Ecology: Populations, Communities, Ecosystems and Human Impact
Numbers and definitions
Ecology items combine arithmetic (growth, energy transfer) with definitional precision (population versus community versus ecosystem) and applied human impact. Define the level of organisation before you answer.
Core rules
- Populations: exponential growth when resources are unlimited; logistic growth with carrying capacity K; density-dependent factors (food, disease) versus density-independent factors (fire, drought).
- Interactions: competition (−/−), predation and parasitism (+/−), mutualism (+/+), commensalism (+/0); fundamental versus realised niche; competitive exclusion.
- Communities and ecosystems: trophic levels, roughly 10 % energy transfer, food webs, keystone species, primary and secondary succession, decomposers returning nutrients.
- Cycles: carbon (photosynthesis, respiration, combustion), nitrogen (fixation, nitrification, denitrification).
- Human impact: greenhouse gases, eutrophication (nutrients → algal bloom → decomposition → oxygen depletion), habitat fragmentation, invasive species, conservation of biodiversity.
Worked example
Producers fix 10,000 kcal m⁻² yr⁻¹. At 10 % transfer, primary consumers receive 1,000, secondary consumers 100 and tertiary consumers 10. Reasoning: energy is lost as heat at every step, which is why food chains are short and top predators are few; matter cycles, while energy flows one way.
Marks lost
Writing that energy cycles; confusing community with ecosystem; assuming predators always control prey numbers regardless of other factors; ignoring that eutrophication kills fish through oxygen depletion after decomposition, not through the nutrients themselves.