Top-down control of prey increases with drying disturbance in ponds: a consequence of non-consumptive interactions?

Biotic interactions are often expected to decrease in intensity as abiotic conditions become more stressful to organisms. However, in many cases, food-web and habitat complexity also change with abiotic stress or disturbance, potentially altering patterns of species interactions across environmental gradients. We used a combination of field assays and mesocosm experiments to investigate how disturbance from desiccation moderates top-down control of prey by predators across a gradient of pond duration in New Zealand. Field manipulations of predator abundance in ponds led to an unexpected decrease in the top-down control of prey biomass by predatory invertebrates as pond duration increased (decreasing abiotic stress). Predatory fish, which are restricted to permanent ponds, had negligible effects on prey biomass. Mesocosm experiments further indicated the consumptive effects of fish are weak; a result that cannot be explained by an increase in physical habitat refugia in relatively more permanent ponds. Manipulations of invertebrate predator diversity in mesocosms (both substitutive and additive treatments), and the addition of olfactory fish cues, revealed that strong non-consumptive effects of fish reduced predation by predatory invertebrates, and these effects overwhelmed the positive influence of invertebrate predator diversity on prey consumption. These results suggest that decreases in top-down control with increasing pond permanence are likely a result of non-consumptive effects of fish weakening predation by invertebrate predators in the more complex food webs of permanent ponds. Therefore, considering non-consumptive effects of predators in complex food webs will likely improve the understanding of biotic interactions across environmental gradients.