Understanding trophic webs in natural communities requires clear understanding of the relative contributions of often non-linear top-down and bottom-up processes. In highly variable continental climates, it is important to know if unpredictable resource availability/quality and abiotic conditions affect species interactions, or alters responses in herbivore performance and its contribution to ecosystem function. Using a simple food chain model (grass-grasshopper-wolf spider), we investigated the relative direct and indirect interactions as they affect both grasshoppers under changing food and weather conditions using field experiments.
In field experiments, one-, two-, and three- level food chains were set up in continental tallgrass prairie in the Nebraska sandhills and at Konza Prairie. Responses in cages with vegetation only (V), vegetation and grasshoppers (VG) and vegetation, grasshoppers and spiders (VGS) were compared. Foliar quality was altered by applying either NH4NO3 (elevated foliar quality) or sugar (depressed quality). In addition to predator and food quality treatments, responses to temperature at Konza Prairie (plastic sheeting to increase temperatures or 50% shade cloth to decrease temperatures) were compared with ambient controls. We measured grasshopper performance (survival, fecundity, body mass) to assess the nature of direct and indirect interactions and measured vegetation biomass to evaluate the presence of trophic cascades.
Results/Conclusions
Strong compensatory outcomes were often observed when multiple factors were manipulated because the relative contributions from bottom-up and top-down forces were altered. Some examples follow The negative impact of spiders on grasshopper performance was ameliorated in a grass-feeding species if foliar protein levels were elevated. Variable temperature also affects arthropod species interactions.Trophic cascades were observed in the shaded treatment with significant predation pressure from spiders, but not in the ambient and warmed treatments. Grasshopper density effects were often observed. For example, in a mixed-feeding grasshopper species, food quality only affected performance at high densities, but trophic cascades only occurred at lower food quality or high densities. Other grasshopper species tested showed no density dependent responses.Egg production rates were not directly affected by the presence of spiders, but lifetime fecundity decreased from spider risk because the nymphal development time was increased and populations crashed in late summer. Studies of this arthropod food chain in a variable continental climate indicated that outcomes are context dependent and multiple factors alter the relative contributions of top-down and bottom-up processes.
