Agroecosystems: farms as simplified ecosystems
An agroecosystem is an ecosystem managed for food, fibre or fuel. Compared with a natural ecosystem it has:
- Low biodiversity: often a monoculture (one crop), with weeds, pests and competitors removed.
- Short food chains: crop → people, or crop → livestock → people. Each step up a food chain loses about 90% of energy, so plant food feeds more people per hectare than meat.
- Extra inputs: fossil-fuel energy (machines, making fertiliser), water, nutrients and pesticides. Energy efficiency = food energy out ÷ energy in; intensive systems often put in more energy than traditional ones.
- Open nutrient cycles: harvest removes nutrients, so they must be replaced.
Farming systems range from extensive (low inputs per hectare, e.g. nomadic herding) to intensive (high inputs, high yields per hectare), and from subsistence (for the family) to commercial (for sale).
Improving crop and livestock genetics
- Selective breeding: choose parents with useful traits (high yield, short stems, disease resistance, more milk) and breed them over many generations.
- High-yielding varieties (HYVs): short-stemmed wheat and rice that put more energy into grain and do not fall over when heavily fertilised.
- Genetic modification (GM): moving a gene from another species, e.g. Bt cotton makes its own insecticide; Golden Rice makes vitamin A.
- Gene banks and seed banks store wild relatives and old varieties as a source of useful genes.
Concerns: loss of genetic diversity (many farms growing the same variety are all hit by one disease), dependence on buying seed each year, gene flow to wild plants, and debate about GM safety and corporate control.
The Green Revolution
The Green Revolution (from the 1940s in Mexico, spreading across Asia and Latin America in the 1960s–70s) was a package of:
- HYV seeds of wheat, rice and maize
- irrigation (canals, tube wells)
- synthetic fertilisers (especially nitrogen)
- pesticides and herbicides
- mechanisation (tractors, threshers)
Benefits: cereal yields roughly doubled or tripled, famines became rarer, food prices fell, and countries like India became self-sufficient in grain. More food from the same land also reduced pressure to clear forests.
Problems: richer farmers with water and credit gained most; small farmers fell into debt; diets narrowed to a few cereals; groundwater fell; soils lost organic matter; pollution and pest resistance rose.
Environmental impacts of farming
- Eutrophication: excess nitrate and phosphate run into lakes and rivers → algal bloom → algae die → bacteria use up oxygen → fish die.
- Salinisation: in hot, dry places irrigation water evaporates and leaves salts in the topsoil, so crops struggle.
- Waterlogging and falling water tables from over-irrigation and over-pumping.
- Soil erosion and degradation: ploughing, bare fields, overgrazing and removing hedgerows let wind and water carry topsoil away; heavy machines compact soil; organic matter falls.
- Pesticide problems: resistance in pests, harm to bees and other non-target species, bioaccumulation and biomagnification up food chains.
- Biodiversity and habitat loss: clearing forests and wetlands, monocultures.
- Greenhouse gases: CO₂ from fuel and land clearing, methane from cattle and flooded rice paddies, nitrous oxide from fertilisers.
Social and economic influences on farming
What farmers grow and how depends on more than soil and climate:
- Markets and prices: high prices for a cash crop encourage farmers to switch to it.
- Government policy: subsidies, guaranteed minimum prices, cheap electricity or fertiliser can encourage over-use of water or one crop.
- Land ownership: tiny or rented plots discourage long-term care of soil.
- Access to credit, technology and knowledge.
- Diets and demand: rising meat demand needs much more land and water than plant foods.
- Trade and supermarkets: demand for uniform, cheap produce.
Making farming more sustainable
- Crop rotation and legumes (beans, lentils, clover) fix nitrogen and break pest cycles.
- Integrated pest management (IPM): monitor pests; use natural predators, resistant varieties and traps; spray only when needed.
- Conservation tillage / no-till, cover crops, mulching, contour ploughing and terraces reduce erosion.
- Efficient irrigation: drip and sprinkler systems instead of flooding; scheduling by soil moisture.
- Precision farming: GPS and sensors apply fertiliser only where needed.
- Agroforestry and hedgerows: trees shelter soil, store carbon and house wildlife.
- Organic farming: no synthetic fertilisers or pesticides; often lower yields but better soil life.
The aim is sustainable intensification: more food from the same land with less damage.
Try it: the fertiliser experiment
Put cress or mustard seeds in four cups of soil. Water them with plain water, and with water containing a tiny, a medium and a large pinch of plant fertiliser. After 10 days compare heights: growth rises, then levels off (diminishing returns), and too much can burn roots. In the 3D free-play step, look for the fertiliser level where yield is high but the pond stays clean.
Key formulas and definitions
- Agroecosystem = managed, simplified ecosystem with human inputs
- Energy efficiency of farming = food energy output ÷ energy input
- Green Revolution package: HYV seeds + irrigation + fertiliser + pesticides + machines
- Eutrophication: nutrients → algal bloom → decomposition → low oxygen → fish die
- Salinisation: irrigation water evaporates → salts left in topsoil
Worked examples
1. Explain why short-stemmed HYV wheat suited heavy fertiliser use.
Fertiliser makes grain heads heavy. Tall traditional wheat would fall over (lodging) and the grain would rot. Short, stiff stems hold the heavy heads up, and the plant puts more energy into grain instead of stalk.
2. A river below farmland turns green in summer and fish die. Explain the chain of events.
Rain washes excess nitrate and phosphate from fields into the river. Algae grow fast (bloom) and block light. When they die, bacteria decompose them and use up dissolved oxygen. Fish and other animals suffocate. This is eutrophication.
3. Evaluate the Green Revolution in India.
Positive: wheat and rice output rose many times, famine was avoided and India became self-sufficient. Negative: gains were concentrated in irrigated regions and richer farmers; groundwater in Punjab fell sharply; soils became saline and poor in organic matter; pesticide use rose. Overall it saved millions from hunger but created sustainability problems that need new methods today.
Common mistakes
- Saying fertiliser always increases yield. Yield levels off (diminishing returns), and the extra mostly pollutes.
- Mixing up salinisation (salt left by evaporating irrigation water) with waterlogging (too much water in soil).
- Writing only benefits or only problems of the Green Revolution. Exam answers need a balanced evaluation.
- Forgetting social and economic factors. Policy, prices and land ownership shape farming as much as climate.