The effects of diversity on stream ecosystem function were investigated by focusing the effects of shredder species richness on litter decomposition in Hong Kong. As part of this study, we conducted a series of laboratory feeding trials, using 3 common leaf-eating invertebrates: Anisocentropus maculatus (Trichoptera: Calamoceratidae), Brotia hainanensis (Gastropoda: Pachychilidae) and Caridina cantonensis (Decapoda: Atyidae). There were 7 experimental treatments (each replicated 6 times), in which species richness was manipulated from 1 to 3. Breakdown rates of Liquidambar formosana (Hamamelidaceae) leaves were quantified as differences between final and initial leaf mass after 10 days. Shredder species composition, rather than species richness, strongly affected leaf breakdown rates. Obvious negative non-additive effects were found in both 2-species treatments containing Trichoptera larvae, but neither in the 2-species treatment without these larvae nor in the 3-species treatment. The results of this study may support the ‘idiosyncratic’ hypothesis: i.e., the direction and magnitude of changes in ecosystem function are unpredictable when biodiversity changes.