Unit 1: How Are Ecosystems Connected?
Core concepts
Unit 1 examines ecosystems as systems of interacting components driven by energy flow and nutrient cycling. You must be able to distinguish biotic (living) from abiotic (non-living) factors, describe producers, consumers, decomposers and detritivores, and construct and interpret food chains, food webs and trophic levels. Know that energy enters most ecosystems as sunlight captured by photosynthesis, is transferred with losses (roughly the ten-percent rule between trophic levels), and is ultimately lost as heat, whereas matter (carbon, nitrogen, phosphorus, water) cycles. Understand productivity (gross vs net primary productivity), biomass, population concepts (size, density, distribution, carrying capacity), and species interactions including competition, predation, mutualism, parasitism and commensalism. Field techniques matter: quadrats for plants, mark-recapture for mobile animals, transects for gradients, and how sampling must be random and repeated to be representative.
Traps
Questions often confuse energy with matter: energy flows one way and is lost; nutrients cycle and are reused. Students mislabel detritivores as decomposers, or assume a food web's top predator has the largest population. Another trap is treating a food chain as fixed when removing one species causes cascading effects. Mark-recapture calculations test whether you remember to use the Lincoln index and recognise its assumptions (no births, deaths, migration, or tag loss between samples).
Worked example / rule of thumb
Lincoln index: N = (M × C) / R, where M = marked in first sample, C = total in second sample, R = marked recaptures. If 40 animals are marked, then 50 are caught later with 8 marked, N = (40 × 50) / 8 = 250. Rule of thumb: always state units and sampling assumptions in your answer.