, water and nutrients, so the original species gets less. The original species may grow less well, reproduce less, and its population may fall, or it could be outcompeted and die out in that area. Markers reward two resources and a sensible consequence (reduced growth or population) for the weaker competitor.
Related dot points
- Food chains and food webs, the roles of producers, consumers and decomposers, the transfer and loss of energy along a food chain, and predator-prey cycles.
A focused answer to the OCR Gateway GCSE Biology A topic B4 on feeding relationships, covering food chains and webs, producers, consumers and decomposers, the loss of energy along a food chain, and predator-prey cycles.
- Using quadrats to estimate population size and to compare two areas, using transects to study how distribution changes across a habitat, random sampling to avoid bias, and improving the reliability of sampling.
A focused answer to the OCR Gateway GCSE Biology A topic B4 on sampling techniques, covering quadrats to estimate population size, transects to study distribution, random sampling to avoid bias, and how to improve reliability, with worked estimation.
- The carbon cycle (photosynthesis, respiration, decomposition and combustion), the water cycle (evaporation, transpiration, condensation and precipitation), and how human activities such as burning fossil fuels affect the carbon cycle.
A focused answer to the OCR Gateway GCSE Biology A topic B4 on the carbon and water cycles, covering photosynthesis, respiration, decomposition and combustion in the carbon cycle, the stages of the water cycle, and the human impact of burning fossil fuels.
- The nitrogen cycle and the roles of nitrogen-fixing, nitrifying, decomposing and denitrifying bacteria, the process of decomposition, and the factors affecting the rate of decay (temperature, water and oxygen).
A focused answer to the OCR Gateway GCSE Biology A topic B4 on the nitrogen cycle and decomposition, covering nitrogen-fixing, nitrifying, decomposing and denitrifying bacteria, the process of decay, and the factors affecting the rate of decomposition.
- Biodiversity and its importance, sampling and monitoring ecosystems using quadrats, transects and indicator species, the human activities that reduce biodiversity, and the methods used to maintain biodiversity such as conservation, reforestation and protecting habitats.
A focused answer to the OCR Gateway GCSE Biology A topic B6 on monitoring and maintaining biodiversity, covering biodiversity and why it matters, sampling and monitoring with quadrats, transects and indicator species, the human activities that reduce biodiversity, and conservation methods to maintain it.