Population size can be estimated using: randomly placed quadrats or quadrats along a belt transect for slow-moving or non-motile organisms; the mark-release-recapture method for motile organisms
Scientists use two main field techniques to count organisms. They use quadrats for plants and slow-moving animals, and mark-release-recapture for animals that move around freely.
Formula
Estimated population = (n₁ × n₂) ÷ n₃
Real World
Ecologists at the Wildfowl & Wetlands Trust use mark-release-recapture with leg rings on migrating Bewick's swans to estimate the total European population each winter.
Exam Focus
In calculation questions, define n₁, n₂, and n₃ clearly before substituting — misidentifying samples is the most common error.
Evaluation Scaffold
A four-step framework for high-quality evaluation. Use this for 'assess', 'evaluate', and 'to what extent' questions.
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