Understanding how biotic and abiotic factors combine to affect species interactions is an important challenge in ecology. Because the effects of biotic and abiotic factors on species interactions are often studied separately, it can be difficult to predict potential non-linear and indirect interactions among biotic and abiotic sources on species interactions. We manipulated three key biotic and abiotic factors (temperature, food quality, predation) in field experiments using a model plant – grasshopper – wandering spider food chain. We evaluated whether temperature manipulations affect grasshopper performance through altered predator-prey interactions, and whether trophic cascades occur.

Field enclosures containing one-, two-, and three- level food chains were set up in continental tallgrass prairie at Konza Prairie Biological Station in Manhattan, KS. Foliar quality was reduced by applying sugar to half of the enclosures every four weeks. Temperature was manipulated with plastic sheeting to increase temperatures or 50% shade cloth to decrease temperatures, which were compared with ambient controls. Warmed treatments increased air temperatures ~3° C, while shaded treatments reduced air temperatures ~2° C relative to ambient. We measured vegetation biomass to identify trophic cascades and also measured grasshopper performance (survival, fecundity, body mass).   

Results/Conclusions

Our results show that temperature can fundamentally alter predator-prey interactions between wandering spiders and grasshoppers, but the way in which predator-prey interactions were influenced depended on whether temperatures were increased or decreased. Grasshopper survival was reduced in the ambient and shaded treatments in the presence of spiders, but not in the warmed treatment, in part because spiders are less active at warmer temperatures than grasshoppers. Trophic cascades were observed in the shaded treatment with significant predation pressure from spiders, but not in the ambient and warmed treatments. Increased temperatures changed the effects of spider predation on grasshopper survival, but not on trophic cascades.  Decreased temperatures altered the occurrence of trophic cascades, but did not alter effects of spiders on grasshopper survival. Manipulations of food quality altered grasshopper performance, but did not influence predator-prey interactions. These data indicate the importance of temperature in structuring species interactions among invertebrates.