Litter decomposition in streams represents an important terrestrial-aquatic link in tropical forests and studies have shown that detritivore shredders are more abundant in temperate than tropical streams because the feeding guild is believed to be more adapted to cool waters where there is greater availability of high quality resources. I hypothesized that this global variation in shredder distribution may be observed at local scales in tropical regions along an altitudinal gradient and investigated the importance of shredders in litter breakdown using field-based techniques in streams flowing through the different forest types of East Malaysia (Borneo) that correspond to different altitudes (21 - 3393 m above sea level). I found evidence of increasing shredder abundance and relative abundance with altitude as a possible function of decreasing temperature and decreasing lignin concentrations in litter. The altitudinal variations in taxonomic composition of shredder assemblages were distinct between lowland streams that were dominated by semi-aquatic cockroach, beetle (Elmidae) and stonefly (Neumouridae and Cryptoperla) shredders, and highland streams that were represented by Cranefly (Tipulidae) and Caddisfly (Lepidostomatidae) shredders. Environmental effects on shredder distribution were detected, but this did not translate into a parallel functional trend of increased shredder-mediated decomposition of local riparian litter with increasing elevation. Initial nitrogen content of litter, which was not altitude related, was a strong and consistent predictor of decomposition rates throughout the study. Diversity of shredders in lowland streams ranged from 0 to 8, and species composition was highly variable. Conversely, in highland streams, diversity was more
stable in terms of both species richness (range 3 to 5) and composition. The high variability, across lowland streams, has typically been observed in other tropical surveys of shredder distribution. We may have captured the effect of this variable distribution on decomposition, since biodiversity effects on decomposition were detected in our study. Whether this relationship is a direct function of species interactions through complementary or facilitative effects, or strongly mediated by the presence of large shredders typical to lowland streams, remains untested.
The relationship between shredder communities and environmental conditions may be general, transcending local factors, because this association is observed at both latitudinal and altitudinal gradients. In contrast, ecosystem functioning may not follow this general trend since biodiversity effects on decomposition are highly specific, reliant on shredder species identity, functional traits, composition and interactions. Shredder community structure may show patterns across broad biological scales, but functional patterns would be reliant on the local interactions within these structures
