The application of glyphosate-based herbicides and nutrient enrichment to natural wetlands has negligible effects on amphibians
University of New Brunswick
The impact some anthropogenic contaminants have on natural systems is understood based largely on the findings of laboratory bioassays and mesocosm experiments. This is problematic because there is a high degree of uncertainty when the results of laboratory and mesocosm studies are extrapolated to natural systems because they are conducted in venues that exclude factors which occur at scales much larger than the unit of replication. Unless the conclusions of laboratory and mesocosm studies are replicated and unequivocal their results should be seen as hypotheses in need of tests by whole-system experiments. Glyphosate-based herbicides are the most commonly used herbicides in the world due to the recent development of glyphosate-tolerant crops and that they are considered relatively benign to non-target organisms. Recent laboratory and mesocosm studies have demonstrated that a wide variety of formulated products can be toxic to amphibian larvae at concentrations that may be found in the environment under worst-case exposure situations. However, the limited number of field studies conducted to date indicate that toxic effects may not occur in nature. In this dissertation the impact of a two glyphosate-based herbicides on larval and juvenile amphibians in natural systems was investigated with three whole system experiments and one laboratory experiment. In the first whole system experiment there was no evidence that the formulated end-use product, VisionMax™, had a negative effect on larval amphibian survival, growth or development, or a negative effect on juvenile survival, body condition or liver somatic index (LSI). There was some evidence to suggest that application of the herbicide had a negative impact on disease incidence in juvenile frogs. In the second whole system experiment there was limited evidence to suggest that the application of the herbicide, Roundup WeatherMax™, alone or in combination with nutrient enrichment had a negative impact on larval survival or development. However, high application rates alone and in combination with nutrient enrichment resulted in a small positive increase in body size. In the third experiment, and in the laboratory there was no evidence to suggest that the herbicide, Roundup WeatherMax™, had an effect on juvenile amphibian survival, LSI, body condition or disease incidence. The results of both whole system experiments on larval amphibians indicate that a high herbicide application rate resulted in an increase in abundance of larval green frogs (Lithobates clamitans). The increase in abundance of this large species that overwinters as larvae could have detrimental impacts on species of amphibians. Our current understanding of glyphosate-based herbicides indicates that they are toxic to amphibian larvae at concentrations that may be found in the environment (based primarily on laboratory and mesocosm studies). However, real world exposures suggest that negative effects are small or absent, probably because the duration of exposure is short and toxicity is mitigated by environmental variables.