Prey vulnerability determines the lethal and non-lethal effects of a predator

Background/Question/Methods

Separating lethal and non-lethal effects of predators reveals the relative importance of each on prey populations and community structure, but lethal effects are rarely tested alongside non-lethal ones.  We tested for differences in these two properties of predators on competing prey species that differ in their vulnerability to the predator with a factorial experiment using lethal water bugs, Belostoma flumineum, non-lethal (caged) water bugs, or no water bugs crossed with either one hundred Hyla chrysoscelis tadpoles, one hundred Physa acutasnails (more vulnerable), or fifty of each. A final treatment contained only primary producers (periphyton) to test for indirect effects (trophic cascades) of water bugs.

Results/Conclusions

Differential vulnerability resulted in 53% and 3% survivorship for tadpoles and snails in lethal water bug treatments compared to other treatments.  Surviving tadpoles grew (mg/d) 27% faster, while surviving snails grew 26% slower compared to other treatments.  Tadpole production (mg/tank/d) was balanced among lethal, non-lethal, and no water bug treatments, but snail production was a 133% lower in lethal tanks compared to other treatments.  Lethal water bugs selected for tadpoles with deeper tail fins and non-lethal water bugs induced deeper tail fins compared to no water bug treatments; the magnitude of shape change from selection was 64% greater than induction.  We found no evidence that water bugs influenced shell shape.  Intraspecific competition was stronger than interspecific; tadpoles and snails grew 27% and 25% faster when together compared to single-species treatments.  Tadpoles developed 3% wider mouths and snails induced wider shells in single species tanks compared to tanks with both species.  We found evidence for a density-mediated trophic cascade for snails but not tadpoles; chlorophyll a concentration was 72% higher in tanks with lethal water bugs and snails compared to other treatment combinations.  Overall, lethal effects were more important than non-lethal ones regardless of prey vulnerability.  Among non-lethal effects, the less vulnerable species (tadpoles) exhibited larger responses.  Prey vulnerability has important consequences for the relative importance of lethal and non-lethal effects of predators that drive community structure.