Fertilizer runoff may be fostering parasite populations and causing an increase in frog deformities, a University of Colorado study published in September suggested.
Deformed frogs first started attracting attention in 1995 when Minnesota schoolchildren found a number of frogs with missing or extra limbs. While theories abound as to possible causes for the malformations, Pieter Johnson of the University of Colorado at Boulder and his team discovered that excess nitrogen and phosphorus initiated a series of events leading to deformed frogs.
Johnson's lab tests demonstrated a chain of effects that resulted in an increase in parasites which interfere with normal frog development. Their study showed that increases in nitrogen and phosphorus lead to increased algae production, a food source for water snail populations. The snails, which feed on the algae, serve as hosts for microscopic parasites known as trematodes.
Ultimately, these parasites are released from the snails and infect tadpoles' cells causing cysts to form. These cysts lead to malformations as the frogs develop, including missing or extra limbs.
If there are more snails, the parasites are more likely to find a snail,"" Johnson said in an interview with New Scientist. ""And if the snails have more food, they survive longer. Once infected, they become zombies whose sole function is to release parasites.""
Scientists have long known that eutrophication, an increase in chemical nutrients like nitrogen and phosphorus, causes algae to proliferate and use up all the oxygen in the water, effectively creating ""dead zones"" which kill fish and other oxygen-dependent species. But this study is one of the first to identify a specific chain of events and pinpoint the culprit harming frog populations.
Teresa Balser, a UW-Madison soil science professor, explained that because aquatic ecosystems are so delicately balanced, this makes buffer zones or ""rain gardens"" all the more important. These areas of plants between watershed runoff and aquatic communities serve to filter the water and help break down toxins such as agricultural and urban chemicals.
""Some people have found evidence that, yes, [pesticides] are responsible for declining amphibian populations, but Piet Johnson doesn't think that is what he is seeing. He thinks he's seeing this trophic cascade productivity effect,"" said Stanley Dodson, UW-Madison professor of zoology. ""If that were true everywhere, that would be good news because then all we would have to do would be to be more careful about how we use nitrogen and phosphorus and use less whenever possible.""
""We've done extensive field work in the western and midwestern United States and have not found any links between pesticide contamination and limb deformities, directly or indirectly,"" Johnson said by e-mail. ""This doesn't mean that other contaminants couldn't be playing a role, but thus far evidence for such a role is largely lacking, both in terms of field studies and laboratory experiments.""
While agricultural runoff is a well-known source of nitrogen and phosphorus pollution, Dodson pointed out that increasingly developed suburban and urban land also contributes high amounts of the chemicals to the watershed.
""Currently, people are putting about three times as much pesticide on their lawn as they put on corn and soy beans. Lawns are heavily treated,"" Dodson said.
After studying aquatic communities in developed areas, Dodson found that areas with more than 30 percent of their watershed coming from lawns had no amphibians. While Dodson did find nearby lawns to be indicators of amphibian presence, when he measured nitrogen and phosphorus levels in water bodies, he did not find a correlation with the size of the frog population.
""So I don't think it's fertilizers in my case, but I think other chemicals put in lawns - herbicides, pesticides, fungicides - are also implicated,"" Dodson said.
Dodson's research indicated that the existence of lawns in the watershed area feeding into pond communities was a bigger indicator of amphibian presence than nitrogen or phosphorus levels.
According to Dodson, Johnson's paper is the first to suggest that rather than directly impacting frogs, agriculture runoff makes it possible for parasites to thrive, thereby disrupting normal frog development.
""So it's a nice complicated story. I think the more complicated it is the more likely it is to be at least partially true.""
Johnson's research could have implications for both amphibian declines and for diseases potentially linked to nutrient pollution worldwide. Other studies have shown that nutrient runoff can increase populations of disease transmitters and their hosts, according to Johnson.
""[These findings] suggest that while nutrient enrichment has enormous benefits in terms of agricultural production, it may also incur costs in terms of human and wildlife disease.""
