Still for other species, digital photographs and automated image analysis of fin scars (whales) or pigment patterns (cheetahs) can be used like fingerprints to identify and track individuals. For some species, a radio tag can be used to continuously track location (white sharks, mountain lions). It might a bracelet (bird banding), a collar (wolves), an ear clip (antelope, deer), a fin clip (fish), a paint dot (insects), or a biodegradable dermal injection (salamanders). The type of “mark” in these studies can vary depending on the species. Ecologists can also use statistics to measure their confidence in the estimated population size. The proportion of recaptured marked individuals is then used to estimate population size. On the second survey, which could be the next day or the next year(s) depending on the study, ecologists capture another set of individuals from the population to determine the proportion that are marked. On the first survey, ecologists capture, mark, and release individuals back into the population. In its simplest form, this type of study requires a pair of surveys. Mark-recapture techniques allow ecologists to track movement of individuals in space and in time through a population. In this study, we will use a third type of census method known as “mark-recapture” that’s used to study highly mobile organisms like fish, birds, and mammals.
Surveys or transects, meanwhile, can be employed to measure larger organisms and larger areas. Smaller areas can be carefully sampled using the quadrat method, which works well for sessile or slowly moving organisms. There are many ways to perform a population census, each method having benefits for particular scenarios as well as underlying assumptions about the study organism. Reliable estimates of population abundance are also at the core of managing natural resources, protecting endangered species, and mitigating environmental impacts of habitat loss to residential, commercial, or industrial uses. An accurate census informs us of changes in population size due to recruitment (births), mortality (deaths), and migration. Estimating the abundance of organisms, especially in naturally occurring populations, is a fundamentally important activity in ecological research.