Some streams in the Santa Monica Mountains have an abundance of non-native crayfish.

The abundance of non-native crayfish (Procambarus clarkii) in Southern California streams can have detrimental effects to stream amphibians. Crayfish, which reproduce rapidly and can survive in a crack of mud, eat native amphibians such as Pacific tree frogs and California newt larvae.

Since 2000, Santa Monica Mountains National Recreation Area - in partnership with Pepperdine University and the Resource Conservation District of the Santa Monica Mountains (RCD) - have annually surveyed park streams for crayfish and other non-native species as part of the aquatic amphibians long-term monitoring program. Monitoring helps park managers understand habitat and population changes in species over time.

Data from the 2009 stream surveys have shown that in crayfish streams there were fewer HYRE tree frog tadpoles (see figure below). Park scientists found that there were more "stops" or habitats with zero tadpoles in streams with crayfish. Also, there were fewer "stops" with tadpoles for all abundance categories.