Summer 2022

Natural Selections

Warbler bird on a tree branch.
One of the rarest forest songbird species, the Kirtland's warbler breeds each summer in Wisconsin and Michigan. The Kirtland's warbler was one of the charter members of the Endangered Species Act of 1973, but it has reached a level of population recovery such that it was taken off the list in 2019. In Wisconsin, it is considered a state-endangered species. Photo by Ashley Olah PhDx'24


As humans continue to drastically alter landscapes across the country, birds suffer the consequences. By at least one estimate, the population of North American birds has dropped by 30% in the last five decades, and declines are predicted to continue. But thanks to a team from CALS, conservationists now have a set of highly detailed and rigorous maps of bird biodiversity that could help them protect rare or threatened species.

Researchers in the Department of Forest and Wildlife Ecology developed the high-resolution maps to help conservation managers focus their efforts where they are most likely to assist birds — in individual counties or forests rather than across whole states or regions.

The maps span the contiguous U.S. and predict the diversity of birds that live in a given area, related by traits such as nesting on the ground or endangered status. Those predictions are based on both detailed observations of birds and environmental factors that affect bird ranges, such as the amount of forest cover or temperature.

“With these maps, managers have a tool they didn’t have before that allows them to get both a broad perspective as well as information at the level of detail that’s necessary for their action plans,” says Anna Pidgeon, Beers-Bascom Professor in Conservation, who worked with professor Volker Radeloff PhD’98, postdoctoral researcher and lead author Kathleen Carroll, and others to develop the maps.

The research was designed to address two outstanding problems in conservation.

Map showing the number of bird species, or overall species richness, found across the contiguous United States.
Researchers mapped the number of bird species, or overall species richness, found across the contiguous United States. Blue areas host fewer bird species than green or yellow areas. Image by Kathleen Carroll and Anna Pidgeon

“Across the world, we’re seeing huge species losses. In North America, 3 billion birds have been lost since 1970. This is across virtually all habitat types,” says Carroll. “And we’re seeing a disconnect between what scientists produce for conservation and how that translates to boots-on-the-ground management.”

Many resources previously available to conservation managers, such as species range maps, lack the detail and rigorous testing that are necessary for them to be useful and accurate.

To overcome those challenges, Carroll and her team wanted to develop data-driven maps of existing bird biodiversity. They produced the maps by extrapolating observations of birds from scientific surveys to mile-by-mile predictions of where different species really live. Those predictions were based on factors including rainfall, the degree of forest cover, and the extent of human influence on the environment, such as the presence of cities or farms.

To improve the maps’ predictive power, the scientists clustered individual species by behavior, habitat, diet, or conservation status — such as fruit eaters or forest dwellers. These groups are called guilds. Many conservation decisions happen at the guild level rather than at the species level. Guilds can also make up for limited information on the most endangered species.

The final maps cover 19 different guilds at resolutions of 0.5, 2.5, and 5 kilometers. While the finest-grained maps were not as accurate, the 2.5-kilometer-resolution maps provided a good balance of accuracy and usefulness for realistic conservation needs, say the scientists. At the 5-kilometer resolution, the maps provide the greatest accuracy and are useful to conservationists operating across large areas.

“We see this being really applicable for things like forest management action plans for the U.S. Forest Service,” says Carroll. “They can pull up these maps for a group of interest, and they can get a very clear indication of areas where they might want to limit human use.”

The maps may also help private land conservancies decide how best to allocate limited resources to maximize biodiversity protections.

Carroll is now working to extend the analysis from guilds of species to individual species. The increased level of detail could help specialist conservation managers improve their work, especially those aiming to protect a single species.

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