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Non-Consumptive Interactions between Grasshoppers (Orthoptera: Acrididae) and Wolf Spiders (Lycosidae) Produce Trophic Cascades in an Old-Field Ecosystem

Abstract Top-down control by spider predators on grasshopper herbivores can produce trophic cascades, which may enhance plant biomass and alter plant community composition. These trophic level effects may be the result of either predator reduction in prey numbers (i.e., consumptive effects) or decreased prey foraging time in response to predator presence (i.e., non-consumptive effects). However, predator-prey interactions can be context dependent and do not always affect the plant trophic level. We conducted a field and laboratory experiment in a Northern Wisconsin (USA) old field ecosystem to uncover whether consumptive or non-consumptive effects of spider predation on grasshopper herbivores result in a trophic cascade, and if so to determine the underlying mechanisms that drive these trophic cascades. In a field experiment, four treatments examined the effects of multiple trophic-level interactions on plant biomass: 1) control treatment of vegetation only, 2) a two trophic-level interaction (grasshoppers and vegetation), and two different three trophic-level interactions: 3) the presence of “predator spiders” to examine consumptive effects, and 4) “risk spiders” with their chelicerae disarmed with beeswax to examine non-consumptive effects. In addition, a lab experiment was conducted to examine behavioral responses by grasshoppers in the presence of both an armed-spider predator and a risk spider to assess whether food quality (high vs low C:N ratios) had an effect on this interaction. Both risk and predator spiders decreased the impact of grasshoppers on plant biomass in the field experiment, and equally reduced overall grasshopper survival, indicating a non-consumptive effect. At the behavioral level, grasshoppers exhibited anti-predator behavior at the expense of reduced food intake. Food quality had no effect on the survival of grasshoppers as foraging was sacrificed for predator avoidance. Taken together, our results indicate that the resulting trophic cascade was the result of non-consumptive effects and that spider presence alone may reduce grasshopper herbivory rates.