Importance of species identity and number for process rates within different stream invertebrate functional feeding groups

1. The effects of decreasing species richness on process rates were investigated in laboratory experiments within three different stream invertebrate functional feeding groups: filter feeders, grazers and predators. 2. We found significant effects in all groups; however, the direction and magnitude varied. The effects were not due to species richness per se as species identity was the only significant factor in all analyses. 3. Filter-feeding collectors showed a decreasing filtration rate with decreasing species richness in a first experimental trial while a second one showed no trend. In contrast, process rates increased with decreasing species richness in both grazers and predators. 4. Comparisons of expected with observed process rates in two- and three-species treatments indicated underlying mechanisms. In grazers resource dominance by one species seemed to negatively affect the other species so that grazing rates decreased when species were combined. In predators interference by two of the species on a smaller predator species seemed to affect predation rates negatively. 5. Our results show that the effects of species loss greatly depend on what type of interactions the lost species participated in and that species identity may be the most important factor generating effects of species loss in cases when the interaction affects species differently.