With little-noticed procedural and policy moves over several years, Bush administration officials have made it substantially more difficult to designate domestic animals and plants for protection under the Endangered Species Act.

Controversies have occasionally flared over Interior Department officials who regularly overruled rank-and-file agency scientists’ recommendations to list new species, but internal documents also suggest that pervasive bureaucratic obstacles were erected to limit the number of species protected under one of the nation’s best-known environmental laws.