An ecologist is monitoring the population of endangered white-clawed crayfish (Austropotamobius pallipesAustropotamobius\ pallipesAustropotamobius pallipes) in a chalk stream. In the first sampling session, she captures 120 crayfish, marks their carapaces with a non-toxic water-insoluble paint, and releases them back into the stream. One week later, she conducts a second sampling session at the same site and captures 90 crayfish, of which 18 are found to have the paint mark.
Using the Lincoln Index, what is the estimated population of crayfish in this section of the stream?
150150150
480480480
600600600
242424
67 exam-style questions on OCR GCSE Biology 4.1 Ecosystems, covering 4.1.1 Cycling of materials in ecosystems, 4.1.2 Microorganisms in cycling materials, 4.1.3 The carbon and water cycles, 4.1.4 Factors affecting decomposition, 4.1.5 Levels of organisation in an ecosystem, 4.1.6 Abiotic and biotic factors affecting communities, 4.1.7 Interdependence and competition, 4.1.8 Trophic levels, 4.1.9 Pyramids of biomass, and 4.1.10 Efficiency of biomass transfers. Each one has a worked solution and a mark scheme showing where the marks go.