Tension between science and the law is a pervading feature of Endangered Species Act (ESA) jurisprudence. Incorporating the scientific discipline of ecology within the legal landscape presents distinct challenges, particularly in comparison with more traditional laboratory sciences. Within the realm of Endangered Species Act liability, the intricacies of nature exacerbate already complicated links of causation, challenging the ability to prove violations of the “take” prohibition. Because uncertainties permeate scientists’ ability to understand complex ecosystem processes, courts should rely on the overarching practicality of common law principles when reviewing ecological testimony.

When evaluating claims that allege violations of the “take” prohibition, the proximate causation standard operates as a threshold to prevent assigning liability to a party or entity that otherwise may be just one insignificant link in an attenuated ecological chain. The proximate causation standard advanced by the Supreme Court in Babbitt v. Sweet Home demonstrates the practicality of maintaining established legal principles, specifically as a limit to relying on scientific testimony as a means of proving causation. More recently, the reasoning in Aransas Project v. Shaw, where an environmental group alleged that the Texas Commission of Environmental Quality caused the “take” of endangered whooping cranes,illustrates the challenges associated with proving the cause of ecological injuries. Although the United States District Court for the Southern District of Texas assigned ESA liability based on scientific testimony, the Fifth Circuit reversed the lower court because this attenuated chain of causation lacked the required proximate cause analysis. In the context of ESA liability, where judges must understand complex ecosystem processes, this dichotomy reflects the reliability of proximate causation as a foundation to ensure equitable results.