A technique for estimating the population density of more elusive or mobile animals. A sample of the population is captured, marked, and released. Assuming that these marked individuals become randomly distributed through the wild population, and that subsequent trapping is random, any new sample should contain a representative proportion of marked to unmarked individuals. From this, the size of the population may be estimated, most simply by multiplying the number in the first sample by the number in the second sample, and dividing the product by the number of marked individuals in the recaptured sample. This calculation is appropriate only when the population is fairly static or where changes (owing to migration, natality, or mortality) are known. It may also be distorted if marked individuals become more vulnerable to predators and if particular individuals become ‘trap-addicted’ or ‘trap-shy’. When a population is fluctuating rapidly, as is common in insect populations, more sophisticated indices, which allow for the probabilities of change, are preferable.
Subjects: Ecology and Conservation.