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16 Variation in Pesticide Tolerance
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93 A growing body of evidence suggests that a number of amphibian populations have declined in recent years (Barinaga, 1990; Blaustein and Wake, 1990; Wake, 1998). The cause of these declines has been difficult to establish because in some instances only a single species is declining while sympatric species are thriving (e.g., K.R. McAllister et al., 1993). Similar variation can be observed within a single species at the population level—there are often instances when some populations of a particular species are declining while others remain unaffected (e.g., northern leopard frogs [Rana pipiens], Corn and Fogleman, 1984; mountain yellow-legged frogs [R. muscosa], Bradford, 1991; western toads [Bufo boreas], Carey, 1993). Because not all populations or species have been affected, it is important for conservation efforts to identify causes for the variability among populations whose environments appear physiographically similar. Research efforts should be focused on addressing what makes some species and populations vulnerable and others resistant to declines. Here we focus on research we have conducted to determine the degree of variation present in amphibians with respect to their response to insecticide exposure. Our goal was to make it possible to establish potential patterns of vulnerability among and within amphibian species. Chemical contaminants are not homogeneous stressors because their presence in the environment can vary over small temporal and spatial scales. Differences in tolerance to a stressor can thus arise from the differential exposure that results from this variability in the presence of the contaminant. Variation in responses may also be due to genetic differences among or within species. In order for a species to adapt to an environment , a trait must be phenotypically variable, and this variation must have a heritable genetic basis. That is, there must be measurable differences among individuals (which ultimately lead to differential survival), and these differences must be passed on from generation to generation. Populations having little or no heritable genetic variability in tolerance may be more susceptible to decline and local extinction because they cannot adapt to the presence of environmental stressors. Assessing Variation Among and Within Frog Species We assessed the degree of variation in response to an anthropogenic stressor among and within species of frogs in the family Ranidae by examining the variation in tolerance of tadpoles to the insecticide carbaryl. Carbaryl is an insecticide that is widely used throughout the United States and Canada, and it can contaminate amphibian habitats via drift from application or in runoff from adjacent agricultural fields or gardens. Carbaryl acts by inhibiting nervous system acetylcholinesterase, which is a common mode of action among insecticides; thus, carbaryl can serve as a model chemical with which to examine amphibian responses. We examined variation in a hierarchical fashion to identify where variation was the greatest: among nine ranid species (crawfish frogs, R. areolata; northern red-legged frogs, R. aurora; plains leopard frogs, R. blairi; foothill yellow-legged frogs, R. boylii; green frogs, R. clamitans; pickerel frogs, R. palustris; Oregon spotted frogs, R. pretiosa; southern leopard frogs, R. sphenocephala ; wood frogs, R. sylvatica), among populations within a single species (southern leopard frogs), and within populations (i.e., among families) of southern leopard frogs (Bridges and Semlitsch, 2000). We used ranid frog species because many declines in the United States have occurred within this family (Hayes and Jennings, 1986). For each group (e.g., family, species) we conducted two assays, both using carbaryl (Bridges, 1999a). The first assay determined sensitivity to a lethal dose of carbaryl, measured by time-to-death. In the second assay, we exposed tadpoles to a sublethal concentration of carbaryl and recorded changes in behavioral activity. Variation Among Ranid Species We detected differences in the tolerance of tadpoles of all nine species tested (Fig. 16-1). Localized declines have been noted in all ranid species in the western United States (Corn and Fogleman, 1984; Hayes and Jennings, 1986; Bradford, 1991; Fellers and Drost, 1993; Bradford et al., 1994a,b; Drost and Fellers, 1996), including the three western U.S. species that we examined: Oregon spotted frogs (USFWS, 1993; K.R. McAllister et al., 1993), California red-legged frogs (R. draytonii; USFWS, 1996c), and foothill yellow-legged frogs (Corn, 1994a). Therefore, we were able to test an important hypothesis of amphibian conservation efforts: that species in decline are more sensitive to environmental stressors (e.g., chemical contamination), thus partially S I XTE E N Variation in Pesticide Tolerance CHRISTINE M. BRIDGES AND RAYMOND D. SEMLITSCH 94 VARIATION IN PESTICIDE TOLERANCE explaining observed patterns of...