Ecology is about how organisms interact with each other and with their surroundings, and about the effects of the very large human population on both.
An ecosystem is the interaction of a community of living organisms with the non-living parts of their environment. A population is all the organisms of one species in a habitat; a community is all the populations together.
Species depend on each other for food, shelter, pollination and seed dispersal, which is interdependence. In a stable community the species and environmental factors are in balance, so populations fluctuate around levels that can be sustained over time.
Organisms compete for resources: plants for light, space, water and mineral ions; animals for food, mates and territory.
Abiotic (non-living) factors include light intensity, temperature, moisture, soil pH and mineral content, wind intensity and direction, and carbon dioxide and oxygen levels.
Biotic (living) factors include availability of food, new predators arriving, new pathogens, and one species out-competing another so its numbers fall too low to breed.
Structural adaptations are features of shape or body, such as a polar bear's thick fur and small surface area to volume ratio. Behavioural adaptations are things organisms do, such as migration. Functional adaptations are internal processes, such as a camel concentrating its urine.
Extremophiles live in conditions most organisms could not tolerate: high temperature, high pressure or high salt concentration. For example, some microorganisms live around deep-sea hydrothermal vents.
Photosynthetic organisms are the producers of biomass for life on Earth. Feeding relationships are shown in a food chain, where the arrows show the direction energy travels.
Trophic levels are numbered: level 1 producers, level 2 primary consumers (herbivores), level 3 secondary consumers, level 4 tertiary consumers. Decomposers break down dead material at every level.
In a stable community, predator and prey populations cycle: prey numbers rise, predators then increase, prey numbers fall, predators then decrease. The predator peak lags behind the prey peak.
Only a small proportion of the biomass at one trophic level is transferred to the next, and around 10% is a commonly used GCSE estimate. The rest is lost: not all of the organism is eaten or digestible, and much of the remainder is used in respiration or lost in waste, in urine and faeces.
Because so little biomass reaches each successive level, food chains rarely have more than four or five levels. A pyramid of biomass is drawn to scale, with the width of each bar representing the biomass at that level, and is widest at the bottom.
All materials in the living world are recycled to provide the building blocks for future organisms.
The water cycle provides fresh water on land: water evaporates, condenses to form clouds, falls as precipitation and drains back to the sea.
The carbon cycle returns carbon to the atmosphere as carbon dioxide, through respiration, decomposition and combustion, and removes it through photosynthesis.
Decomposition is faster when it is warmer, when there is more oxygen and when there is more moisture, because those conditions suit the microorganisms doing the work. Compost is decayed material used as a natural fertiliser; biogas generators use anaerobic decay to produce methane.
Plants need nitrates to make proteins but cannot use nitrogen gas directly. Nitrogen-fixing bacteria, some free-living and some in the root nodules of legumes, convert nitrogen into compounds plants can use; decomposers release ammonia from dead material; nitrifying bacteria convert that to nitrates; and denitrifying bacteria return nitrogen to the air. Farmers add nitrates as fertiliser or use crop rotation with legumes.
Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem.
High biodiversity can make an ecosystem more resilient to change, because ecological roles are shared among a wider range of species. Human activities can reduce biodiversity through habitat destruction, pollution, climate change and overexploitation.
The rapid growth of the human population and rising standards of living mean more resources used and more waste produced. Unless waste is handled properly, it pollutes water with sewage, fertiliser and toxic chemicals; land with landfill and toxic chemicals; and air with smoke and acidic gases.
Humans use land for building, quarrying, farming and dumping waste, which reduces the land available for other organisms. The destruction of peat bogs reduces habitats and releases carbon dioxide as the peat decays or is burned.
Deforestation in tropical areas, to provide land for cattle and rice fields and to grow crops for biofuels, reduces biodiversity, reduces the amount of carbon dioxide taken up, and releases carbon dioxide from burning and from the decay of what remains.
Increases in carbon dioxide and methane contribute to global warming. Consequences include rising sea levels from melting ice and thermal expansion, changes in the distribution of species as temperature and rainfall shift, and changes to migration patterns.
Environmental change affects the distribution of species. A change in temperature, in availability of water or in the composition of atmospheric gases can make an area suitable for a species that could not live there before, or unsuitable for one that could.
Some species are useful as indicators of pollution because they only survive in particular conditions: lichens are sensitive to sulfur dioxide in the air, and invertebrates such as mayfly larvae indicate clean water because they need high oxygen levels.
Programmes to protect biodiversity include breeding programmes for endangered species, protection and regeneration of rare habitats, reintroduction of field margins and hedgerows on farmland, reduction of deforestation and carbon dioxide emissions, and recycling rather than landfill.
These often conflict with other pressures, including cost, the demand for land and food, and local economic interests, so they are contested.
Populations are estimated by sampling rather than counting everything. A quadrat is a square frame placed at random positions, and the mean number of organisms per quadrat is scaled up to the whole area.
A transect is a line along which quadrats are placed at intervals, used to see how distribution changes across an environmental gradient, for example from the water's edge up a shore.
More samples give a more reliable estimate, and positions must be genuinely random to avoid bias.
Food security means having enough food to feed a population. It is threatened by the increasing birth rate, by changing diets in wealthier countries taking scarce food resources further, by new pests and pathogens, by environmental changes such as drought, by the cost of farming inputs, and by conflict.
Restricting the movement of farm animals and keeping them warm reduces the energy transferred through movement and through maintaining body temperature, so a greater proportion of the food they eat contributes to growth. That is effective but raises welfare concerns.
Overfishing can cause fish stocks to decline, and if numbers fall too low a species may disappear from an area altogether. Minimum net sizes and fishing quotas can help populations recover.
Biotechnology contributes too: the fungus Fusarium is grown on glucose syrup in aerobic conditions to produce mycoprotein, a protein-rich food for vegetarians; genetically modified bacteria produce human insulin; and GM crops such as golden rice have been developed to address specific deficiencies.
