CHADRON – Chadron State College Assistant Professor Dr. Aaron Field explained the historical distribution of black-tailed prairie dogs, attempted management practices, and criteria that qualify them as a keystone species in his Graves Lecture April 23.
Field said before European settlement, black-tailed prairie dog colonies covered approximately 150 million-acres across the middle of the U.S. from Canada to Mexico. Since then, due to grassland conversion and widespread poisoning, their range has decreased to about 1.5 million acres in 1960. Field estimated that their current range might be approaching 3 million acres.
Although keystone species is a poorly understood term, Field said scientific prairie dogs qualify for the designation because the species defines an ecosystem and it promotes bio-diversity.
“We have to ask when determining a keystone species if another species or humans replicate what they contribute to the ecosystem,” Field said.
A 1999 study Field cited found 200 species use the burrows of prairie dogs, 117 have some relationship with them by either using burrows or being their predators or prey, and eight species depend on them. The black-footed ferret, an endangered species, is being bred in captivity, and burrowing owls, listed as a species of conservation concern, both use the burrows made by prairie dogs. Field asked the audience if it makes sense to reduce the prairie dogs while trying to protect two other species that interrelate with them.
A prairie dog's diet is up to 80 percent grass, a fact that places them in competition with livestock. This competition has been a main point used by livestock producers to justify poisoning prairie dogs, but Field said this impact has not been adequately studied.
A 1982 study in Oklahoma determined that loss of forage to prairie dogs resulted in less weight gain that translated into $14 to $24 less per steer at market. In 2006, research conducted near Cheyenne, Wyoming, indicated that where 20 percent of grazing land was occupied by both cattle and prairie dogs, steers gained less weight resulting in a $15 loss per head at market and a loss of $38 per head where 60 percent of grazing land was occupied by both species.
Field said these two studies still represent a lack of thorough and extensive research.
“There are many factors to consider including scale, grazing density, livestock's ability to move to range not occupied by prairie dogs and the number of livestock grazing the area,” Field said.
He said benefits from the prairie dogs include increasing floral abundance by spreading seed, reintroducing nitrogen into the soil from their urine and droppings and, assuming adequate rainfall, stimulate new, more palatable plant growth available for cattle by eating old growth.
Since prairie dogs need to live in a short grass setting to see and avoid predators, they don't pose competition in areas with tall grass, Field said.
He offered two scenarios of poison application, with different sets of variables. One that might be financially viable with the cost of poison paid back in seven years, and a second that would not provide a financial return. He said poisons can also kill untargeted animals and affect ground nesting birds and that recreational shooting of prairie dogs is not practical on a large scale.
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