• Rick Evans

Mountains Restoration Trust Still Battling Invasive Crayfish in Santa Monica Mountains


(OAK PARK, CALIFORNIA) The water flowing through Medea Creek is overrun with invasive crayfish.

The red swamp crayfish is an invasive species in Southern California, and they continue to destroy the ecosystem in many of the creeks and streams found in the region. They consume almost all life in these important waterways like fish, amphibians, even plants.


Their consumption of dragonfly nymphs, which are the aquatic form of dragonflies before they mature, is specifically alarming. These small insects are one of the main predators of mosquito larvae. These dragonfly nymphs help control the mosquito population, along with other animals like frogs. This, in turn, aids in prevention of diseases spreading around the local housing communities nearby.


(OAK PARK, CALIFORNIA) A volunteer holds up one of the crayfish removed from Medea Creek.

Studies have shown that when crayfish are present in streams dragonfly nymph numbers are lower than when there are no crayfish. Results of these studies ultimately reveal how this relationship directly affects the mosquito population. The likeliness of mosquito-borne diseases spreading is increased when the crayfish consume valuable species like dragonfly nymphs and frogs.


Data from the scientific journal Conservation Biology (Graphic by Rick Evans)

The Mountains Restoration Trust (MRT) organization, has been dedicated to addressing this issue for several years in the areas around the Santa Monica mountains near Los Angeles. One location they have recently been working on is Medea Creek in Oak Park. They hold regular volunteer events where they invite people from the community to come out and help remove the invasive crayfish from the stream.


(OAK PARK, CALIFORNIA) MRT volunteers gather to remove crayfish from Medea Creek.

Areas like Oak Park lost a lot of vegetation due to a massive wildfire in November. Following that, the Southern California region received a heavy amount of rainfall during the rainy winter season. These two events coupled together caused significant changes to some of the natural areas.


Erik Sode, Aquatic Supervisor for Mountains Restoration Trust, says, "[The rain] has definitely changed a lot of characteristics in the creek, in particular the areas that did see a lot of fire." Without the vegetation, the soil left behind after the fire washed into sections of the creek bed, filling it in with sediment. But, the rain had a positive effect on the creek as well. "I have seen a decrease in crayfish population which is not out of the ordinary after a large rainstorm," says Sode. "So, I find it really important that we start trapping immediately. That way we can really control the population," he adds. Rain has a tendency to flush the crayfish out of the streams and into the ocean where they cannot survive.


(OAK PARK, CALIFORNIA) Baited metal traps are strategically placed along the creek to catch crayfish for removal.

MRT's crayfish removal projects, along with the benefits from the rain, appear to be having a positive impact on the local ecosystem. One way this is evidenced is by the presence of amphibians like frogs and tadpoles, which had previously become rare due to crayfish predation. Sode says, "[T]hese are the very tangible results from crayfish removal; You see an increase in tadpoles and frogs either as bycatch or just seeing them normally in the stream." He says the creek will still need a lot more work besides crayfish removal, but seeing a resurgence in the amphibian population is a definite plus. "[T]hese frogs are going to start controlling a lot of the mosquitoes and gnat fly populations, which will benefit everybody in general," he adds.


Projects like crayfish removals are important to rebuild and maintain habitats for hundreds of plants and animals. Making sure invasive species are removed to allow native ones to thrive is a major step in enriching these habitats. MRT has a variety of volunteer events like crayfish removals as well as planting native vegetation in other areas. They encourage people of all ages to come out to learn about the environmental issues and take part in the conservation process.



(OAK PARK, CALIFORNIA) Volunteers gather to listen to a procedure and safety briefing from MRT's Erik Sode.







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