BILLINGS, Mont. — Black-tailed prairie dogs were denied protection under the Endangered Species Act after federal officials concluded the once-prevalent species shows signs of rebounding.

Decades of poisoning, shootings, the plague and loss of habitat to agriculture are blamed for a dramatic drop in prairie dog numbers since the early 1900s, from roughly 1 billion animals to an estimated 24 million today.

In 2007, the New Mexico-based environmental activist group WildEarth Guardians petitioned the U.S. Fish and Wildlife Service to list the animal as threatened or endangered.

But the agency said last week that the population is slowly spreading, despite continued pressure from sickness and deliberate killings. “They reached a low point in approximately 1961 and have bounced back pretty good since then,” said Joy Gober, the biologist who drafted the decision.

A representative of WildEarth Guardians said a federal court challenge to the ruling was likely.