Roundup® and amphibians: The importance of concentration, application time, and stratification

Abstract

The widespread use of pesticides raises the possibility that non-target organisms might also be affected. To assess this, the traditional approach has been to conduct short-term laboratory experiments spanning a range of lethal concentrations and some longer-duration experiments at sublethal concentrations. While this approach has been very useful, less attention has been paid to the timing of exposure and the impacts of multiple, small exposures versus single, large exposures. We examined the role of application amount, timing, and frequency using outdoor mesocosm communities containing larval amphibians (Rana sylvatica and Bufo americanus) and using a commercial formulation of the herbicide glyphosate (Roundup Original MAX(R)). Consistent with past studies, exposures of up to 3 mg acid equivalent (a.e.)/L caused substantial amphibian death. However, the amount of death was considerably higher when the herbicide was applied earlier in the experiment than later in the experiment. Single, large applications (at different times) had larger effects on tadpole mortality and growth than multiple, small applications (of the same total amount). The results may reflect an acclimation to the herbicide over time. In treatments with high tadpole mortality, there was no resulting increase in periphyton, suggesting that the reduction in tadpole herbivory might have been offset by direct negative impacts of the herbicide. We also discovered that temperature stratification caused herbicide stratification, with higher concentrations near the surface. Such stratification has important implications to the habitat choices of ectotherms that might prefer surface waters for thermoregulation or prefer deeper waters to avoid predators. Collectively, the present study demonstrates the importance of examining multiple applications times and frequencies to understand the impacts of pesticides on organisms.

Competitive stress can make the herbicide Roundup® more deadly to larval amphibians

“…On average, however, the top and bottom combined to produce a test value very close to our nominal concentration. Thermal stratification causing greater pesticide concentrations near the water's surface has been observed in previous studies [39,40], including a recent study examining the sensitivity of tadpoles to glyphosate over ontogeny [31]. Given that the present study only sampled stratification early in the experiment, future studies should examine how much this stratification persists over time.…”

“…For the highest concentration (3 mg a.e./L), the sample taken from the top of the water column broke during shipping. However, based on the precision of the other two measured concentrations relative to the nominal concentration, and based on the fact that similar patterns of stratification were found in another mesocosm study [31], it seems likely that the nominal concentration of 3 mg a.e./L, which had a measured bottom concentration of 2 mg a.e./L, had a top concentration of approximately 4 mg a.e./L. For simplicity, we will refer to the four herbicide treatments as 0, 1, 2, and 3 mg a.e./L.…”

Measuring the impacts of Roundup Original® on fluctuating asymmetry and mortality in a Neotropical tadpole

“…In some Brazilian regions, this amount of contamination can be related to a pesticide and herbicide use that is up to 3.2 times higher than the global mean (Pignati & Machado, 2007). Thus, it is not unrealistic for tadpoles to be exposed to glyphosate contamination levels comparable to the concentrations used in our experiments and in toxicity bioassays conducted with amphibian eggs and tadpoles in the wild (e.g., Mann & Bidwell, 1999;Lajmanovich et al, 2003;Relyea, 2005b, c;Relyea & Jones, 2009;Jones et al, 2010Jones et al, , 2011Relyea, 2012;Simioni et al, 2013;Figueiredo & Rodrigues, 2014). The LC50 96h for P. cuvieri was 2.13 mg a.i./l, which is considered moderately toxic according to glyphosate toxicity classification on aquatic organisms (Giesy et al, 2000;U.S.EPA., 2008) and was lower than that recorded for other congeneric species (P. albonotatus, LC50 96h = 5.38 mg a.i./l, moderately toxic, Simioni et al, 2013; P. centralis, LC50 96h = 19.7 mg a.i./l, slightly toxic, Figueiredo & Rodrigues, 2014).…”

“…Characteristics such as permeable skin and water dependency increase the susceptibility to pollutants Allentoft & O'Brien, 2010), especially for species with indirect development (Altig & McDiarmid, 1999a, b). Tadpoles have high phenotypic plasticity and can rapidly respond to environmental changes (Alford, 1999), such as contaminant exposure (e.g., Bridges, 1999;Griffis-Kyle, 2005, 2007Relyea, 2005a, b, c;Snodgrass et al, 2008;Jones et al, 2010;Relyea, 2012;Lajmanovich et al, 2013). According to Relyea (2012), the majority of studies that have investigated contaminant effects on tadpoles were based on experiments with a single species in a short time period (e.g., 1-4 days), resulting in a lack of empirical evidence of contaminant impact over a long exposure time.…”

Amphibians as indicators of changes in aquatic and terrestrial ecosystems following GM crop cultivation: a monitoring guideline

“…This may lead to significant effects, such as deformities and increased mortality as observed in anuran larvae (Mann et al 2009;Paganelli et al 2010;Relyea 2005b). While interactions between toxic effects and other ecological factors can amplify lethal effects of pesticides (Relyea and Mills 2001), even sublethal effects on larval growth and development can impair amphibian populations permanently (Jones et al 2010;Relyea 2005a;Relyea 2005b). In agricultural systems, amphibians may be particularly susceptible to the toxic effects of pesticides because their breeding sites are often shallow ponds or temporary pools that may accumulate pollutants without substantial dilution (Howe et al 2004;Mann et al 2003).…”