Species interactions depend on the plasticity of species traits, including induced life history response. Abiotic and biotic factors, such as resource availability and predation, mediate plasticity in life history traits, but metabolic costs and energetic limitations constrain life history response. Life history trade-offs also affect stoichiometric composition: elemental content reflects relative investment in growth and reproduction. These factors do not operate independently, instead often acting simultaneously and synergistically to affect life history and stoichiometry. To understand the potential effects of life history trade-offs on species interactions, we explore the strength of the independent effects of resource quality and predation, and their interactive effects on Daphnia life history and elemental composition. We focus on Daphnia because it plays a central role in trophic cascades, and the independent effects of resource availability and predation on Daphnia life history and stoichiometry are well characterized. To test how multiple selection pressures interact to shape life history and stoichiometry, we reared daphniids from birth to release of their fifth clutch in three food treatments ranging from P-deficient to P-sufficient algae, and exposed them to four predator kairomone treatments (no predator, Chaoborus, fish, and fish+Chaoborus) known to elicit different Daphnia life history responses.
Results/Conclusions

Predator pressure had significant effects on Daphnia life history traits, including juvenile growth rate, size at first reproduction (SFR), age at first reproduction (AFR), and fecundity; effects of food quality on Daphnia life history were smaller. In the presence of Chaoborus, increased P-availability resulted in increased SFR, decreased AFR, and increased reproductive output, but in the presence of fish, food quality had no effect on timing of or investment in reproduction. As a result, the interaction of food and Chaoborus significantly affected Daphnia fecundity, but the interaction of food and fish did not significantly affect growth or reproduction. In contrast, both resource quality and predator had significant effects on Daphnia stoichiometry, but the interaction of food and predator did not affect elemental composition. Compared to the no predator treatment, animals exposed to Chaoborus had significantly decreased C and N content, and animals exposed to fish had significantly increased P content. Resource quality significantly affected Daphnia N content and C:N ratio, but not P content. These results indicate that predation plays a more important role than resource availability in mediating plasticity in Daphnia life history traits, but that both food quality and predator pressure influence Daphnia stoichiometry.