Drought rewires an old field food web through shifts in plant nutrient content and herbivore feeding behaviors

Food web rewiring is becoming more likely as climate change continues, yet few experimental studies have focused on it and even fewer have examined the effects of two or more climate variables simultaneously. To help fill this gap the current study examined the effects of warming and drought, both alone and in combination, on herbivore feeding behaviors in a well-known old field food web consisting of two plants (grass and goldenrod), one grasshopper herbivore (Melanoplus femurrubrum), and one arachnid predator (Pisaurina mira). Drought had much stronger effects than warming on goldenrod mortality and flowering, goldenrod nutrient content, herbivore feeding preferences, and live goldenrod biomass remaining at the end of the experiment, while grass was largely unaffected. Drought combined with warming to almost completely suppress goldenrod because of increased goldenrod mortality rates and the drought-stressed grasshoppers’ clear preference for consuming goldenrod with high foliar carbon concentrations. When compared with previous studies that have focused on warming in this system, the current study suggests that food web rewiring is very likely in old fields but the type of rewiring that may occur will be dependent on which climate variables shift more strongly.