Biodiversity and ecosystem functioning in stream detritivore systems : patterns and mechanisms.

Global changes in biodiversity have prompted ecologists to examine the relationship between biodiversity and the functioning of ecosystems. The occurrence and form of such a relationship, and the mechanisms driving it, are critical for predicting the effects of biodiversity loss. The aim of this study was to address whether species diversity is important for ecosystem function in streams, focusing on the relationship between the diversity of leaf-shredding macroinvertebrates and detritus processing. An initial field study revealed that there was no simple positive relationship between shredder diversity and leaf processing at a regional scale (i.e. between streams), but when combined with other biotic and abiotic factors did contribute significantly to explaining leaf-processing rate. The presence of particular species also appeared to be important for leaf processing. A second field study tested whether an increased variety of leaf types might lead to increased detritus processing in high, compared to low, diversity shredder communities. Again, particular species, or species combinations, appeared to be important in processing a mixed leaf resource, but shredder diversity was unimportant. Artificial streams were then used to examine experimentally the effects of shredder identity and species number on detritus processing. Once more shredder diversity had little effect on processing rates, while shredder identity was important, on single and mixed leaf resources. A final experiment quantified the extent of resource-use complementarity among a set of shredder species, and then tested whether differences in complementarity among species pairs influenced leaf-processing rates. Although shredders did show differences in their leaf diets, experimental increases in complementarity did not translate into positive effects on leaf processing rates. Both field and experimental studies provided little clear evidence for a positive diversity-function relationship, or the mechanisms proposed to underpin it. Results imply that the consequences of biodiversity loss for stream ecosystem function are dependent on which species are lost.