The upper San Marcos River is one of the most biologically diverse aquatic ecosystems known in the southwestern United States and has a number of endemic species that are isolated to the upper 4.5 miles of the river. Consequently, the U.S. Fish & Wildlife Department and Texas Parks and Wildlife have designated the San Marcos Springs and Spring Lake 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 and that may require special efforts for their management and protection.
Species are listed threatened or endangered based on declining numbers or loss of habitat. There are 8 known species listed as endangered or threatened by the U.S. Fish & Wildlife Service and Texas Parks & Wildlife Department that live in the San Marcos region of the Edwards Aquifer, Spring Lake and the upper 4 miles of the San Marcos River. They include: