To understand how different physical or biological factors influence the distribution or abundance of species, we usually need to measure changes in population abundances over time. Ecologists interested in questions about populations and communities frequently face a very basic question: how best to estimate the size of a population of organisms in the field? However, it usually is not possible to obtain a complete count or census of a natural population of animals (and it is often difficult even for plants!). For this reason, ecologists generally have to rely on some kind of estimate of abundance or density. A variety of methods exist to do the job; the utility of each varies depending on such factors as the type of organism, the habitat in which the population occurs, and the time and equipment available.
By far the most popular way to measure the size of a population is called the Mark and Recapture Technique. This technique is commonly used by fish and wildlife managers to estimate population sizes before fishing or hunting seasons. The marking recapture method involves marking number of individuals in the natural population, returning them to that population, and subsequently recapturing some of them as a basis for estimating the size of the population at the time of marking and release. This procedure was first used by CJG Peterson in studies of marine fishes and FC Lincoln in studies of waterfowl populations, and is often referred to as the Lincoln Index or the Peterson Index. It is based on the principle that if the proportion of the population was marked in someway, returned the original population and then, after complete mixing, a second sample was taken, the proportion of marked individuals in the second sample would be the same as was marked initially in the total population.