Relationships of organisms with one another and with the environment
Food production
Greenhouses and polythene tunnels raise crop yields by controlling limiting factors: higher temperature, extra CO₂ (from paraffin heaters) and artificial light. Fertilisers supply nitrate, phosphate and potassium. Pesticides kill pests but can harm other species and accumulate in food chains; biological control uses a natural predator instead. Fish farming keeps fish in controlled conditions with fed diets and disease control. Monocultures and intensive livestock increase output but reduce biodiversity and can spread disease and pollution.
≈ 35 minFood chains and energy flow
Energy enters ecosystems as sunlight captured by producers (plants) in photosynthesis. It passes along food chains to primary consumers (herbivores), secondary consumers and tertiary consumers (carnivores). Each feeding level is a trophic level. Many chains link into a food web. At each transfer only about 10% of the energy passes on; the rest is lost in respiration, movement, heat, and uneaten or undigested parts. So food chains are short, and pyramids of energy and biomass narrow upwards. Decomposers release energy from dead material.
≈ 40 minNutrient cycles
Elements are recycled in ecosystems. Carbon cycle: CO₂ is removed from air by photosynthesis; returned by respiration (plants, animals, decomposers) and combustion of fuels; passed along food chains by feeding; trapped in fossil fuels if decomposition doesn't happen. Nitrogen cycle: nitrogen-fixing bacteria (in soil and root nodules of legumes) turn N₂ into nitrogen compounds; decomposers turn proteins and urea into ammonium; nitrifying bacteria convert ammonium → nitrite → nitrate, absorbed by plants; denitrifying bacteria return nitrate to N₂ in waterlogged soil. Water cycle: evaporation, transpiration, condensation, precipitation.
≈ 40 minPollution and conservation
Humans destroy habitats for farming, building, mining and logging. Deforestation reduces biodiversity, increases CO₂ (less photosynthesis, burning), causes soil erosion and flooding. Eutrophication: fertilisers or sewage add nutrients → algal bloom → algae block light, plants die → decomposers respire and use up oxygen → fish die. Greenhouse gases (CO₂, methane) cause climate change. Plastics don't biodegrade and harm wildlife. Conservation protects species and habitats with reserves, laws, seed banks, captive breeding and education; sustainable use includes fishing quotas, net mesh sizes and replanting forests.
≈ 45 minPopulations and sampling
A population is all the organisms of one species in an area; a community is all the populations of different species; an ecosystem is a community and its physical environment. Population size depends on biotic factors (food, predators, disease, competition) and abiotic factors (temperature, light, water). Populations often follow a sigmoid curve: lag, exponential (log) growth, stationary (births = deaths) and sometimes death phase. Population size is estimated with quadrats: count organisms in random quadrats, find the mean, and scale up to the total area.
≈ 40 min