Before humans arrived, natural predators such as garter snakes, racoons and great blue herons threatened the frog on its journey from egg to adult. But now, pollution, increased ultraviolet radiation, loss of habitat, roads and non-native predators introduced by humans further reduce the frog's chance of survival. These central coast frogs lay their eggs in streams and pools, which are being damaged by draining, damming and cattle grazing. Also, suburban sprawl and roads fragment breeding sites, making the frogs traveling to and from breeding ponds vulnerable to cars.
Roads bring people, and people bring non-native species of predators.
Bullfrogs, for example, eat the native frogs. Mosquitofish, introduced to control mosquitoes, are a particular threat because they eat the eggs of the red-legged frog. The U.S. Fish and Wildlife Service has proposed placing the red-legged frog on the endangered species list. The Service based its decision in part on research done on the UCSC campus by illustrator, environmental consultant and UCSC alumnus Randy Schmieder.