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We found amphibian chytrid fungus (Bd = Batrachochytrium dendrobatidis) to be widespread within a coastal watershed at Point Reyes National Seashore, California and within two high elevation watersheds at Yosemite National Park, California. Bd was associated with all six species that we sampled (Bufo boreas, B. canorus, Pseudacris regilla, Rana draytonii, R. sierrae, and Lithobates catesbeianus). For those species sampled at 10 or more sites within a watershed, the percent of Bd-positive sites varied from a low of 20.7% for P. regilla at one Yosemite watershed to a high of 79.6% for P. regilla at the Olema watershed at Point Reyes. At Olema, the percent of Bd-positive water bodies declined each year of our study (2005-2007). Because P. regilla was the only species found in all watersheds, we used that species to evaluate habitat variables related to the sites where P. regilla was Bd-positive. At Olema, significant variables were year, length of shoreline (perimeter), percent cover of rooted vegetation, and water depth. At the two Yosemite watersheds, water depth, water temperature, and silt/mud were the most important covariates, though the importance of these three factors differed between the two watersheds. The presence of Bd in species that are not declining suggests that some of the amphibians in our study were innately resistant to Bd, or had developed resistance after Bd became established.

New findings from USGS and Taiwanese researchers suggest that butachlor has significant reproductive impacts on frogs even below the recommended application concentration. Butachlor (N-butoxy-methyl-2-chloro-20,60-diethyl-actanilide) is the most commonly used herbicide on rice paddy fields in Taiwan and throughout Southeast Asia. Paddy fields are man-made habitats that are commonly used for reproduction by many species of frogs, but, little is known regarding the ecological and physiological effects of butachlor on frogs.

A recent study published in Ecotoxicology (Liu et al. 2011) examined acute and chronic effects of butachlor on tadpoles of the alpine cricket frog (Fejervarya limnocharis), an Asian species that breeds in rice paddies when they are first flooded. The timing coincides with typical butachlor application regimes, and because of the timing of their breeding behavior cricket frogs can be exposed to higher concentrations of butachlor than other frogs.

Tadpoles were hatched from egg masses collected from paddy fields in Taiwan, and then assigned to treatments of zero, 0.05, 0.1, 0.2, 0.4, 0.8, 1.6 and 3.2 mg/l of butachlor. All concentrations were below the recommended application rate of 4.8 mg/l of butachlor in paddy water. The experiments showed that 0.87 mg/l of butachlor would kill 50% of the tadpoles after 96 hours of exposure -- a concentration far below the recommended application rate.

Although the results suggested that cricket frog tadpoles were less sensitive than other amphibians that breed in rice paddy fields -- such as the narrow-mouthed toad (Microhyla ornata; 0.53 mg/l) and Guenther’s frog (Rana guentheri; 0.74 mg/l) -- the surviving tadpoles still exhibited a range of impacts, including delayed metamorphosis. Also, butachlor was genotoxic to tadpoles; the number of DNA strand breaks in the red blood cells of cricket frog tadpoles increased with increasing butachlor concentrations.

Butachlor likely has widespread negative impacts on many species of amphibians, although the severity depends on each species’ sensitivity, butachlor's short half-life and the pattern and timing of its application. Staggered spraying in adjacent fields may create refugia with lower butachlor concentrations, which adult frogs may be able to detect.

This study was supported by a Taiwan National Science Council Grant. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the US Government. The study is contribution 417 of the USGS Amphibian Research and Monitoring Initiative (ARMI).

For further reading:

Liu Wan-Yi, Wang Ching-Yuh, Wang Tsu-Shing, Gary M. Fellers, Lai Bo-Chi, Kam Yeong-Choy. 2011. Impacts of the Herbicide Butachlor on the Larvae of a Paddy Field Breeding Frog (Fejervarya limnocharis) in Subtropical Taiwan. Ecotoxicology 20(2): 377-394. doi: 10.1007/s10646-010-0589-6 USGS: http://www.werc.usgs.gov/ProductDetails.aspx?ID=4239

WERC Publication Brief: Rice Field Herbicide Butachlor Is Toxic to Taiwanese Frog Tadpoles. Updated April 2011. http://www.werc.usgs.gov/ProductDetails.aspx?ID=4243

WERC Point Reyes Field Station. http://www.werc.usgs.gov/pointreyes

Butachlor is the most commonly used herbicide on paddy fields in Taiwan and throughout Southeast Asia. Since paddy fields provide habitat for pond breeding amphibians, we examined growth, development, time to metamorphosis, and survival of alpine cricket frog tadpoles (Fejervarya limnocharis) exposed to environmentally realistic concentrations of butachlor. We documented negative impacts of butachlor on survival, development, and time to metamorphosis, but not on tadpole growth. The 96-h LC50 for tadpoles was https://0.87 mg/L, much lower than the 4.8 mg/L recommended dosage for application to paddy fields. Even given the rapid breakdown of butachlor, tadpoles would be exposed to concentrations in excess of their 96 h LC50 for an estimated 126 h. We also documented DNA damage (genotoxicity) in tadpoles exposed to butachlor at concentrations an order of magnitude less than the 4.8 mg/L recommended application rate. We did not find that butachlor depressed cholinesterase activity of tadpoles, unlike most organophosphorus insecticides. We conclude that butachlor is likely to have widespread negative impacts on amphibians occupying paddy fields with traditional herbicide application.

Most research to assess amphibian declines has focused on local-scale projects on one or a few species. The Amphibian Research and Monitoring Initiative (ARMI) is a national program in the United States mandated by congressional directive and implemented by the U.S. Department of the Interior (specifically the U.S. Geological Survey, USGS). Program goals are to monitor changes in populations of amphibians across U.S. Department of the Interior lands and to address research questions related to amphibian declines using a hierarchical framework of base-, mid- and apex-level monitoring sites. ARMI is currently monitoring 83 amphibian species (29% of species in the U.S.) at mid- and apex-level areas. We chart the progress of this 5-year-old program and provide an example of mid-level monitoring from 1 of the 7 ARMI regions.