Estimating Wildlife Population Numbers

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Recent estimates put the population of white-tailed deer between 26-28 million in the United States.  The worldwide polar bear population is between 20,000 – 25,000 animals and the wild tiger population is down to only about 3,000 animals. Like the white-tailed deer, polar bears, and tigers, biologists try to have an estimate of how many individuals are in every species population. Estimates are needed for biologists to properly manage species, set harvest numbers, and have some indication of wildlife health. However, completing a census of every individual in a population is neither possible nor practical.

Many animals are secretive, well camouflaged, and difficult to count. Therefore, most wildlife agencies monitor relative abundance, not absolute or actual numbers of animals. When monitoring relative abundance, the most important consideration is whether the population trend is increasing, decreasing, or stable; not how many are in an area. Biologists often use mathematical models to help generate a representation of the population.

Wildlife biologists rely on various sample census techniques to estimate wild populations. Different types of surveys are completed for different species in order for the relative abundance to be measured. Some of these techniques include mark-recapture, aerial surveys, audible (howl, roar, call) surveys, footprint surveys, roadside wildlife counts, road kill counts, DNA capture, and trail camera surveys. Each of these methods has its own limitations and options that can influence the population estimate.

Mark-recapture techniques are most commonly used for migrating birds, but can be used for various species, including white-tailed deer.  Animals are first captured, marked for identification by banding, tagging, or painting, and then released back into the population. Later, another capture is completed and the proportion of marked individuals is counted. With this information, the total population size can be estimated by dividing the number of marked individuals by the proportion of marked individuals in the second sample.

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Aerial surveys are particularly useful for species that are widely spread over large areas. Elephants are often surveyed using this method. These counts are completed by flying aircraft over a known distance and counting the number of individuals seen according to various methods. This gives biologist a measure of density for the area surveyed, which can be expanded to a larger area of land. However, errors in counting can occur, thus leading to an over or under estimation of the population.

For white-tailed deer and other game species, biological information of animals is taken at the time of harvest and used to assess population size and trends. For example, age from teeth, average diameter of antlers, body fat thickness, and even same year pregnancy from ovarian scars can all assist with determining whether a population is healthy or unhealthy, increasing or decreasing. The sheer numbers of harvested animals within a given period of time, compared with numbers of hunting licenses sold in state, provide an understanding of trends and hunter success rate. For example, with a fixed number of hunting licenses sold, harvest generally increases when there are more animals available to be harvested.

As you can see, the available techniques to assess the numbers of wild animals are extensive. Each method has several variations based on the species, time of year and habitat type. Before conducting a survey, biologist need to understand what information is needed, for what purpose the information will be used, how precise an estimate is needed and the time and cost of conducting the survey.  The key to deriving population abundance estimates is the ability to select a method that fits a particular situation. And, we’ve learned that long term datasets, where we have collected the same kind of information from populations over a long period of time, give us the best understanding of what is happening.  Overall well planned surveys enable wildlife biologist to manage the needs of wildlife and people using the best available science.

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