Shredders: species richness, abundance, and role in litter breakdown in tropical Hong Kong streams

Shredders appear to be scarce in many tropical streams, but few data are available about their abundance or role in litter breakdown in these systems. Shredder species richness, abundance, and biomass were investigated in 10 small streams (5 shaded, 5 unshaded) in Hong Kong as a step toward understanding the role of shredders in tropical Asian streams. In addition, breakdown of Liquidambar formosana (Hamamelidaceae) leaves was investigated to determine if breakdown rates changed in response to shredder species richness, abundance, or biomass in these 10 streams. Shredders were scarce in terms of species richness (a total of 6 obligate shredders and 2 facultative shredders) and abundance (mean = 2.0%, range = 0-7.7% of total macroinvertebrate abundance). Obligate shredder biomass was not high across the 10 streams (mean = 13% of total biomass), and it was variable in shaded streams (0.6-38.7%); all high values were attributable to the presence of a few large individuals (<0.2% of total abundance). Relative abundance and biomass of obligate shredders were higher in shaded than in unshaded streams (abundance: 1.9% vs 0.1%; biomass: 22.3% vs 4.3%). Shading did not affect the densities or relative abundances of functional feeding groups (FFGs) other than shredders. Litter breakdown rates did not vary in response to species richness, abundance, or biomass of shredders among the 10 streams. However, when the 2 moderately nutrient-enriched streams were excluded, breakdown rates were positively related to obligate shredder densities, indicating a possible contribution of shredders to litter breakdown. The potential importance of microbes was indicated by a relationship between breakdown rates and stream PO4 concentrations.