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Estimating Abundance Using Genetic Mark-Recapture

A key component of sound conservation and management of bears is knowing how many bears live in each of Florida’s seven subpopulations. Bears cannot be directly counted because they are generally secretive, live in dense vegetation, lack distinguishing characteristics, and wander over large areas. Instead, the FWC uses a statistical method to estimate bear numbers known as mark-recapture.

To illustrate this method, imagine a large bucket of red marbles for which we need to estimate the number. The only marbles visible are the ones on the surface so a direct count is not possible. However, if we removed, say, 100 marbles, marked them with white paint, put them all back in the bucket and shook up the bucket, then removed a second sample of 100 marbles, some may be marked white. In this example, 10 of 100 marbles (10%) are marked white. By observing the proportion of “marked” marbles to “unmarked” marbles in the second “recaptured” sample, we can estimate the number of marbles in the bucket by asking: 10% of what number equals 100? In this case, we can estimate that we have 1,000 marbles in the bucket.

The estimate of marbles in the bucket is a statistic, because the true abundance of marbles is unknown and our estimate includes some level of error. To reduce error and get closer to the true abundance, we can repeatedly recapture 100 marbles many times so that the average of the abundance estimates will get closer and closer to the true number. This is how we estimate bear abundance in its simplest form. If we can “mark” a sample of bears, then “recapture” some of the marked bears repeatedly, we can estimate how many bears are in a region.

The FWC marks and recaptures bears noninvasively using genetics. Researchers attract bears to a small barbed wire enclosure known as a hair corral, obtain tufts of hair left behind, and analyze the DNA in the hair follicles to identify individual bears. Identified bears are marked bears. They then refine the abundance estimate by using sophisticated statistical models (spatially explicit capture-recapture, or SECR) that can incorporate the probability that a bear was detected, certain bear home range characteristics, behavioral differences among bears, and habitat types surrounding the hair snare.

The FWC research methods result in rigorous and reproducible bear abundance estimates to inform the FWC Bear Management Program and interested stakeholders. The FWC plans to repeat estimates of abundance for each bear subpopulation every 10 years so that trends in abundance can be identified and quantified.

Counting Bears

This video outlines the process for surveying Florida’s black bear population and provides a simplified example of the mathematical equation scientists use to estimate bear numbers.