Capture-Mark-Release-Recapture Estimates Populations of Moving AnimalsMotile animals such as birds, fish, insects, and mammals move constantly, so they cannot be counted in place.Ecologists instead use the capture-mark-release-recapture (CMR) method to estimate population size.It gives a reliable estimate from a sample and lets researchers track a species over time.AnalogyImagine trying to count every butterfly in a forest by eye.They never stay still, so a direct count is hopeless.CMR and the Lincoln index give a workable answer instead.The Four Steps of the MethodCaptureCatch a first sample using traps, nets, or similar techniques.Record the number caught and marked, called n1.MarkMark each individual in a harmless, recognisable way, such as leg bands, non-toxic dye, or tags.The mark must not affect survival, behaviour, or the chance of being caught again.It must also last until the second capture.ReleaseReturn the marked individuals and allow time for them to mix fully back into the population.RecaptureTake a second sample and record its total size, called n2.Count how many of these are marked, called n3.ExampleResearchers estimate sea turtle numbers by tagging individuals during nesting and recapturing them in later years.The data guides conservation decisions.Capture-mark-release-recapture method.The Lincoln Index Turns the Counts Into a Population EstimateThe Lincoln index estimates total population size N from the three counts.N = (n1 x n2) / n3n1 is the number marked in the first capture.n2 is the total number in the second capture.n3 is the number of marked individuals found in the second capture.ExampleMark 100 individuals, then catch 50 of which 10 are marked.N = (100 x 50) / 10 = 500 individuals.Getting the Calculation RightTipDouble-check n3, the number of marked recaptures.Miscounting it throws off the whole estimate.The Estimate Relies on Assumptions That Must HoldNo change to the population: no births, deaths, immigration, or emigration occur during the study.Marks do not affect the animal: the mark does not harm the individual or change its chance of being recaptured.Marked individuals mix fully: released individuals spread evenly through the population before the second capture.Marks last: no marks are lost between the two captures.Common MistakeDo not assume this method works for all organisms.It only suits motile organisms that can be safely captured and released.Why the Method Matters for ConservationNon-invasive: it estimates numbers without harming individuals.Useful for conservation: it tracks endangered species such as tigers and whales.Suited to moving animals: it works where quadrat sampling of sessile organisms cannot.ExampleScientists estimate tiger numbers in India using camera traps, where each tiger's unique stripe pattern acts as its mark.Active recallWhy is CMR used for motile animals rather than quadrat sampling?Write the Lincoln index formula and define n1, n2, and n3.If 100 are marked and a second sample of 50 contains 10 marked, what is the estimated population?State three assumptions the Lincoln index depends on.Why must a mark not affect an animal's chance of being recaptured?
