Population index

Population indexes are usually species-specific and are based on the number of animals, or their sign, detected for each unit of effort. The effort can be measured in many ways, including number of miles/kilometers walked, number of traps set, number of trees examined, or number of hours observed. The index itself is usually tied to specific traits of the species: for example, claw marks on trees made by Asiatic black bears (Ursus thibetanus), calls by howler monkeys (Alouatta spp.), night nests of gorillas (Gorilla spp.), and latrines of rhinos are indexes that are not broadly applicable to other species. Indexes based on sign are conducted along transects of known length, with all sign within a set distance of transect recorded, or a predetermined area is searched for sign. When interpreting signs, one must have observed how such signs are actually created by the animal, as well as which activities do not leave any signs at all (i.e., a deer feeding on dead leaves shed by trees or small bits and pieces of conifer branches or lichen or small fruit leaves no trace at all). When the signs are not permanent, such as fecal pellets or tracks, and the area is to be resurveyed at a later date, the boundaries of the area can be marked and all sign cleared from the area at the end of each survey. When traps or cameras are used during a survey, the measure of effort is usually expressed as trap-nights (or camera-nights). This is the number of traps (or cameras) set each night multiplied by the number of nights. Using this criterion, a small number of traps used for a long period would be roughly equivalent to the effort of a large number of traps used for a short period.

It is the rare index for which the number of signs can be directly converted into the number of animals. This is because there are assumptions that must be used for each conversion. For instance, to convert the number of pellet groups detected to the number of deer, there must be estimates of how many pellet groups each deer produces daily and how rapidly the pellet groups degrade. Both these measures have a variance that is so large any resulting density estimate is meaningless. For most indexes, it is also difficult to determine if increased signs indicate increased numbers or increased activity. For example, would the detection of six pellet groups mean six deer used the area once, or that one deer used the area six times?

As it is tailored to a specific species, a good index can be quantified and it is likely that two biologists can compare results. The power of an index is to give relative comparisons between sites or periods for a minimal amount of work. Some effort must be made to verify that the index used reflects changes in density over the range measured. There is also the danger that an increased number of sign does not reflect more animals but rather shifts in habits such as diet or habitat. One must question whether seasonal increases in deer pellet groups in an old field represent an increase in the number of deer or a shift in habitat use by the same number of deer. If all habitats are being monitored simultaneously, it is possible to differentiate between shifts in habitat use and shifts in abundance.

No index can work under all circumstances, so a pilot study that measures both density and the index is preferable to making assumptions of correlation. Few indexes have a linear relationship with density over its entire range, as usually an index flattens out as density increases beyond a certain range. For example, an increase in the number of subadult or non-reproductive individuals may not be reflected in an index based on the number of morning calls by adult howler monkeys. It is important for the research or monitoring to demonstrate that the index is responsive to changes in density over the range. For many species, verification of an index has already been accomplished and a review of relevant literature

A biologist weighs a bear cub to record its weight. (Photo by © Raymond Gehman/Corbis. Reproduced by permission.)

is recommended before undertaking an index survey. In short, good preparation by observing animals is required before collecting data and making inferences from them.

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