64 Fla. L. Rev. 141 (2012)| | |

This Article investigates how the U.S. Fish and Wildlife Service, the National Marine Fisheries Service, and the courts are implementing the Endangered Species Act’s prohibition on “adverse modification” of “critical habitat.” That prohibition appears to be one of environmental law’s most ambitious mandates, but its actual meaning and effect are contested. Using a database of over 4,000 “biological opinions,” interviews with agency staff, and a review of judicial decisions considering the adverse modification prohibition, this Article assesses the extent to which the Fish and Wildlife Service, the National Marine Fisheries Service, and the courts are relying on the adverse modification prohibition to provide habitat protection. It also assesses the extent to which these groups are providing habitat protection by invoking other Endangered Species Act provisions. This Article concludes that although agency practice and some judicial decisions substantially depart from statutory requirements, with problematic results, the agencies are still providing substantial habitat protection through other means. It then considers the implications of these findings, first for ongoing debates about Endangered Species Act implementation and reform and then for broader discussions about legal strategies for responding to small environmental harms and the incremental degradation they cause.