Document Title:

Determination of impacts on the endangered Wyoming toad (Bufo baxteri) at Mortenson National Wildlife Refuge from ammonium nitrate concentrations.

Kim Dickerson Edward Little Robin D. Calfee

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The endangered Wyoming toad (Bufo baxteri) is a glacial relict species confined to the Laramie Plains of southeastern Wyoming. Once common in the area, it declined dramatically in the 1970's and was subsequently listed under the Endangered Species Act in 1984 (Lewis et al. 1985). This prompted the development of a recovery plan and captive breeding program to establish viable toad populations at Mortenson and Hutton Lake National Wildlife Refuges (NWRs). In order for the effort to be successful, the effects of several environmental stressors (e.g. red-leg disease, predation, habitat alteration) have been explored. Because none of the variables were considered a serious threat, toads were reintroduced to both refuges in 1995. However, it remains of concern that Wyoming toad breeding sites may be contaminated with agricultural chemicals, which could lower fecundity and jeopardize the establishment of viable populations. Amphibian eggs and larvae are the most sensitive stages to chemical contamination and reproductive success may be reduced if exposure results in slower growth or increased mortality. Aquatic habitat at Mortenson NWR is fed by the Laramie River, but also receives snowmelt and irrigation runoff from nearby agriculture fields and pastures. The timing of this input coincides with the primary growth and development period of pre-metamorphic toads (Stone 1991). Fertilizer runoff is a major source of nitrogen pollution in aquatic habitat. Common ingredients include ammonium sulfate, sodium nitrate, and ammonium nitrate. Reduced feeding, weight loss, low activity, and physical abnormalities are among the reported effects of nitrogen toxicity (Baker & Waights 1993; Hecnar 1995; Xu & Oldham 1997). The use of nitrogen fertilizers in the United States has increased dramatically in the last fifty years and currently more than 9.98 million metric tons are applied annually (Lanyon 1996). Application typically occurs in the late spring and early summer when watershed runoff from snowmelt and rainfall may cause pulses of elevated nitrate and ammonia concentrations that can degrade the water quality and pose a risk to aquatic species. Nitrogen pollution of aquatic habitat has been linked to the decline or disappearance of some amphibian populations (Wederkinch 1988; Berger 1989). Exceedences of the current United States drinking water guideline of 10 mg/L NO3-N (EPA 1986) have been observed in the field (Schuytema & Nebeker 1999) and may not adequately protect some organisms (Kincheloe et al. 1979; Hecnar 1995). A laboratory study was performed using ammonium nitrate to assess the effects of aquatic nitrogen pollution on the reestablishment of B. baxteri at Mortenson NWR. The Woodhouse’s toad (B. woodhousii) was tested as a surrogate because of the endangered status of the Wyoming toad. Water chemistry conditions modeled those found at Mortenson NWR during the B. baxteri breeding season (late May – early July). Because of a limited Project #: 99-6-6N41-GC

U.S. Fish and Wildlife Service



U.S. Fish and Wildlife Service - Mountain-Prairie Region, Contaminants Program

Last updated: February 13, 2013