Ecology — AQA Combined Science: Trilogy

Ecology is the study of how organisms interact with each other and with their environment. This unit covers ecosystems, the cycling of materials, biodiversity and the impact of humans on the environment.

Levels of organisation

Population — all the organisms of one species in a habitat.
Community — all the populations of different species in a habitat.
Ecosystem — the interaction of a community with the non-living (abiotic) parts of the environment.

Organisms compete for resources. Plants compete for light, space, water and mineral ions. Animals compete for food, mates and territory.

Interdependence

Species in a community depend on each other for food, shelter, pollination and seed dispersal — this is interdependence. If one species is removed it can affect the whole community. A stable community is one where the species and environmental factors are in balance so population sizes remain roughly constant.

Abiotic and biotic factors

Abiotic (non-living) factors that affect communities:

light intensity, temperature, moisture, soil pH and mineral content, wind, CO₂ (for plants) and oxygen (for aquatic animals).

Biotic (living) factors:

availability of food, new predators, new pathogens, and competition between species.

Adaptations

Organisms have adaptations that allow them to survive in their conditions:

Structural — physical features (e.g. thick fur, large surface area).
Behavioural — actions (e.g. migration, hibernation).
Functional — internal processes (e.g. producing little urine).

Extremophiles are organisms (often bacteria) adapted to extreme conditions such as high temperature, pressure or salt concentration.

Feeding relationships

Food chains show the transfer of energy and biomass between organisms, always starting with a producer (usually a green plant or alga that makes its own food by photosynthesis).

producer → primary consumer → secondary consumer → tertiary consumer

Herbivores eat plants; carnivores eat other animals; predators hunt prey.
In a stable community, predator and prey populations rise and fall in cycles: more prey → more predators → fewer prey → fewer predators, and so on.

Required practical: measuring the population size and distribution of a species using quadrats (for abundance) and transects (to study how distribution changes across an area). You should be able to calculate means, medians, modes and estimate population sizes.

Cycling of materials

Materials in the living world are recycled to provide the building blocks for future organisms.

The carbon cycle

Carbon dioxide is removed from the air by photosynthesis in plants and algae.
Carbon passes along food chains as animals eat plants.
Carbon dioxide is returned to the air by respiration (of plants, animals and decomposers) and by combustion of fuels.
Decomposers (microorganisms) break down dead organisms and waste, releasing CO₂.

The water cycle

The water cycle provides fresh water for organisms on land before draining back to the seas. Energy from the Sun drives evaporation; water vapour rises, cools and condenses to form clouds, then falls as precipitation.

Decomposition

Decay is carried out by microorganisms. The rate of decay is faster when it is warm, moist and has plenty of oxygen. Gardeners and farmers use this knowledge in compost production. Anaerobic decay produces methane (biogas), which can be used as a fuel.

Biodiversity and human impact

Biodiversity is the variety of all the different species of organisms on Earth or in an ecosystem. High biodiversity makes ecosystems more stable. The future of the human species relies on maintaining good biodiversity.

Human impacts

A rapidly growing human population and higher standards of living mean more resources are used and more waste is produced.

Negative impacts:

Pollution of water (sewage, fertiliser), air (smoke, acidic gases) and land (landfill, toxic chemicals).
Land use for building, quarrying, farming and waste reduces the land available for other organisms.
Deforestation (for timber and farmland) reduces biodiversity and removes trees that store carbon.
Peat bog destruction releases stored CO₂ and destroys habitats.
Global warming caused by increasing levels of carbon dioxide and methane (greenhouse gases).

Positive actions to maintain biodiversity:

Breeding programmes for endangered species.
Protection and regeneration of rare habitats.
Reintroduction of field margins and hedgerows on farms.
Reducing deforestation and carbon emissions.
Recycling rather than dumping waste in landfill.

These often involve a conflict between human needs and conservation, which you should be able to evaluate.

Exam tips

Learn the difference between abiotic and biotic factors with examples.
Be able to explain predator–prey cycles in terms of food availability.
For the carbon cycle, name the processes: photosynthesis (removes CO₂), respiration and combustion (return CO₂), decomposition.
For quadrat questions, practise calculating means and estimating total population size from a sample.
Use data to evaluate the benefits and costs of human activities on biodiversity.

Biology: Ecology