Scientists have long known that high concentrations of pesticides can be toxic to frogs, toads, and salamanders. What is less well known is the effect of low, repeated doses of pesticides on these amphibians.
With this in mind, Rick Relyea and Nicole Diecks, at the University of Pittsburgh, conducted a series of experiments on the matter. What they found is that small doses of pesticides are ultimately fatal to some amphibians – not directly, but through a cascade of ecosystem events.
Relyea and Diecks studied the insecticide malathion, one of the most common insecticides in use in the United States. Malathion is used to kill virus-carrying insects, including mosquitoes. Directly applied, it is only moderately toxic to amphibians in their sensitive larval stage, but it is highly toxic to aquatic invertebrates.
To evaluate the indirect effects of malathion on tadpoles, the larval stage of frogs, Relyea and Diecks first created artificial ponds to which wood frog and leopard frog tadpoles were added, in both high and low densities.
Some tanks then received two low doses of malathion, one at the start of the experiment and a second when the wood frog tadpoles – which develop quickly – had already begun to metamorphose into frogs but before the slower to develop leopard frog tadpoles had advanced as far.
Other tanks received smaller, but more frequent, doses of malathion. Toward the end of the experiment, the scientists artificially dried the ponds to simulate the seasonal progression of spring into summer that the tadpoles would experience in nature.
While the low doses of malathion didn’t affect the tadpoles directly, Relyea and Diecks found that they did kill zooplankton (microscopic animals that float in pond water). These zooplankton rebounded if malathion additions stopped, but declined further if small, multiple applications continued.
Zooplankton eat phytoplankton (microscopic plants), and with zooplankton populations suppressed by the frequent malathion applications, phytoplankton populations soared and the ponds experienced phytoplankton blooms.
Phytoplankton float near the water’s surface, and as their populations exploded, the blooms blocked out light from reaching the artificial ponds’ bottoms where periphyton – plants that are a tadpole food staple – grow. Periphyton declined, both in small- and frequent-dose ponds and in ponds that received two big doses early on.
The wood frog tadpoles – the fast developers – were able to “outpace” periphyton decline and pond drying. In fact, the only variable that affected their survival was tadpole density.
The slow-growing leopard frog tadpoles, however, were affected by both pesticides and tadpole density, and the interaction of those two variables. These tadpoles still relied on periphyton after it had gone into decline and this food scarcity was exacerbated by high tadpole densities. The leopard frog tadpoles, therefore, grew more slowly, ultimately exposing them to the lethal consequence of still being tadpoles when the ponds dried up.
This study highlights the effects of a “trophic cascade,” whereby a web of ecosystem interactions can link seemingly far-removed causes and effects. When testing a pesticide, scientists will usually test the direct toxicity of the pesticide on an organism in a controlled environment. But life is messy, and real-world, long-term experiments are necessary in order for us to truly determine a chemical’s (or an action’s) widespread consequences.