Ecosystems and Environmental Issues
What the exam tests
Ecosystems and environmental issues cover ecosystem structure, energy flow, ecological pyramids, nutrient cycles, pollution, greenhouse effect, ozone depletion, biodiversity loss, and conservation.
Core rules
- Ecosystems have biotic components (producers, consumers, decomposers) and abiotic components. Energy flows unidirectionally from the sun through trophic levels; the 10% law states that about 10% of energy transfers to the next level.
- Gross primary productivity (GPP) minus respiration = net primary productivity (NPP). Ecological pyramids can be of numbers, biomass, or energy; the pyramid of energy is always upright.
- Nutrient cycles include carbon, nitrogen, phosphorus, and water. Nitrogen fixation, nitrification, and denitrification are key steps.
- Pollution includes air, water, soil, and noise pollution. Biomagnification increases toxin concentration at higher trophic levels. Eutrophication is nutrient enrichment causing algal blooms.
- Greenhouse gases (CO2, CH4, N2O, CFCs) trap heat; ozone depletion is caused by CFCs. Conservation includes in-situ (national parks) and ex-situ (seed banks, zoos) methods.
Worked example
If GPP is 1000 kcal/m2/year and plant respiration is 400 kcal/m2/year, NPP = 600 kcal/m2/year. If herbivores assimilate 10% of NPP, secondary productivity = 60 kcal/m2/year. Reasoning: energy is lost as heat at each trophic level, so higher levels receive less energy. This explains why food chains rarely have more than four or five trophic levels.
Traps
Do not confuse GPP with NPP. The pyramid of biomass can be inverted in aquatic ecosystems, but the pyramid of energy is never inverted. Biomagnification is not the same as bioaccumulation. Greenhouse effect is not the same as ozone depletion. In-situ conservation protects organisms in their natural habitat.