Print PageBiodiversity can strongly influence disease transmission in wildlife populations and the dilution effect is a well-studied example of this phenomenon. We examined the potential for biodiversity among predators of trematode cercariae to reduce the risk of trematode infections in amphibians. Free swimming cercariae are vulnerable to predation from a wide variety of aquatic organisms, but larval odonates appear to be particularly voracious consumers of cercariae. We used a multi-factorial approach to assess the independent and combined effects of larval odonate richness and density on the survival, behavior, and trematode infection load of Rana clamitans tadpoles. The experiment included two treatment variables, odonate diversity (8 levels) and odonate density (2 levels). The eight odonate diversity levels included a full-factorial treatment design with the presence/absence of three early-instar odonates: Ischnura verticalis, Pachydiplax longipennis and Erythemis simplicicolis. The eight odonate treatments were crossed with two levels of odonate density resulting in a 2 x 2 x 2 x 2 design and 16 treatments. A treatment with no odonate predators served as a control.
Results/Conclusions
There were no significant main effects of odonate density or richness on R. clamitans survival. Trematode infection load was significantly influenced by both odonate density and richness with lower trematode infection loads occurring at higher levels of odonate richness and density. However, there was also an interaction between odonate density and richness driven by an unexpectedly elevated trematode infection load among tadpoles at the highest odonate density and richness treatment. This latter result may be partially explained by a significant reduction in activity level of tadpoles in this treatment which made tadpoles more vulnerable to infection from cercariae. These experimental results clearly indicate that non-host predators of cercariae, such as larval odonates, have the potential to modulate trematode infection risk for amphibians and corroborate our field observations showing that insect species richness is a significant negative predictor of trematode loads in amphibian metamorphs. Taken together these results emphasize the importance of community context in understanding the dynamics of cercarial transmission.
