Deformed Frog Hotspots Found in Central Valley Wildlife Refuges

California's Central Valley is a bad place to be a frog, according to a study published Monday in the scientific journal PLOS ONE: frogs living in the valleys' remaining wetlands are more likely to have stunted, missing, or extra limbs than their counterparts in most other parts of the country.

The ten-year study, which examined nearly 63,000 frogs sampled from wetlands throughout the United States, showed that the Sacramento and San Joaquin valleys in Central California seem to be one of three large hotspots for frog deformities nationwide. The other large hotspots were found in the Mississippi River Valley and in Alaska. Smaller hotspots were noted in the Great Lakes and the New York City area.

That's actually good news, in a way: herpetologists have been studying the increasing prevalence of frog deformities for decades, and some had suggested global phenomena such as pervasive environmental chemicals, ozone depletion, or climate change as culprits. The fact that those deformities seem tied to geography suggests that the causes of the deformities may be local in nature, which may mean they'll be easier to correct.

Among the 152 National Wildlife Refuges (NWRs) nationwide from which the researchers studied representative frogs were 18 in California, including the Colusa, Delevan, Kern, Merced, Sacramento, Stone Lakes, San Luis, San Joaquin River, and Sutter NWRs in the Central Valley. The research team included scientists from UC Davis, the University of Colorado, Indiana University, and the U.S. Fish and Wildlife Service.

Of 4,704 frogs sampled from California NWRs over the 10-year study period, 347 showed some form of abnormality either in their skeletal structure or in missing or undeveloped eyes. Among the species exhibiting deformities were 90 Pacific chorus frogs, 117 northern red-legged frogs, 134 Cascades frogs, and five California red-legged frogs, the last species being best known for its starring role in Mark Twain's "The Celebrated Jumping Frog of Calaveras County."

The string of NWRs along the Sacramento and San Joaquin rivers showed especially high percentages of deformed frogs: of 1,077 frogs sampled from the nine Central Valley NWRs, 95 -- or 8.8 percent -- showed abnormalities.

Some of the refuges in the Central Valley fared worse than others with regard to frog health. A bit of number crunching of the study's raw data reveals that the Colusa National Wildlife Refuge near Williams in Colusa County might well be one of the worst places to be a frog in California: of 62 frogs examined during the study, eight -- or 12.9 percent -- showed signs of deformities. Merced National Wildlife Refuge in the northern San Joaquin Valley was a close second, with 30 of 239 sampled frogs showing deformities, or 12.55 percent of the total. Given Merced's larger sample size and the reduced likelihood of statistical error, Merced might well prove on further study to be a worse place for frogs than Colusa.

Contrast those figures with the national average abnormality rate of 2 percent found by the researchers, and it's easy to see why they named the Central Valley a hotspot for deformed frogs..

Oddly, Delevan NWR -- just a few miles upstream from Colusa on the Sacramento River -- had only half as many abnormal frogs as its neighbor reckoned by percentages, with five deformed frogs in a sample size of 80, for a total of 6.25 percent. This may well underscore the importance of very local influences in frog development.

What sort of influences might those be? The Central Valley's wildlife refuges are surrounded by one of the most intensively managed agricultural landscapes in the world, so a range of related factors from pesticides to fertilizers, and increasing urban and residential development throughout the Valley may well expose amphibians to a broad range of other mutagens. Deformations in frogs have been tentatively linked to parasite infestations and blooms of toxic cyanobacteria in the frogs' watery habitats, and both of those biological factors might be heightened by nutrient runoff from farms.

Whatever turns out to be the case for the Central Valley, the study's authors recommend taking a close look to see what's going on in the neighborhood that might be hurting the frogs. "We suggest that findings of high percentages of abnormal frogs, particularly those that cluster together spatially, warrant targeted investigation of the local factors causing abnormalities and their consequences for the sustainability of affected amphibian populations."