Detritivore Diversity or Dominant Species: What Drives Detrital Processing?

The role that biodiversity plays in maintaining ecosystem functioning has been debated for nearly two decades. Previous research suggested that a dominant species (Pycnopsyche gentilis) and not detritivore diversity determined leaf breakdown in a southern Appalachian stream. However, these previous studies could not directly compare the effects of other large detritivores (Tallaperla and Tipula) to that of Pycnopsyche. Here I report the results of a field experiment where I created monocultures, 2- and 3-species combinations of these three species and examined their effect on leaf breakdown. This experimental design allowed me to determine if these other taxa altered the effect of Pycnopsyche. Treatments containing Pycnopsyche exhibited the highest levels of leaf breakdown. Leaf mass lost (LML) in the 3-species polyculture was not significantly different from the Pycnopsyche monoculture. LML in the Tallaperla monocultures was indistinguishable from microbial breakdown. Tipula monocultures had intermediate LML. Neither Tallaperla nor Tipula facilitated or inhibited Pycnopsyche. However, when present together, Tallaperla inhibited breakdown by Tipula. My results confirm that Pycnopsyche is the functionally dominant detritivore in this system and suggest a novel mechanism that can influence diversity-function relationships. Inhibitory interactions among functionally subordinate species may result in reduced ecosystem function despite increases in species richness.