🌍 Ecology

Adaptations & competition

Competition occurs when organisms compete for limited resources. Plants compete for light, space, water and minerals, while animals compete for food, mates and territory. Species also depend on one another, for food, shelter, pollination and seed dispersal, and this interdependence means that removing one species can affect many others. A stable community is one in which the species and the environment remain in balance.

What shapes a community

• Abiotic factors are non-living factors, such as light intensity, temperature, water, soil pH and mineral content, wind, CO₂ level for plants, and O₂ level for aquatic animals.
• Biotic factors are living factors, such as food availability, new predators, new pathogens, and competition from other species.

Adaptations

Extremophiles thrive in conditions where almost nothing else can survive, such as deep-sea volcanic vents with extreme heat and pressure, salt lakes, and acidic springs. Many extremophiles are archaea bacteria.

Organisation of an ecosystem

Every food chain starts with a producer, a green plant or alga that photosynthesises, locking the Sun's energy into biomass. Food chains show which organisms eat which:

Consumers that hunt are called predators, and what they catch is called prey. In a stable community, their population sizes cycle. When prey is plentiful, predators feed well and increase in number, which causes the prey population to fall. The predator population then falls too, from lack of food, and the prey population recovers. This pattern repeats, with each population's peak slightly behind the other's.

The carbon cycle

Carbon moves in a cycle: photosynthesis takes CO₂ in, while respiration, by plants, animals and decomposers, together with combustion of wood and fossil fuels, releases it back into the air. Microorganisms are crucial to this cycle, since by decaying dead bodies and waste they return carbon, along with mineral ions, so they can be used again. The water cycle runs alongside the carbon cycle: water evaporates, condenses into clouds, falls as precipitation, and returns through rivers, plants and transpiration.

Decay (Triple)

Decay is caused by microbial digestion, so it speeds up in conditions where microbes thrive: warmth, moisture and oxygen. Gardeners create these conditions deliberately in a compost heap, to recycle nutrients quickly. Without oxygen, anaerobic decay produces biogas, which is mostly methane, and biogas generators use this process to turn farm and food waste into fuel.

Biodiversity and ecosystems

Biodiversity is the variety of species in an ecosystem. High biodiversity tends to bring stability, because species are less dependent on any single other species. Human food, air and water all depend on healthy ecosystems, so protecting biodiversity serves human interests directly, rather than being purely an act of charity.

Human pressure on ecosystems is rising because the population is growing and each person is using more resources. This pressure causes several kinds of damage:

• Water pollution from sewage, fertiliser run-off and toxic chemicals kills aquatic life.
• Air pollution from smoke and acidic gases causes acid rain.
• Land pollution comes from landfill, toxic waste, pesticides and herbicides.
• Land use for building, quarrying, farming and dumping shrinks wild habitats.
• Peat destruction causes a double loss, since a rare habitat is destroyed and the burning or decay of peat releases the stored CO₂.
• Deforestation clears tropical forests for cattle, rice and biofuel crops, causing habitat loss as well as reducing the amount of CO₂ absorbed.

Global warming happens because rising levels of CO₂ and methane trap more heat. This has several biological effects: species shift their ranges or migrate at the wrong times, ice habitats shrink, coral reefs bleach, and some species cannot move or adapt quickly enough, leading to extinction.

Biodiversity can be maintained through breeding programmes for endangered species, protecting rare habitats, hedgerows and field margins on farms, governments limiting deforestation and emissions, and recycling instead of using landfill. These measures have real costs in money, jobs and food output, so exam answers should weigh both sides of the argument.

Trophic levels & food production (Triple) Separate science only

Trophic levels number the stages of a food chain: level 1 is producers, level 2 is herbivores, level 3 is carnivores that eat herbivores, and level 4 is apex predators. Decomposers sit outside this numbering, feeding on dead organisms at every level.

A pyramid of biomass represents living material at successive trophic levels. About 10% is transferred to the next level. Some material is not eaten or digested, and some leaves in faeces or excretory products such as urea. Respiration produces carbon dioxide and water that leave the organism. Respiration also transfers energy for life processes and to the surroundings as heat: biomass and energy are different quantities.

Feeding the world

Food security means having enough food for everyone, reliably. It is threatened by a rising population, changing diets that include more meat, new pests and pathogens, climate change, war, and the rising cost of farming inputs.

Intensive farming raises efficiency by making use of the 10% rule: restricting animals' movement and keeping them warm means less biomass is respired away, so more growth is produced per unit of feed. The main objection is ethical, concerning the animals' quality of life, and disease also spreads faster in crowded conditions. Fish stocks are managed by enforcing catch quotas and using larger net mesh sizes, which let young fish escape to breed, so that populations stay above the point of collapse.

Biotechnology offers other options for food production: mycoprotein, sold as Quorn, is made from the fungus *Fusarium*, grown on cheap glucose syrup in aerobic fermenters and harvested as a protein-rich food; genetically modified bacteria are used to make human insulin; and genetically modified crops such as golden rice are engineered to fight vitamin A deficiency.