The search for what's causing widespread deformities in amphibians has drummed up several suspects, including agricultural chemicals and parasitic flatworms. Now a study that combines field surveys and aquarium experiments bolsters the idea that a widely used herbicide conspires with the parasites to harm frogs. According to the new study, the herbicide atrazine not only weakens the frogs' immune system, but it also provides a nutritional boost to the ecosystem that leads to more parasites.
Frogs and other amphibians are some of the most endangered vertebrates on the planet, facing threats such as fungal pathogens and pollution. Near farms, for example, atrazine afflicts frogs by somehow suppressing the immune system. It may also interfere with their reproductive systems, although researchers disagree on that point (Science, 1 November 2002, p. 938). There is no doubt, however, about the damage that parasitic flatworms called trematodes can inflict, causing cysts that injure the kidneys and grotesquely deform limbs (ScienceNOW, 29 April 1999). In 2002, researchers showed in pond experiments that frogs exposed to herbicides and pesticides tended to have more parasites than those that weren't exposed to these chemicals (ScienceNOW, 12 July 2002). And last year, a study by another group showed that phosphate fertilizer can lead to more algae, more trematode-infected snails that eat it, and consequently more infections among amphibians.
The new study began almost a decade ago. Led by Val Beasley of the University of Illinois, Urbana-Champaign, researchers measured many characteristics of 19 wetlands in Minnesota, including trematode abundance, chemical concentrations, and frog health. Beasley's group later teamed up with Jason Rohr, an ecologist now with the University of South Florida, Tampa, who started a statistical analysis called structural equation modeling. Even though atrazine levels were relatively low, Rohr discovered that atrazine and phosphate fertilizer were by far the most important factors behind trematode abundance. The effect was so strong that "it almost made me jump out of my seat," Rohr recalls. Apparently, atrazine and phosphate each stimulate algal growth, boosting the population of the snails that host the trematodes before they infect amphibians.
Experiments the team conducted in 1100-liter tanks of water strengthened that conclusion. Tanks with atrazine had four times more snails than did tanks without the herbicide. Two kinds of frogs that lived in the herbicide-containing water showed signs of suppressed immune systems. One species, Rana clamitans, carried almost three times as many trematode cysts in the atrazine-laced tanks, the team reports today in Nature. And for another species, R. palustris, the mortality rate increased from 30% to 52%. The herbicide appears to increase frogs' susceptibility to the trematode by suppressing the immune system, while snails benefit from increased algae, says Rohr.
Ecologist Pieter Johnson of the University of Colorado, Boulder, says it's surprising that atrazine would stimulate algal growth. "It's an example of complex ecological interactions that might not be anticipated," he says. Why would an herbicide lead to more algae? Apparently, the kind of algae favored by the snails recovers faster than competing algae. Although the aquarium studies don't prove that an increase in infected snails would lead to more frog deaths, Johnson says it's a reasonable hypothesis.
The bottom line of the study is that agricultural runoff can be bad for frogs, says ecologist Janet Koprivnikar of the University of the Pacific in Stockton, California. "It points out that it's not just nutrients, it's not just atrazine," she says. "We need to be thinking about all of them if we want to address remediation."