Insect herbivory increases litter quality and decomposition: An extension of the acceleration hypothesis

Herbivore alteration of litter inputs may change litter decomposition rates and influence ecosystem nutrient cycling. In a semiarid woodland at Sunset Crater National Monument, Arizona, long-term insect herbivore removal experiments and the presence of herbivore resistant and susceptible pinyon pines (Pinus edulis) have allowed characterization of the population- and community-level effects of herbivory. Here we report how these same two herbivores, the mesophyll-feeding scale insect Matsucoccus acalyptus and the stem-boring moth Dioryctria albovittella alter litter quality, dynamics, and decomposition in this ecosystem. We measured aboveground litterfall, litter chemical composition, and first-year litter decomposition rates for trees resistant and susceptible to both herbivores and for susceptible trees from which herbivores had been experimentally removed for 16-18 years. Both herbivores significantly increased nitrogen concentration and decreased lignin:nitrogen and carbon:nitrogen ratios of aboveground litter. Herbivory by scale insects also increased litter phosphorus concentration and annual needle litterfall mass. Consistent with its increased chemical quality, litter from herbivore-susceptible trees decomposed more rapidly in the first year. These results suggest that herbivory may increase nutrient cycling rates in this system by altering the chemical quality of litter, a mechanism that augments the nutrient acceleration hypothesis. The distribution of trees resistant and susceptible to the two herbivores at our site, and their associated variation in litter quality and decomposition, likely creates a mosaic of litter quality and nutrient cycling rates at the landscape scale. Further, because the differences in litter quality are associated with tree resistance and susceptibility traits, our findings are among the first to establish that intraspecific genetic variation may affect ecosystem function.