Ecology
Ecology examines ecosystems, energy flow, productivity, nutrient cycling, population dynamics, species interactions and succession. Questions often ask you to interpret pyramids, graphs and energy budgets.
Core rules
- Energy enters most ecosystems as light and is converted to chemical energy by photosynthesis; it is lost as heat at each trophic level.
- Gross primary productivity (GPP) is total photosynthesis; net primary productivity (NPP) = GPP − respiration.
- Ecological efficiency is the percentage of energy transferred between trophic levels; it is usually low (about 10%).
- Nutrient cycles are biogeochemical: carbon, nitrogen and phosphorus move between organisms and abiotic reservoirs; decomposers return nutrients.
- Population growth can be exponential when resources are unlimited, but logistic growth slows as carrying capacity is approached.
- Interactions include competition, predation, parasitism, mutualism and commensalism.
Worked example
A grassland has GPP of 50,000 kJ m−2 yr−1 and plant respiration of 20,000 kJ m−2 yr−1. NPP = 50,000 − 20,000 = 30,000 kJ m−2 yr−1. If herbivores assimilate 3,000 kJ m−2 yr−1, ecological efficiency = (3,000/30,000) × 100% = 10%. Reasoning: always divide by the energy available at the lower trophic level, not by GPP, when calculating transfer efficiency.
Traps
- A pyramid of numbers can be inverted (e.g., one tree, many insects); an energy pyramid cannot.
- NPP is not the same as biomass; biomass is standing crop.
- Exponential growth has a constant per capita rate; logistic growth has a decreasing rate as N approaches K.