Quadrat and Mark-Recapture Sampling Methods
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Ecologists estimate population size by sampling: a quadrat counts individuals in marked-off areas for immobile organisms; mark-recapture estimates size from the ratio of marked to unmarked in a second sample.
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Tagging 80 deer, then finding 20 tagged among 100 recaptured, estimates roughly 400 deer total.
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Mark-recapture assumes marked individuals mix randomly back in — trap-avoidance after marking biases the estimate.