Toxicity assessments on nontarget organisms have largely been addressed using short-term, single-species laboratory experiments. Although extremely helpful, these experiments inherently lack many pervasive ecological stressors found in nature. Though a substantial challenge, incorporating these ecological stressors in contaminant studies would shed light on potential synergistic effects. For the world's leading herbicide, glyphosate, we know little about how natural stressors affect the toxicity to nontarget organisms. To explore how the natural stress of competition might interact with a glyphosate-based herbicide, we used outdoor mesocosms containing three tadpole species that were exposed to a factorial combination of three glyphosate concentrations (0, 1, 2, or 3 mg acid equivalent (a.e.)/L of the commercial formulation Roundup Original MAX®) and three tadpole densities (low, medium, or high). We found that increased tadpole density caused declines in tadpole growth, but also made the herbicide significantly more lethal to one species. Whereas the median lethal concentration (LC50) values were similar across all densities for gray treefrogs (Hyla versicolor; 1.7–2.3 mg a.e./L) and green frogs (Rana clamitans; 2.2–2.6 mg a.e./L), the LC50 values for bullfrogs (R. catesbeiana) were 2.1 to 2.2 mg a.e./L at low and medium densities, but declined to 1.6 mg a.e./L at high densities. The large decrease in amphibian survival with increased herbicide concentration was associated with increases in periphyton abundance. We also found evidence that temperature stratification lead to herbicide stratification in the water column, confirming the results of a previous study and raising important questions about exposure risk in natural systems. Environ. Toxicol. Chem. 2011;30:446–454. © 2010 SETAC
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