The Upper San Marcos River is one of the most biologically diverse aquatic ecosystems known in the southwestern United States. It has several endemic species that are isolated to the upper four miles of the river. Consequently, the U.S. Fish and Wildlife Department and Texas Parks and Wildlife have designated the San Marcos Springs and Spring Lake as critical habitat. Critical habitat refers to a particular geographical area that contains all of the physical, chemical, and biological attributes needed for the continued success of an endangered plant or animal that may require special efforts for their management and protection.
Species are classified as threatened or endangered due to declining numbers or habitat loss. There are seven known species listed as threatened or endangered by the U.S. Fish & Wildlife Service and Texas Parks and Wildlife Department that reside in the San Marcos region of the Edwards Aquifer, Spring Lake, and the upper four miles of the San Marcos River. These species include: