Species interactions in a warming climate: Examining the direct and indirect effects of climate change on New England grassland food webs

Global climate is predicted to change significantly during the next century. Effects of climate change are already evident as shifts in the geographic distribution of species and altered species phenology and life-history. However, predicting the particular fate of species in food webs is difficult because the net effects of climate change result from multiple interacting species responding to multiple changes within their environment. My research contributes to our understanding of climate change effects on species through a series of experiments that revealed the mechanisms by which climate warming alters food web interactions in a grassland community comprised of herbaceous plants, grasshopper herbivores and spider predators. Pisaurina mira spiders are indirect keystone predators that, when present, cause grasshoppers to shift from a grass-based diet to an herb-based diet. Consequently, P. mira has a positive indirect effect on grass biomass and a negative indirect effect on herb biomass. Synthesis of data spanning 15 years revealed that the magnitude of the top-down effect increased with warning. This happened because warming had no affect on grasshopper location but caused P. mira to seek thermal refuge lower in the plant canopy. Thus, by decreasing the degree of overlap between predator and prey, warming decreased predation risk and allowed grasshoppers to increase daily feeding time on herbs. A second spider predator, Phidippus rimator, cohabits these grassland communities by occupying positions lower in the plant canopy, spatially segregated from P. mira. Experimental warming forced these two spider predators to interact within the lower plant canopy, resulting in intraguild predation by P. mira and the local extinction of P. rimator. The effects of climate change on food webs may be reduced if species respond to warming via phenotypic plasticity or evolutionary responses. I tested this prediction by examining P. mira spiders along a 4.8°C temperature gradient and found that spiders from warm study sites had higher tolerance to warming, which allowed them to remain high in the plant canopy and precluded warming from increasing top-down effects. Thus, this dissertation not only identified a mechanism by which warming can alter species interactions, but also demonstrated that species responses to chronic climate change may mitigate some of the effects of warming on food webs.