Population estimation techniques combine a count of animals with an estimate of the fraction that have been missed and produce an estimate of the total number of animals out there along with a set of statistically determined upper and lower bounds to that estimate.

Many of what are considered the traditional techniques for estimating population size require a series of periods in which animals are captured and marked or captured and removed (temporarily). Such techniques are still very much valid, but often are often too costly to implement solely as a means of tracking changes over time.

Newer approaches now often use less invasive and intensive approaches. Examples are the estimated distance from observer to animal, sampling by more than one observer at a time, or the recording by time interval in which an animal was detected. In addition to not having to deal with the problems and trauma of handling animals directly these techniques can be applied to many of the traditional visual counting techniques such as point counts or transects. The results are estimates of differences in species detectability or observer differences. While usually less time-consuming than the traditional mark-and-recapture techniques, these techniques all require at least some additional time in the field or the office than the comparable index. Additionally, for many small projects or monitoring programs there may not be sufficient samples to enable calculation of useful estimates of detectability. Finally, as with using an index, each population estimation technique comes with a series of assumptions that must be met in order to produce a valid estimate.

There are many estimation techniques available, as well as a very large literature. Below is a list of texts that summarize the current techniques and can provide a guide to their use. Below that are links to web sites that carry software packages for analyzing these data as well as further information about the techniques themselves.

Bibliography

Buckland, S. T., D. R. Anderson, K. P. Burnham, J. L. Laake, D.L. Borchers, and L. Thomas. 2001. Introduction to distance sampling. Oxford University Press, Oxford, UK.

Lancia, R.A., J.D. Nichols, and K.H. Pollock. 1994. Estimating the number of animals in wildlife populations, p. 215-253. In T. Bookhout (ed.) Research and Management Techniques for Wildlife and Habitats. The Wildlife Society, Bethesda, Maryland.

Otis, D.L., K. P. Burnham, G. C. White, G. C. and D. R. Anderson. 1978. Statistical inference from capture data on closed animal populations. Wildlife Monographs 62.

Pollock, K. H., Nichols, J. D., Brownie, C. & Hines, J.E. (1990) Statistical inference for
capture-recapture experiments. Wildlife Monographs 107.

Seber, G. A. F. 1982. Estimation of animal abundance and related parameters. Macmillan, New York.
Thompson, W. L., G.C. White & C. Gowan. 1998. Monitoring vertebrate populations Academic Press, San Diego, CA.

Williams, B. K., J. D. Nichols and M. J. Conroy. 2002. Analysis and management of animal populations. Academic Press, San Diego, California, USA.

Population Estimation Software Sites

Patuxent Wildlife Research Center Software Archive

Colorado State University Software Archive

Population Analysis Software Group

PhiDot Links to Population Software Sources

Pisces Conservation Software

Clearinghouse for Ecological Software