Investigating the role of insect predators on trematode infection in tadpoles

Background/Question/Methods

Parasite transmission is a community-level phenomenon and can be influenced by a number of factors including the diversity and abundance of predators that consume parasites. Recent studies with amphibians and trematodes demonstrate that insect predators that consume free-living trematode cercariae can reduce trematode infections in tadpoles.  Larval odonates have received a considerable amount of attention as predators of trematode cercariae, but most studies have focused on relatively small-bodied predators. There is considerable size structure in odonate assemblages both within- and between-species which can lead to cannibalism and intraguild predation, respectively. Therefore, it is important to consider how interactions among larval odonate predators that differ in size and ecological function influence the survival, behavior, and trematode infection load of tadpoles. In this mesocosm experiment, I manipulated the presence of large-bodied (Anax junius) and small-bodied (Ischnura verticalis) predators using a factorial design to examine their direct and indirect effects on infection load of Rana clamitans tadpoles. 

Results/Conclusions

There were significant negative effects of A. junius on the survival and activity level of R. clamitans. While reduced activity can be an effective antipredator strategy for tadpoles, it can also increase susceptibility to metacercarial infections. A. junius also caused a reduction in the survival of I. verticalis, a small-bodied odonate previously shown to consume cercariae. The highest level of metacercarial infections in tadpoles were in treatments with A. junius. This can likely be explained by a combination of both direct effects such as the reduction in activity level of tadpoles in the presence of A. junius and indirect effects generated by the consumption of cercarial predators by A. junius. These results highlight the importance of considering top predators in the dynamics of parasite transmission; however, further research is needed to explore these effects in complex food webs.