Dirty water

Johnson, P., Chase, J., Dosch, K., Hartson, R., Gross, J., Larson, D., Sutherland, D., & Carpenter, S. (2007). Aquatic eutrophication promotes pathogenic infection in amphibians Proceedings of the National Academy of Sciences, 104 (40), 15781-15786 DOI: 10.1073/pnas.0707763104

You might notice that your local lake or pond or river is a lot less clear than it used to be. More green, more brown, more algae growing on the rocks. This is eutrophication.

How much algae can grow in freshwater is typically limited by how much available nitrogen or phosphorus there is in the water. Human have been putting in a lot more of these chemicals into watersheds for lots of reasons. They're in soaps, for instance. More nutrients, more algae. More algae, more things that eat the alage, like snails. More snails, fewer frogs.

Wait. Fewer frogs? That's what this paper from P.T.J. Johnson and colleagues attempts to show.

The missing link in the little story is a parasite, a trematode fluke named Ribeiroia ondatrae (right). This parasite has a complex life cycle requiring three hosts: snails, frogs, and birds. It initially infects snails, castrating them when they do so.

They next infect tadpoles of frogs. This infection doesn't kill the tadpole or the adult frog, but when the infection is heavy, they do cause quite serious limb deformities: extra appendages, and so forth.

The final stages of this life cycle weren't studied by this paper, but it seems that the limb deformations would make the frogs more likely to be eaten by birds, which is the final host for this parasite and will be where the trematodes reproduce sexually. They pass out in the feces of the bird, back into water to infect more snails.

So when you have eutrophication generating more snails (more food for them), there's more hosts for the parasites, which then go on to really hammer the tadpoles, which are not benefiting quite so much from all the extra algae.

The downsides to this study are that it covers a very short time span: only two months. And these are artificially created pools the experimenters set up, so they're really only documenting the establishment of a small ecosystem. It's much harder to tell what a final, steady state condition might be. Considering that the trematodes castrate the snails before they infect tadpoles, it's possible that the big differences they see between low, medium, and high levels of water eutrophication in tadpole infection might get smoothed out somewhat over time. That is, as the parasite population jumps, the snail population might crash, removing some of the pressure on the tadpoles.

A very clean, straightforward experiment, but, nothing terribly surprising here. It probably is only published in a major journal because of the concerns over amphibian decline. If one of the hosts was something other than an amphibian, it might not be have been published in PNAS.