Population SizeA population is a localized group of individuals of the same species. In somesituations populations have easily defined boundaries (e.g., Burr Oaks in Thatcher Woods or Northern Pike in the Des Plaines River); whereas, in other instances, the boundaries are almost impossible to define so they are arbitrarily set by the investigator's convenience (e.g., Grey Squirrels on Triton's campus). Once the boundaries are set, the next challenge is determining a population's size. Occasionally you may be able to actually count all the individuals of a population, but this is rarely done. Usually population size is estimated by counting all the individuals from a smaller sample area, and then extrapolated to the set boundaries. Another common method is mark-recapture. Using this method, a small random sample of the population is captured, marked, and then released to disperse within the general population. Next a subsequent random sample of the population is recaptured. The ratio of marked to recaptured individuals in the second
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