What you'll learn
Ecology is the unit of AQA GCSE Combined Science: Trilogy that studies living organisms in their environment — how they interact with each other and with the non-living world, and what happens when human activity disturbs those relationships. It is the most quantitative of the biology units, because it involves sampling, estimating population sizes and reading graphs of environmental data, and it is also the unit where scientific evidence meets public argument. By the end you should be able to define the levels of organisation in an ecosystem, distinguish biotic from abiotic factors, explain the adaptations of organisms to extreme environments, use quadrats and transects to sample a population and calculate an estimate from the data, describe the carbon and water cycles, explain the conditions affecting decay, and evaluate the causes and consequences of pollution, deforestation and global warming. This unit is assessed on Biology Paper 2 and includes the required practical on sampling with quadrats.
Key terms and definitions
Ecosystem — the interaction of a community of living organisms with the non-living parts of their environment
Community — all the populations of different species living in the same habitat
Population — all the organisms of one species living in a habitat
Habitat — the place where an organism lives
Abiotic factor — a non-living factor affecting a community, such as light intensity, temperature, moisture, pH or wind
Biotic factor — a living factor affecting a community, such as food availability, predators, pathogens or competition
Interdependence — the reliance of each species in a community on other species for food, shelter, pollination or seed dispersal
Stable community — a community in which all the species and environmental factors are in balance so that population sizes remain roughly constant
Quadrat — a square frame of known area placed on the ground to sample the organisms within it
Transect — a line along which samples are taken, used to study how distribution changes across a habitat
Biodiversity — the variety of all the different species of organisms on Earth, or within an ecosystem
Eutrophication — the process in which fertiliser washed into water causes algae to grow rapidly, blocking light and leading to the death of plants and animals as oxygen is used up
Core concepts
Levels of organisation and interdependence
An ecosystem is built from levels. Individual organisms of one species form a population, the populations of all species together form a community, and the community interacting with the abiotic environment forms the ecosystem.
Within a community, species depend on one another for food, shelter, pollination and seed dispersal. This interdependence means that removing one species can affect many others. A stable community is one in which all the species and environmental factors are in balance, so population sizes remain roughly constant over time; ancient oak woodland and tropical rainforest are standard examples.
Competition
Plants compete with each other for light and space, and for water and mineral ions from the soil. Animals compete for food, mates and territory. To survive and reproduce, an organism needs a supply of the materials it requires from its surroundings and from the other species living there.
Where two species need the same limited resource, the better-adapted one tends to increase in number while the other declines. This is the basis of many exam questions about what happens when a new species is introduced to a habitat.
Abiotic and biotic factors
Abiotic factors are the non-living conditions: light intensity, temperature, moisture levels, soil pH and mineral content, wind intensity and direction, and the concentration of carbon dioxide for plants or of oxygen for aquatic animals. A change in any of these can affect the size of a population.
Biotic factors are the living influences: availability of food, new predators arriving, new pathogens, and competition between species where one outcompetes another to the point that numbers are no longer sufficient to breed.
Adaptations
Adaptations are features that enable an organism to survive in the conditions in which it normally lives. They may be structural, such as the thick fur and small surface area to volume ratio of an arctic animal; behavioural, such as migration to avoid cold seasons; or functional, such as producing very concentrated urine to conserve water.
Some organisms, called extremophiles, live in environments that are very extreme in temperature, pressure or salt concentration. Bacteria living in deep sea vents are the standard example.
Desert plants illustrate the principle neatly. Many have a thick waxy cuticle and reduced leaves or spines to lower water loss by evaporation, a shallow but very widespread root system to capture occasional rain, and tissue that stores water.
Sampling and estimating population size
Counting every organism in a habitat is rarely possible, so ecologists sample. A quadrat of known area is placed on the ground and the organisms within it are counted. To avoid bias, quadrats must be placed randomly, which is usually done by generating random coordinates on a grid.
To estimate the total population, calculate the mean number per quadrat, then multiply by the total area divided by the area of one quadrat. The reliability of the estimate depends on the number of quadrats used: more samples give a mean closer to the true value.
A transect is used when the question is about distribution rather than total number. A line is laid across the habitat, typically running from one condition to another such as from the sea towards land, and quadrats are placed at regular intervals along it. This shows how species change as an abiotic factor changes.
The carbon cycle
Carbon is constantly recycled. Carbon dioxide is removed from the atmosphere by photosynthesis in green plants and algae, and the carbon becomes part of the carbohydrates, proteins and fats making up their bodies. When animals eat plants, the carbon is passed along the food chain.
Carbon returns to the atmosphere as carbon dioxide in three ways: respiration by plants, animals and microorganisms; combustion of wood and fossil fuels; and the decay of dead organisms by decomposers, which respire as they feed.
The essential point to state in an exam answer is that decomposers return carbon to the atmosphere and mineral ions to the soil, so the cycle continues and materials are constantly reused.
The water cycle
The water cycle provides fresh water for plants and animals on land before draining into the seas. Energy from the Sun evaporates water from the surface of the sea and from land, and plants lose water vapour by transpiration. The water vapour rises, cools and condenses to form clouds, and then falls as precipitation, which provides fresh water. Water then drains through the soil and along rivers back to the sea.
Decay and decomposition
Decay is carried out by microorganisms, and its rate depends on three conditions: temperature, because microorganisms respire and their enzymes work faster when warm but are denatured if too hot; the availability of water, because microorganisms need moisture; and the availability of oxygen, because most decomposers respire aerobically.
Gardeners and farmers apply this deliberately. Compost heaps are kept moist and turned to admit air, which speeds up decay and produces a natural fertiliser that returns mineral ions to the soil. Biogas generators use the opposite conditions, working anaerobically to produce methane gas that can be burned as a fuel.
Human impacts on biodiversity
The rapid growth of the human population and the rising standard of living mean that more resources are used and more waste is produced. Unless waste is properly handled, pollution follows. Water is polluted by sewage, fertiliser and toxic chemicals; air by smoke and acidic gases; and land by landfill and by toxic chemicals including pesticides and herbicides, which may then wash into water.
Humans reduce the land available for other animals and plants by building, quarrying, farming and dumping waste. Peat bogs in particular are destroyed to produce garden compost, which both reduces the area of this habitat and releases carbon dioxide as the peat decays or is burned.
Large-scale deforestation in tropical areas has been carried out to provide land for cattle and rice fields and to grow crops for biofuels. The consequences are increased carbon dioxide in the atmosphere from burning and from the decay of felled trees, less carbon dioxide removed by photosynthesis, and reduced biodiversity.
Biodiversity matters because a greater variety of species makes an ecosystem more stable, since species depend on one another for shelter and for the maintenance of the physical environment. The future of the human species relies on maintaining it.
Global warming
Levels of carbon dioxide and methane in the atmosphere are increasing, and this contributes to global warming. The biological consequences include loss of habitat where low-lying areas are flooded by rising sea levels, changes to the distribution of species as temperature and rainfall patterns change in different regions, and changes to migration patterns.
Programmes to reduce the negative effects on ecosystems and biodiversity include breeding programmes for endangered species, protection and regeneration of rare habitats, reintroduction of field margins and hedgerows on farms where only one crop is grown, reduction of deforestation and of carbon dioxide emissions by governments, and recycling rather than dumping waste in landfill.
There are conflicting pressures on these programmes, and questions often ask you to explain them. Protecting a habitat may restrict farming or development, and reducing emissions has an economic cost, so decisions balance environmental benefit against local livelihoods and national economies.
Worked examples
Example 1: Estimating a population from quadrat data (4 marks)
A student places ten quadrats, each 0.25 square metres, at random in a field measuring 50 metres by 40 metres. The mean number of daisy plants per quadrat is 6. Estimate the total number of daisies in the field.
The total area of the field is 50 multiplied by 40, which is 2,000 square metres. The number of quadrat-sized areas in the field is 2,000 divided by 0.25, which is 8,000. Multiplying by the mean of 6 plants per quadrat gives an estimate of 48,000 daisies. The estimate assumes the quadrats were placed randomly and that the daisies are evenly distributed across the field.
Example 2: Explaining eutrophication (4 marks)
Explain how fertiliser running off a field can kill the fish in a nearby river.
The fertiliser adds mineral ions to the water, which causes algae to grow rapidly and form a layer on the surface. This blocks light from reaching the plants below, so they cannot photosynthesise and they die. Decomposing microorganisms feed on the dead plants and multiply, and because they respire aerobically they use up the oxygen dissolved in the water. With too little dissolved oxygen, the fish cannot respire and they die.
Example 3: Explaining a desert adaptation (3 marks)
Explain how having spines instead of broad leaves helps a cactus survive in a desert.
Spines have a much smaller surface area than broad leaves, so less water is lost by evaporation from the plant's surface. Conserving water is essential where rainfall is very low and infrequent. The spines also deter animals from eating the plant to obtain the water stored in its tissue.
Common mistakes and how to avoid them
The most common error is confusing habitat, population, community and ecosystem. Learn them as a nested sequence: one species is a population, all the species together are a community, and the community plus its non-living surroundings is the ecosystem.
In quadrat calculations, students frequently forget that the quadrat is not one square metre. Always divide the total area by the area of the quadrat, not by one.
Many answers on eutrophication stop at the algae blocking the light. The marks are in the chain that follows: plants die, decomposers multiply, oxygen is used up, fish die. Write the whole chain.
Another frequent slip is saying that plants adapt to their environment during their lifetime. Adaptations arise over generations through natural selection; an individual does not change to suit its habitat.
Finally, in global warming questions, avoid vague statements such as it harms the environment. Name a specific biological consequence: loss of habitat, change in species distribution, or change in migration patterns.
Exam technique for "Biology: Ecology"
Sampling questions almost always include a mark for random placement and a mark for repeating the sample. Mention both even when the question seems to be only about the calculation.
For the required practical, be ready to state the independent variable as distance along the transect or position in the field, the dependent variable as the number of organisms per quadrat, and to explain why a mean is used.
Cycle questions want processes named. Writing that carbon goes into the air is worth nothing; writing that carbon dioxide is returned to the atmosphere by respiration, combustion and the decay of dead organisms by decomposers scores three.
Where a question asks you to evaluate a programme such as a breeding programme or habitat protection, give a benefit, give a cost or conflict, and conclude. The conflicting pressures are explicitly in the specification, so the examiner expects them.
Quick revision summary
An ecosystem is a community interacting with its abiotic environment; populations make up the community. Organisms compete for light, space, water and mineral ions, or for food, mates and territory, and they are interdependent. Abiotic factors are non-living conditions and biotic factors are living influences. Adaptations may be structural, behavioural or functional, and extremophiles live in extreme conditions. Quadrats placed randomly estimate population size by scaling the mean per quadrat to the total area, while transects show distribution. Carbon is removed from the atmosphere by photosynthesis and returned by respiration, combustion and decay; the water cycle runs through evaporation, transpiration, condensation and precipitation. Decay depends on temperature, water and oxygen, used in compost heaps and, anaerobically, in biogas generators. Human population growth causes pollution of water, air and land, and deforestation and peat destruction reduce biodiversity and raise carbon dioxide. Global warming causes habitat loss and changes to species distribution and migration.