When Congress passed the Endangered Species Act in 1973, it did so in the belief that threats to emblematic creatures like bison or commercial crops such as fish could be resolved simply by placing limits on hunting and trade. Few lawmakers could have foreseen how the implementation of that law would change over the years, as the directive to save seemingly insignificant species would extend to the protection of entire ecosystems. As the courts have interpreted the law in the light of a better understanding of science, the ESA increasingly has become a tool used to block development projects...