Counting Alaska's Brown Bears from the Air, for Less

Counting brown bears across the vast parks of northwestern Alaska is notoriously expensive and difficult. A study by National Park Service scientists found a way to do it for about 20 to 30 percent less, without losing accuracy — by paying attention to where bears are likely to be.

A brown bear and cub seen from the air. Credit: Jared Hughey, National Park Service.

Mark-resight from the air

The Arctic Inventory and Monitoring Network counts brown bears with a non-invasive, aerial "mark-resight" survey. Observers "mark" bear groups by their location, coloring and group makeup, so the same groups can be recognized — "resighted" — on a second pass. The resightings show how many groups the survey missed overall, giving an estimate of how many bears there are without capturing or tagging any.

Adding the map

Standard surveys ignore spatial patterns. The researchers extended the model to include information about the landscape and to account for unexplained clustering, using survey data from northwestern Alaska from 2015 to 2021.

• Seeing bears: bears were more likely to be detected in more rugged terrain.
• Finding bears: bears were more likely to occur at middle elevations, where there is more denning habitat, and in areas with more salmon streams.
• Something else too: the leftover clustering showed that other, unmeasured factors also shape where bears live.

None of this was surprising, the authors say, but it is very useful for planning.

A cheaper survey

In a hypothetical redesign of the 2021 survey, they dropped half of the survey units with below-average elevation, where bears were least likely to be. The estimates stayed unbiased. Adopting the spatial approach could cut project costs by about 20 to 30 percent — making brown bear monitoring more sustainable, and giving more information about the populations.

Sources

Based on "Mark-Resight Brown Bear Survey Methods," National Park Service Arctic Inventory and Monitoring Network, a work of the United States government in the public domain. The study: J. H. Schmidt, W. W. Deacy, L. J. Hughes and D. T. Schertz, "Non-invasive Mark-Resight Surveys for Brown Bears: Incorporating Spatial Information to Improve Landscape-Scale Monitoring of Density and Distribution," Ecological Solutions and Evidence 4, no. 4 (2023): e12288.