Quantifying consumptive and non-consumptive effects of predators in the midst of nutrient enrichment provides metrics for interpreting the biotic and abiotic interactions that structure communities.  I used a food-web fragment consisting of crayfish (predators), snails (prey), and periphyton (resources) in a 2 × 3 × 2 factorial experiment that separated the effects of predators (present / absent), prey density (present / removal / absent), and nutrient additions (addition / ambient) on multiple prey traits.  I also examined the effects of nutrients and the cascading effects of predators on prey resources. 

Results/Conclusions

Nutrient additions had strong positive effects, while predator cues had negative effects on prey growth.  Total snail production was highest in treatments with nutrient additions and no predator cues.  In the presence of predator cues, snails were less active and allocated more resources to shell growth compared to snails without predator cues.  However, snail growth was unaffected by density reduction treatments suggesting that snail density had little effect on growth.  Additionally, nutrient additions caused a reduction in periphyton volume compared to ambient treatments indicating an increase in resource quality.  Therefore, snail populations in the Everglades appear most limited by nutrients but non-consumptive predator effects are also important.