Helpful invaders: Can cane toads reduce the parasite burdens of native frogs?

• Presence of toads or frogs reduces frog larvae available to infect subsequent anurans.
• As toads kill frog parasites, these results support assumptions of a ‘sink’ mechanism.
• Exposure to frog parasites does not enhance a toad's immunity to toad parasites.

Many invading species have brought devastating parasites and diseases to their new homes, thereby imperiling native taxa. Potentially, though, invaders might have the opposite effect. If they take up parasites that otherwise would infect native taxa, but those parasites fail to develop in the invader, the introduced species might reduce parasite burdens of the native fauna. Similarly, earlier exposure to the other taxon's parasites might ‘prime’ an anuran's immune system such that it is then able to reject subsequent infection by its own parasite species. Field surveys suggest that lungworm counts in native Australian frogs decrease after the arrival of invasive cane toads (Rhinella marina), and laboratory studies confirm that native lungworm larvae enter, but do not survive in, the toads. In laboratory trials, we confirmed that the presence of anurans (either frogs or toads) in an experimental arena reduced uptake rates of lungworm larvae by anurans that were later added to the same arena. However, experimental exposure to lungworms from native frogs did not enhance a toad's ability to reject subsequent infection by its own lungworm species.

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