European researchers studying the effects of seven common agricultural pesticides on frogs report that exposing the amphibians to the chemicals resulted in rates of mortality from 40 to 100 percent.
The study, published in Scientific Reports, replicated agricultural “overspray” onto the frogs that would likely occur within croplands. The scientists used chemicals diluted at typical application strengths, and at more dilute and more concentrated levels. The exposures killed some of the lab animals within an hour. Others died within days.
Noting the high mortality rates, the research team speculated that agricultural pesticides could be an under-appreciated factor in the worldwide decline of frogs. About one-third of frog species are threatened or endangered with extinction, according to the advocacy Save the Frogs, based in California.
“The demonstrated toxicity is alarming and a large-scale negative effect of terrestrial pesticide exposure on amphibian populations seems likely. Terrestrial pesticide exposure might be underestimated [in previous literature] as a driver of their [the frog's] decline,” they wrote.
The scientists said their study highlighted the need for stronger conservation efforts and risk assessments to protecting “this vanishing animal group.”
The decline of frogs from various places across the planet has been attributed to ultraviolet radiation, the loss of habitat and global warming. Pesticides have been identified as harmful to amphibians, and reports have shown that atrazine, for instance, causes the feminization of male frogs.
But this study proposed that pesticides play an even bigger role in the overall loss of frog populations. The scientists contended that frogs have been overlooked in setting standards for acceptable pesticide dosages, noting that many chemicals must be tested for their effects on birds and fish, but not amphibians. Yet amphibians, with their permeable skin and aquatic habitats, are highly vulnerable to chemical pollution. From the report:
In contrast to birds and mammals, for amphibians to date no specific risk assessment is required for the registration of a new pesticide product. However, negative effects of pesticides on amphibians are probable since their skin is highly permeable to allow gas, water, and electrolyte exchange with the environment. Indeed dermal uptake processes of chemicals have been shown to be two orders of magnitude faster than in mammals, suggesting that for terrestrial amphibian life stages present in crop fields pesticide uptake through the skin might represent a likely exposure route. Furthermore, with a life cycle that encompasses aquatic and terrestrial phases as well as migrations to and from spawning waters, amphibians are exposed to toxicants in two environments.
The chemicals in the study that proved most deadly to frogs were the top-selling fungicide Headline, which is used on 90 different crops such as soybeans and wheat around the world, according to the researchers. Headline, applied at “label rates”, produced 40 to 100 percent mortality rates over seven days in the young frogs tested.
Those results “indicate that existing risk assessment procedures for pesticide regulation are not protecting amphibians,” the scientists wrote.
Since amphibians are considered sentinel species for environmental and human health, our results might even have implications for other taxa or entire ecosystems. It is therefore imperative to understand the underlying mechanisms of the toxicity of pesticides for amphibians to obtain a realistic estimate of the extent of their impact and to reconcile agricultural practice and amphibian conservation efforts.
The study’s authors work with public and private science institutions in Germany and Switzerland.