Ramifications of predator avoidance: Predator and heavy-metal-mediated competition between tadpoles and snails

We examined how the effects of competition between snails (Lymnaea pulustri s) and Columbia spotted frog (Rana luteiventris) tadpoles are mediated by p redatory bluegill sunfish (Lepomis macrochirus) and heavy metals. We raised snails and tadpoles in outdoor mini-ecosystems containing lead-, zinc-, an d cadmium-contaminated soil from the EPA Superfund site in the Silver Valle y of northern Idaho. To some treatments we added water from the tanks of su nfish that had fed on tadpoles and snails. Unexpected indirect effects occurred in polluted environments. We found a h abitat shift by snails and tadpoles due to the response of tadpoles to pred aceous fish odor. This shift decreased tadpole ingestion of metal-rich sedi ments and increased metal ingestion by snails. Heavy metals and fish odor t herefore altered competitive interactions between snails and tadpoles. Meta l-exposed tadpoles metamorphosed at an older age than non-metal-exposed ani mals. Metal exposure did not affect snail survival, growth, or recruitment. In th e absence of heavy metals tadpoles reduced snail recruitment. Because heavy metals disproportionately harmed tadpoles, the negative effects of tadpole s on snails were ameliorated in polluted environments. The presence of snai ls did not alter tadpole development but did increase metal loads in tadpol es. We also tested the ability of snails to detect and respond to chemical cues emanating from crushed snails, an extract of crushed snails, and sunfish. Snails changed their behavior in response to crushed snails, but not to sun fish. Heavy-metal-exposed snails did not alter their behavior. Tadpoles alt ered their behavior when exposed to sunfish that had fed on tadpoles but di d not respond to sunfish that had not fed on tadpoles. We found that heavy metal toxicants and predator odor cause behavioral chan ges and can alter competitive interactions between snails and tadpoles. We also showed that, due to indirect effects, pollution exposure for a given s pecies may be modified both by the presence of competitors, and also by spe cies that are neither competitors nor perceived as predators.