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Deer Population Surveys - How Good Are They?

Posted Oct. 31, 2007

Chances are that if you are managing deer on your property, you have probably been encouraged to conduct some type of population survey.

There are numerous types of surveys, including track counts, infrared-triggered camera surveys, aerial surveys with helicopters and, probably the most often used around here, spotlight surveys. These techniques result in collecting data relative to deer numbers (density), sex ratios, reproductive rates, etc. These techniques can provide valuable insight about deer populations, but it is important to remember that these techniques are surveys - the numbers generated using the data are estimates of population parameters, not exact numbers. Accuracy varies by technique and is not even constant for a given technique. There is typically so much "noise" in survey data that it is difficult to detect small population changes from year to year. However, over time, significant changes may be detected using trend analysis, which can facilitate qualitative assessments of the population. Bottom line - a manager needs to realize the limitations of population surveys when interpreting results.

I am going to limit my remarks here to deer density estimates generated from spotlight surveys. Many people interpret the numbers as absolutes, when, in fact, they are generalities at best.

For example, the following figures depict deer density estimates derived from spotlight survey data on two hypothetical ranches. Chart 1 depicts a population with some fluctuations in the density estimates from year to year. A literal interpretation of the data would imply that in year one the deer density was one deer per 20 acres. The following year, the density declined to one deer per 23 acres. The next year, the density increased to one deer per 19 acres, followed by another increase to one deer per 18 acres and so on. Unfortunately, the technique is not sensitive enough to accurately detect small changes over a short time span. Since the magnitudes of the differences are relatively small and the differences are not sustained over a period of time, the logical conclusion is that the deer population in this example is relatively stable. The differences reflected in the estimates may be real, or they may be artifacts of the inherent inaccuracies of the technique. The fact is, we just cannot determine the nature of the differences based on the data collected.

Chart 2 depicts a somewhat different scenario. The overall trend is a decline in deer density estimates. There are still some fluctuations among years, but, relative to the population estimates for the first several years, subsequent estimates reflect a decline in deer density, and that decline is sustained over a period of time. From these data, a manager still does not know exactly how many deer are on an area or the exact magnitude of the population decline, but there is strong evidence of a decline. This is important information for a manager whose goal was to reduce herd size. This indicates that the management strategy is working.

Population surveys cannot determine actual deer numbers, but, over time, they can provide important population trend information. They are particularly important for someone initiating a management program in that they provide some insight of where to begin with your harvest strategy. Just be careful not to read too much into them.

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