Abstract Little is known about the mechanisms behind positive effects of species richness on ecosystem functioning. In a previous study that showed a positive effect of aquatic detritivore species richness on leaf litter breakdown (process) rates, we proposed that facilitation and release from intra-specific interference were the two most likely mechanisms. To test the interference hypothesis, we performed an experiment using three densities of each of three detritivore species and found varying effects on leaf breakdown rates across species: one species showed no effect, one a positive, marginally insignificant, effect, and a third species showed a significant, positive effect of decreasing density. The density (interference) effect thus partly explained the results from our previous study. The facilitation hypothesis was tested by sequentially introducing and removing two species. We predicted that, if this hypothesis were true, facilitation would be expressed in higher process rates than when replacing with individuals of the same species. We found that process rate per unit biomass did increase when one species was introduced after the other species, while the opposite sequence did not show any increase. Hence, this result was also confirmative of our previous results. Therefore, we conclude that both intra-specific interference and inter-specific facilitation may explain the positive effect of species richness observed in our system. Since many species exhibit intra-specific interference that inhibits foraging efficiency, this may be a general mechanism generating effects of species richness per se. If facilitation is unidirectional, or if it involves few species, it is more likely to be species specific with species identities being more important than species richness per se. We conclude that species loss may be expected to have negative consequences on ecosystem functioning if any species is lost, with additional effects in the event of losing "facilitator" species.
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