Effects of abiotic conditions and herbivore density on tritrophic interactions in an old field food chain

An important challenge for ecologists is to understand how variation in climatological factors caused by global warming will affect biotic communities. Climatological factors such as temperature may have important impacts on species interactions and community processes, including the occurrence of trophic cascades. This study investigated the effects of temperature on species interactions in a model plant--grasshopper--spider food chain. Three main questions were addressed: 1) How do grasshoppers behaviorally respond to temperature and predation risk? 2) How do grasshopper performance traits (survival, fecundity, body mass) respond to variation in temperature, and can grasshopper density modify these responses? 3) How does temperature and grasshopper density affect the occurrence of trophic cascades in this system? Manipulative field experiments were conducted in an old field at the University of Notre Dame Environmental Research Center (East) in northern Wisconsin from 2002 - 2005. Food chain length, grasshopper density, and temperature were manipulated in field enclosures. Temperature was varied using shade cloth or plastic greenhouses placed over enclosures. Grasshopper behavior and performance traits were measured, as well as plant biomass. Spider presence caused grasshoppers to shift feeding to warmer parts of the day, and to increase overall time spent feeding. This is surprising because most studies find that grasshoppers decrease activity in response to predator presence. It is hypothesized that increased feeding in response to spider presence may be a mechanism to compensate for increased metabolic costs of foraging in warmer parts of the day. Both temperature and grasshopper density were found to be important factors affecting predator-prey dynamics. Furthermore, spiders modified grasshopper response to temperature. For example, grasshopper survival varied with temperature when spiders were not present. This was best explained by the effects of temperature on net resource intake. In contrast, when spiders were present, climate treatment had no effect on grasshopper survival. This indicates that spiders can buffer grasshopper survival from the effects of temperature change. Furthermore, the occurrence of trophic cascades varied with both temperature and grasshopper density. This research indicates that global climate change may have profound impacts on biotic communities, affecting species performance and species interactions.