The influence of macroinvertebrate shredders, leaf type and composition on litter breakdown in a Hong Kong stream

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The contribution of shredders to litter breakdown in a tropical stream in Hong Kong was investigated by using coarse- and fine-mesh bags to include or exclude large macroinvertebrates. Three native species, plus a mixture of all of them, were used to examine the effects of leaf type and species composition on breakdown. A well-studied temperate species, Alnus glutinosa, was included to benchmark breakdown rates in Hong Kong with counterpart streams outside the tropics. Shredders constituted 13% of macroinvertebrates in coarse-mesh bags but were virtually absent from fine-mesh bags. However, abundance of macroinvertebrates (especially chironomids) in fine-mesh bags was almost twice that in coarse-mesh bags. Despite interspecific differences, litter breakdown rates were consistently faster in coarse-mesh bags, indicating a probable effect of shredders, although current velocity also influenced breakdown of one native species. Alnus glutinosa breakdown rates were within the range reported in temperate streams. The litter mixture broke down at a rate approximately equal to the mean of the constituent species and non-additive compositional effects (indicative of biodiversity-ecosystem functioning relationships) were not apparent. Litter breakdown in fine-mesh bags was rapid relative to rates reported from temperate streams, despite the exclusion of shredders. Evidently, non-shredding macroinvertebrates and microbes played a significant role in litter breakdown in Hong Kong.