ally needs to be observable or caught on a regular basis. It also helps if natural or added markings allow individuals to be identified. The two main techniques to estimate absolute density without changing the density are either mark/recapture or distance sampling. Mark/recapture uses an initial capture period that results in a known number of marked animals. A second capture period then looks at the ratio of marked to unmarked animals and estimates how many animals the population contains. Mark/recapture is based on the two assumptions that all animals can be captured and capture does not influence the probability of future captures and that animals can be marked and the marks will not fall off or influence the probability of the animal to be recaptured. There is also an assumption that no mortality, natality, or immigration occurs between capture sessions, but it is possible to relax this assumption and still estimate density.
Removal sampling allows the observer to estimate a population's density post-hoc. The number of animals removed for each unit of effort, or a change in the ratio of two classes of animals (i.e., antlered and antlerless deer), is the basis for most surveys of harvested large mammals. For both calculations, the advantage is that the researcher is not necessarily the one doing the removal. Large areas and large populations can be estimated using volunteer hunters that agree to follow simple rules such as harvesting only antlered animals. The disadvantage is that there are limited species that attract a large number of volunteer hunters and the removal is limited to populations that are robust enough to sustain a harvest. It is possible to use the catch-per-unit-effort technique without harvesting animals, but the observer must be able to identify animals that have been previously sighted. There are several statistical programs that are commonly used to analyze this type of data and details of the programs and their assumptions are available at <http://www.cnr.colostate.edu/~gwhite/ software.html>.
How to calculate species density estimates?