Species interactions are often context-dependent, where biotic and abiotic components of the environment combine to influence the outcome of interactions.  Modifications of the abiotic context, such as those projected with climate change, are expected to impact species interactions.  However, the extent to which alterations to the abiotic environment affect species interactions may depend on the biotic context.  We examined how key biotic and abiotic factors (temperature, predation, herbivore density) affect herbivore performance and predator-prey interactions using a model plant – grasshopper – wolf spider food chain and two grasshopper species (Melanoplus bivittatus and Orphulella speciosa).  Predator-prey interactions between grasshoppers and wolf spiders may be especially sensitive to temperature because grasshoppers and wolf spiders have different preferred temperature ranges.  We created one-, two-, and three- level food chains in field enclosures at the Konza Prairie Biological Station in Manhattan, KS.  Temperature was manipulated by surrounding field enclosures with plastic sheeting to increase temperatures or with 50% shade cloth to decrease temperatures, which were compared with ambient controls.  Grasshoppers were stocked at high and low densities.  We measured grasshopper performance (survival, body size) and vegetation biomass to identify trophic cascades.   

Results/Conclusions

We find that grasshopper performance and predator-prey interactions in our system are temperature dependent, but the effect of temperature is mediated by the biotic context.  For O. speciosa, temperature had direct effects on grasshopper performance, as survival varied with temperature treatment in each year of the experiment.  However, temperature treatment had no direct effects on M. bivittatus performance.  Temperature indirectly affected both species by altering predator-prey interactions.  The effects of wolf spiders, which prefer cooler temperatures than grasshoppers, was intensified in the shaded treatments, and diminished in the ambient and warmed treatments.  Trophic cascades were only observed in shaded treatments where top-down processes were strengthened.  However, for M. bivittatus, which exhibited high density dependence, the effect of predators was only observed at the high density treatments.  Effects of wolf spiders on O. speciosa were observed at both density treatments.  Our work indicates that temperature can alter species interactions and highlights the importance of considering the biotic and abiotic context when predicting how species interactions will respond to temperature change.