Multiple stressors and salamanders: Effects of an herbicide, food limitation, and hydroperiod

Amphibian populations can be affected adversely by multiple biotic and abiotic stressors that together can contribute to their local and global decline. We focused on the combined effects of food limitation, drying conditions, and exposure to possibly the most abundant and widely used herbicide in the world, atrazine. We used a factorial design to evaluate the effects of exposure to four ecologically relevant doses of atrazine (approximate measured doses: 0, 4, 40, and 400 mug/L), two food levels (limited and unlimited food), and two hydroperiods (presence or absence of a dry down) on the survival, life history, and behavior of the streamside salamander, Ambystoma barbouri, from the embryo stage through metamorphosis. In general, food and atrazine levels did not interact statistically, and atrazine affected dependent variables in a standard, dose-dependent manner. Exposure to 400 mug/L of atrazine decreased embryo survival and increased time to hatching. Drying conditions and food limitation decreased larval survival, while 400 mug/L of atrazine only reduced larval survival in one of the two years tested, Suggesting that the lethality of atrazine may be condition dependent. Sublethal effects included elevated activity and reduced shelter use associated with increasing concentrations of atrazine and food limitation. The larval period was lengthened by food limitation and shortened by 400 mug/L of atrazine. Drying conditions accelerated metamorphosis for larvae exposed to 0 and 4 mug/L of atrazine but did not affect timing of metamorphosis for larvae exposed to 40 or 400 mug/L of atrazine. Food limitation, drying conditions, and 400 mug/L of atrazine reduced size at metamorphosis without affecting body condition (relationship between mass and length), even though feeding rates did not differ significantly among atrazine concentrations at any time during development. This suggests that high atrazine levels may have increased larval energy expenditures. Because smaller size at metamorphosis can lower terrestrial survival and lifetime reproduction, resource limitations, drying conditions, and environmentally realistic concentrations of atrazine have the potential to contribute to amphibian declines in impacted systems.