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By Wildlife Biologist Mark Allaback
Although it has taken a decade and is (somewhat) ongoing, I believe I have been instrumental in eradicating bullfrogs (Rana catesbeiana) after they were introduced to the Big Sur Watershed in 2000 at a California Red-Legged Frog (Rana draytonii) breeding pond. Of course, the bullfrog eradication effort required lots of work, including 2 pond drainings and endless hunting. But since there were no known bullfrog sources from the relatively isolated area, it was a worthwhile effort.
Unfortunately, we have also lost the California Red-Legged Frog breeding population from the site, almost certainly due to Louisiana Red Swamp Crayfish (Procambarus clarkii). Although present when I arrived in 2000, after draining the pond, they completely took over, effectively removing all submergent vegetative cover and presumably feasting on eggs and tadpoles (scattered detections of tiny numbers of late-stage tadpoles were nearly all hammered by pincers). The pond is perennial. Daily crayfish trapping does nothing, although 5000-6000 crayfish are removed each year.
This year we may essentially take the pond out of commission, at least half the year, by dewatering in late May and keeping it dry until the rains begin in Oct. Perhaps if we do this, the crayfish reproductive cycle will break, and Red-Legged Frogs will one day re-colonize. However, Red-Legged Frogs have not bred successfully since about 2002, although they attempted to for many years (egg mass counts have dropped from ~45 in 2004 to 1 this year).
The crayfish burrow but maybe reducing the hydro-period to 6-7 months for 1 or more years will take them out. My fear is that they will return, since I have seen them disperse overland and found them under debris far from water.