Ecology, ecosystems, biodiversity and human impact
Ecology studies how organisms interact with each other and their environment. The exam tests levels of organisation, food webs, energy flow, nutrient cycling, biodiversity and the effects of human activity.
Levels of organisation
Organism → population (one species in an area) → community (all the populations) → ecosystem (community plus the physical environment). A habitat is where an organism lives. Biotic factors include competition, predation and disease; abiotic factors include light, temperature, pH, water and oxygen. Organisms show adaptations to these conditions.
Feeding and energy flow
Producers photosynthesise; consumers eat; decomposers break down dead matter and recycle nutrients. A food chain arrow points in the direction of energy flow (grass → rabbit → fox). About 10% of energy passes to the next trophic level; the rest is lost as heat from respiration, in uneaten parts and in waste. Food chains are therefore short and top predators are few. Pyramids of numbers show numbers at each level.
Nutrient cycles
Carbon moves between the atmosphere (CO₂), photosynthesis, respiration, combustion of fossil fuels and decomposition. Nitrogen is fixed by bacteria and returned to the soil by decomposers.
Worked example
A grassland contains 20,000 kJ of energy in grass. At 10% transfer: rabbits receive 2,000 kJ, foxes 200 kJ and a top predator 20 kJ. This explains why a top predator needs a large territory.
Biodiversity and human impact
Biodiversity is the variety of species and genetic variation. Threats include habitat loss, invasive species, pollution, overexploitation and climate change. Conservation includes protected areas, hedgerows, reducing fertiliser runoff to prevent eutrophication, and reforestation.
Traps
- Arrows drawn in the wrong direction.
- Blaming all energy loss on 'not being eaten' rather than respiration and waste.
- Confusing a population with a community.