How to Estimate the Consumer Population in an Ecosystem

Mark-recapture is used to estimate populations of many species, including fish.

Every ecosystem, from grasslands to forests to wetlands to deserts, has many "consumer" species. Compiling population data about these creatures allows ecologists to ascertain the relative health of the overall ecosystem, develop appropriate management plans and investigate human impacts. Unlike producers (plants), consumers are mobile populations. Counting is a difficult process, and arriving at a precise population is impractical. One common estimation method used by ecologists and other field scientists is the mark-recapture method. This method creates a reasonably accurate population estimate with a modest investment of resources.

Things You'll Need

• Wildlife capturing equipment
• Wildlife tagging equipment

Instructions

1. Capture a predetermined number of the species being estimated. For example, shoot 20 white-tailed deer with tranquilizer darts.

2. Mark the captured animals with species-appropriate markers. Ear tags are a good choice for deer. Estimating populations using the mark-recapture method assumes the marks remain attached to the animals during the study period.

3. Release the tagged specimens back to the original area under study. Allow enough time for the tagged animals to re-integrate themselves with the animal community. This may take several hours or several days, depending on the size of the study area and the type of species.

4. Capture a larger number of the study species a second time. A larger second capture size increases the estimate's accuracy. For example, capture 200 white-tailed deer.

5. Tally the number of untagged and previously tagged individuals in the second capture. For example, of the 200 white-tailed deer, let's say 15 were previously tagged and the remaining 185 were not.

6. Calculate the population size by setting up a ratio. The number of recaptured, tagged animals divided by the total size of the second capture equals the total number of tagged animals divided by the estimated population size "x". For example, 15 divided by 200 equals 20 divided by "x". Cross-multiplication pegs "x" at approximately 266 white-tailed deer.

Tips & Warnings

• Averaging several trial estimates further improves the accuracy of the estimate.

• Estimate species not currently migrating.

• Capturing and tagging requires government permits in most locales.

• Only trained professionals should attempt capturing large and/or dangerous animals.

References

• University of Pittsburgh: Mark-Recapture History
• Northeastern Illinois University: Estimating... Animal Populations

Resources

• ExploreLearning: Estimating Population Gizmo