Effects of leaf toughness and nitrogen content on litter breakdown and macroinvertebrates in a tropical stream

High plant diversity in the tropics results in leaf litter of widely differing quality that may affect litter breakdown rates in streams. The effects of leaf toughness and nitrogen content on litter breakdown rates and macroinvertebrate colonization were investigated in Tai Po Kau Forest Stream, Hong Kong during dry seasons in 3 different years, using 7 species of leaves (Arundinaria shiuyingiana, Ficus fistulosa, Indocalamus sinicus, Liquidambar formosana, Livistona chinensis, Melaleuca quinquenervia, Pandanus furcatus) with contrasting characteristics. Liquidambar formosana (Hamamelidaceae) was included as a benchmark species (for comparison) in all 3 dry seasons, but the identity of the other leaf species varied among years. There was no significant inter-year difference in breakdown rates of L. formosana. Leaf toughness was the primary determinant of breakdown rates, which were negatively correlated with leaf strength. Soft leaves had faster breakdown rates than tough leaves, irrespective of their nitrogen content. Collector-gatherers dominated (> 60% of individuals) macroinvertebrate assemblages on litter, and obligate shredders were scarce (1-6%). Toughness and nitrogen content of leaves were not good predictors of the composition of litter-associated macroinvertebrate assemblages, which were similar among leaf species studied within the same season. There were, however, small but significant inter-year variations in macroinvertebrate assemblages on L. formosana litter, mainly due to changes in predator abundance. The absence of a deterministic link between leaf quality and macroinvertebrate assemblage composition suggests that litter served as a substrate rather than a food source for macroinvertebrates, and may account for the scarcity of shredders in tropical streams.