Numerical and per capita responses to species loss: mechanisms maintaining ecosystem function in a community of stream insect detritivores

We experimentally reduced the diversity of detritivorous stream insects in field enclosures, and measured the effects on an ecosystem function, processing of leaf litter. Two dominant species were removed separately, the stonefly Pteronarcys californica and the caddisfly Lepidostoma unicolor. In principle, processing could be maintained after species loss in two ways: the remaining species could increase their rates of shredding (per capita response), or they could increase in abundance (numerical response). We imposed a numerical response in some treatments by experimentally increasing abundances of either all the remaining species or the other dominant species so that expected metabolic capacity of the assemblage returned to full-diversity levels. Numerical responses were generally effective in maintaining leaf breakdown when either Lepidostoma or Pteronarcys was removed, except that the treatment in which Lepidostoma was replaced by an equivalent metabolic capacity of all remaining species showed less leaf loss than the full-diversity treatment. Per capita responses by other species appeared effective in compensating for the removal of Pteronarcys (although there were other explanations) but were not effective in compensating for the removal of Lepidostoma. In summary, the consequences of reduced biodiversity varied with which species was lost and how the remainder responded. Thus there was no simple relationship between biodiversity and ecosystem functioning. However, when numerical or per capita compensation does occur, stability of function should rise with diversity in such "interactive" assemblages.