Leaf litter composition and community structure: Translating regional species changes into local dynamics

The temperate deciduous forest of the northeastern United States is believed to be undergoing a shift from domination by mixed oak (Quercus sp.) to red maple (Acer rubrum); a shift termed "The Red Maple Paradox." This shift in the composition of the forest has the potential to affect ecosystems within the forest that use leaf litter as an energy source. To assess the impact of the red maple paradox on detrital food webs, we conducted a mesocosm experiment investigating the influence of red maple litter on the food web of temporary woodland ponds. Amphibian biomass and survival were significantly lower in experimental ponds that contained only maple litter. The negative effects of maple litter were most pronounced in larval wood frogs (Rana sylvatica). We hypothesize that this effect resulted from a temporary shift from a grazing-based food web in oak mesocosms to a microbial-based food web in maple mesocosms, as oak mesocosms initially had higher levels of phytoplankton biomass and lower levels of bacterial production than maple mesocosms. This bottom-up effect of maple litter on wood frogs then appeared to affect other amphibians and zooplankton through a complex series of interactions. These data suggest that litter composition impacted the performance of most trophic groups by altering the processing of energy in this system, thereby destabilizing species interactions throughout the food web. Therefore, "subtle" compositional shifts in the forest have the potential to influence communities that rely upon leaf litter inputs as a primary source of energy.