Ecology is the study of how living organisms interact with one another and with their non-living surroundings. Every organism depends on others and on its physical environment for resources such as food, water, light and shelter. This topic explains how populations are organised into communities and ecosystems, how energy and matter flow through them, and how human activity disturbs these natural balances. Several parts are Higher tier only or biology-only (8461), so check the labels as you revise.
An ecosystem is the interaction of a community of living (biotic) organisms with the non-living (abiotic) parts of their environment. The levels build up as follows:
Organisms need materials and energy from their surroundings and from other living things. To survive a species must compete for the resources it needs.
Within a community each species depends on others for food, shelter, pollination, seed dispersal and more. This is called interdependence. If one species is removed it can affect the whole community. A community where all species and environmental factors are in balance so population sizes remain roughly constant is described as a stable community.
Plants compete for light, space, water and mineral ions in the soil. Animals compete for food, mates and territory. Organisms best adapted to obtain these resources outcompete their rivals and are more likely to survive and reproduce.
Abiotic (non-living) factors that can affect a community include: light intensity, temperature, moisture level, soil pH and mineral content, wind intensity and direction, and carbon dioxide level (for plants) and oxygen level (for aquatic animals). A change in any of these can change population sizes.
Biotic (living) factors include: availability of food, new predators arriving, new pathogens, and one species outcompeting another so its numbers fall too low to breed. Recognising whether a factor is living or non-living is a common exam requirement.
Adaptations are features that enable organisms to survive in the conditions where they normally live. There are three types:
Extremophiles are organisms living in very extreme environments such as high temperature, high pressure or high salt concentration. Many extremophiles are bacteria found near deep-sea hydrothermal vents.
It is impossible to count every organism, so ecologists sample and estimate. A quadrat is a square frame used to measure the abundance of slow-moving or non-moving organisms. Placing quadrats at random (e.g. using random number coordinates) avoids bias, then results are scaled up to the whole area. The mean, median and mode can be calculated from quadrat counts.
To study how distribution changes across an environmental gradient (e.g. from a path into a field), quadrats are placed along a line called a transect. This is a systematic, non-random method used to investigate the effect of an abiotic factor. You should be able to calculate population size = mean number per quadrat × total area / area of quadrat.
The carbon cycle returns carbon from organisms back to the atmosphere as carbon dioxide to be used by plants again. Key processes:
Microorganisms are vital recyclers, releasing carbon and mineral ions back into the environment.
The water cycle provides fresh water for plants and animals on land before draining into the seas. Energy from the Sun drives evaporation of water from land and oceans, producing water vapour. This water vapour rises, cools and condenses to form clouds. Water then returns to the land as precipitation (rain, snow). The water provides fresh water for organisms, then runs off and drains back to the sea, where the cycle repeats. Water is therefore continually recycled.
Decomposers, mainly bacteria and fungi, break down dead plants, animals and waste. The rate of decay is affected by:
Gardeners and farmers use compost heaps, providing the optimum conditions for decay to produce a natural fertiliser. Decay without oxygen (anaerobic decay) produces biogas, mostly methane, which can be burned as a fuel. You should be able to calculate rate of decay and explain how changing a factor changes the rate.
Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. High biodiversity makes ecosystems more stable because species are less dependent on any single one. The future of the human species relies on maintaining good biodiversity, but many human activities are reducing it.
The rapidly rising human population and rising standard of living mean more resources are used and more waste is produced. Unless this is managed, pollution increases. Pollution occurs in water (sewage, fertiliser, toxic chemicals), in air (smoke and acidic gases), and on land (landfill and toxic chemicals such as pesticides and herbicides). Pollution kills plants and animals, reducing biodiversity.
Humans reduce the land available for other organisms by using it for building, quarrying, farming and dumping waste. The destruction of peat bogs to produce garden compost reduces the area of this habitat and releases stored carbon dioxide (peat is decaying plant material), adding to greenhouse gases.
Large-scale deforestation in tropical areas has occurred to provide land for cattle and rice fields and to grow crops for biofuels. Deforestation reduces biodiversity, reduces the amount of carbon dioxide absorbed by photosynthesis, and releases carbon dioxide through burning and decay of wood.
Levels of carbon dioxide and methane in the atmosphere are increasing, and many scientists believe this is causing global warming. Biological consequences include loss of habitat (e.g. ice melting, low-lying areas flooding), changes to the distribution of species where temperature or rainfall changes, and changes to migration patterns.
Programmes that aim to reduce the negative effects on ecosystems and maintain biodiversity include: breeding programmes for endangered species, protection and regeneration of rare habitats, reintroduction of field margins and hedgerows on farms, reducing deforestation and carbon dioxide emissions, and recycling resources rather than dumping waste in landfill. There are often conflicting pressures, for example between conservation and the needs of farmers and developers.
Trophic levels can be represented by numbers, starting at level 1 with plants and algae (producers) that make their own food by photosynthesis. Herbivores that eat plants are level 2 (primary consumers); carnivores that eat herbivores are level 3 (secondary consumers); carnivores that eat other carnivores are level 4 (tertiary consumers). Apex predators are at the top with no predators. Decomposers break down dead material, releasing substances back into the ecosystem.
A pyramid of biomass shows the relative amount of biomass at each trophic level. Only about 10% of the biomass from each level is transferred to the level above. Losses occur because not all of an organism is eaten or digested, energy is used in respiration and movement, and material is lost in waste (faeces, urine) and as heat. You should be able to calculate efficiency of biomass transfer = biomass transferred / biomass available × 100.
Food security means having enough food to feed a population. Threats include the increasing human population, changing diets in developed countries, new pests and pathogens, environmental changes affecting farming, the cost of farming, and conflicts affecting water and food supply.
To produce more efficiently, the energy transferred from livestock to the environment can be limited by restricting their movement and keeping them warm (intensive farming), though this raises animal welfare concerns. Overfishing threatens fish stocks, so fishing quotas and net size controls help maintain breeding populations and biodiversity. Biotechnology can help: a fungus, Fusarium, is grown to produce mycoprotein, a protein-rich food, in aerobic conditions on glucose syrup; the biomass is harvested and purified. Genetically modified bacteria are also used to produce human insulin. GM crops can give higher yields or added nutrients (e.g. extra vitamins).
Practise exam-style questions on this topic.