Date of Award


Document Type

Master's Thesis

Degree Name

Master of Science in Environmental and Energy Policy (MS)

College, School or Department Name

Department of Social Sciences


Audrey L. Mayer


The Endangered Species Act (ESA) requires that the “best available scientific and commercial data” be used to enable the protection of critically imperiled species from extinction and preserve biodiversity. However, the ESA does not provide specific guidance on how to apply this mandate. In addition, the interpretation of scientific data can be uncertain and controversial, particularly regarding species delineation and hybridization issues. US Fish and Wildlife Service (FWS) field biologists must decide what the best science is and how to interpret and apply it in their recommendations. As a result, FWS field biologists often have considerable discretion when it comes to making recommendations for what species to list and how to recover them. My study has examined how FWS field biologists’ knowledge and beliefs about species concepts and hybridization may impact their discretion to use the best available science to protect and recover imperiled species. I used semi structured interviews to help uncover how much discretion FWS field biologists believe they have, and their knowledge and beliefs about species concepts and hybridization related to ESA implementation. I found that they have a large amount of discretion to determine what the best available science is and how to interpret it. However, their recommendations are subjected to multiple levels of peer review and generally they defer to the scientific consensus on the taxonomic status of an organism. Hybridization was viewed primarily as a problem in the context of the ESA, which likely reflects the tumultuous history the FWS has had with this issue. However, FWS field biologists who had experience with hybridization issues were more likely to describe it as a complex evolutionary force with varied outcomes rather than wholly negative, as compared to those with little to no experience. Overall, resource limitations and “listing by litigation” impacted ESA implementation more than biologists’ knowledge and beliefs concerning species concepts and hybridization.