Ecology and Environment
Ecology and Environment tests how organisms interact with each other and with their surroundings, how energy and nutrients move through ecosystems, and how human activity alters them. Expect food webs, pyramids, population graphs, sampling calculations and questions on fertiliser pollution or conservation.
Energy flow and trophic levels
- Energy enters as sunlight, is fixed by producers and passes to consumers; only about 10% transfers to the next trophic level.
- Losses occur as heat from respiration, plus egestion and undigested material.
- Pyramids of energy are always upright; pyramids of numbers can invert, for example one oak tree supporting many insects.
- Net primary productivity = gross primary productivity − respiration.
- Decomposers release nutrients back into the soil; food webs show interdependence, and removing one species has knock-on effects.
Nutrient cycles
- Carbon: photosynthesis, respiration, combustion, decomposition and fossilisation.
- Nitrogen: fixation (Rhizobium, lightning, industrial), nitrification by Nitrosomonas and Nitrobacter, assimilation, ammonification, denitrification.
- Phosphorus has no gaseous phase; it is released slowly by rock weathering.
Populations
- Exponential growth gives a J-curve; logistic growth levels off at the carrying capacity.
- Density-dependent factors include disease and competition; density-independent factors include fire and drought.
- Predator and prey populations cycle with a lag.
- Sampling uses random quadrats, transects, and mark–recapture for mobile animals.
Succession and human impact
- Primary succession: pioneer species colonise bare rock, soil forms, diversity rises towards a climax community; secondary succession is faster after a disturbance.
- Eutrophication: nitrate runoff → algal bloom → algae die → decomposers multiply and respire → oxygen falls → fish die.
- Other impacts: greenhouse gases, deforestation, invasive species, overexploitation; conservation strategies protect biodiversity.
Worked example
40 woodlice are caught, marked and released. Next day 50 are caught and 8 are marked. Lincoln index N = (40 × 50) ÷ 8 = 250. State the assumptions for full marks: marks do not affect survival or catchability, marked animals mix randomly, and there is no birth, death or migration between samples. If marked animals were easier to recatch, the estimate would be too low.
Traps
- Claiming energy pyramids can be inverted.
- Confusing nitrogen fixation with nitrification.
- Saying 10% of energy is lost rather than transferred.
- Forgetting the assumptions behind mark–recapture, or the order of the eutrophication sequence.