2017-10-05 / Opinions

Estimating population abundance

Agriculture and Natural Resources Agent

The first step behind the science of wildlife management is observation. In order for managers to determine what management techniques they need to implement a relative population abundance of the species being managed for needs to be known. While complete or partial direct counts are favored where terrain, visibility, manpower, and budgets permit, determining absolute densities of animals is complex and often controversial. Furthermore, many species are simply too elusive or cryptic for humans to observe so direct counts are impractical; therefore researchers must rely on indirect signs, such as tracks, scat/pellet, or den sites to estimate abundance.

In order to estimate a relative abundance per unit area managers and researchers often use random sampling, stratified sampling, or systematic sampling. A relative abundance is not an absolute count; rather, it is an index used to measure the relative population density from one given point to another. The type of sampling techniques utilized must be able provide a representative sample while reducing variation between observation events and is therefore dependent upon the species being sampled. For those more secretive and elusive animals – like coyotes, foxes, and bobcats – systematic sampling methods like line transects and belt transects are most often used in which scats/pellets, tracks, or den sites are collected or counted.

While the observation/collection of tracks, scats/pellets, or den sites are imperative for determining relative abundance of those most elusive species, these observations are not made randomly. In order to estimate a population abundance per unit area a transect of known length is chosen that covers all habitat types in the given study area. Often times game trails or ATV trails/roads can be used as a transect since these trajectories tend to cross the entire study area: wildlife tend to use these areas as travel corridors because they provide a path of least resistance. Scat/pellet surveys are as simple as walking the transect and collecting all the scats/ pellets found on the transect. The process is repeated two weeks later. If your transect is one mile long and after two weeks you found five new scats, your scat deposition rate would be 5 scats/14 days = 0.36 scats per day, per mile. The number of scats collected is used as an index of relative abundance for the species being sampled. Using the same transect, active denning sites can be identified and recorded, providing an index of species abundance.

Similar to scat/pellet surveys, track counts also use transects. Stations are set up at predetermined intervals along the transect and a one meter circle is established at each station. The circle is most often coated with powdered lime or sand so that tracts can easily be visible. This type of survey often uses scent tablets to attract the species being sampled. Stations should not be placed directly on the transect in order to prevent bias and unwanted tracks. Additionally, each location should be checked every day for 3-5 days, individual tracts counted, and sand or lime being smoothed out each time to provide a clean slate for the next day. The study area can also be gridded and random grids can be chosen and scent stations established in the center of these areas. All data is recorded in the same manner as it would be in a line transect survey.

It is important to understand that the surveys that have been described here only provide relative abundance, not an absolute number. Keeping up with the data collected from year to year can allow managers to establish trends in a given species population. For example, if in one year five scat samples were along a transect and the following year eight samples were collected, it could be assumed that there was an increase in population density. If over a span of five or 10 years the number of samples collected seemed to rise and fall it may be assumed that the population was cycling above and below the carrying capacity for that area. So not only is it important to keep up with the data, but it is also crucial to conduct counts over the same transects at approximately the same time each year so as to reduce variation and bias.

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