| Abstract: | Several studies have reported a positive relationship between species richness and ecosystem functioning. However, if much of a particular ecosystem function is performed by one species (i.e. a functionally dominant species) and this species is also a competitive dominant that excludes other taxa from a habitat, then it is possible to obtain a negative relationship between richness and ecosystem functioning. Results of a leaf pack breakdown experiment in a small stream suggested that the caddisfly Pycnopsyche gentilis , a common detritivorous insect in North American headwater streams, was both a functional and competitive dominant. In a second experiment we compared the effect of Pycnopsyche on leaf breakdown to that of other detritivore taxa by enclosing them with leaf packs in a section of headwater stream in which they were uncommon ( Pycnopsyche transplant experiment). Final leaf pack mass was significantly lower in the Pycnopsyche enclosure treatment; leaves exposed to a greater diversity of detritivores displayed little reduction in leaf mass. These results demonstrated that Pycnopsyche was a functionally dominant detritivore. In a third experiment ( Pycnopsyche density experiment) we found that Pycnopsyche was also a competitively dominant species. Leaf packs and large Pycnopsyche were placed in enclosures that were permeable to the majority of other detritivores but not Pycnopsyche . Leaf mass lost increased with increasing Pycnopsyche density. Leaf packs exposed to Pycnopsyche , however, contained fewer detritivore taxa which suggested that Pycnopsyche was also a competitive dominant. There was a negative relationship between three measures of diversity and leaf litter breakdown in the Pycnopsyche density experiment. Experiments conducted in natural communities that incorporate important species interactions may produce diversity-ecosystem function relationships other than the positive ones that are commonly reported. |
