Shredders in Malaysia: abundance and richness are higher in cool upland tropical streams

Stream shredders have been reported as scarce in several tropical areas. This pattern is in contrast to observations in temperate streams, which support an abundant and diverse shredder fauna. Two possible explanations for this pattern are that most shredders are adapted to cool conditions and that temperate riparian vegetation often produces more palatable and more nutritious leaves than do the more diverse, tropical rainforests. In peninsular Malaysia, most streams flow through lowland dipterocarp forests that are characterized by strikingly high tree diversity and by many species with tough leathery leaves that are high in lignin and toxic secondary compounds and low in protein. In contrast, highland streams flow through montane rain forests and are more similar to temperate streams. We hypothesized that shredder fauna would be distributed along an altitudinal gradient, with more abundant and diverse assemblages in highland streams. We sampled leaf litter in 12 sites at altitudes from 55 to 1560 m above sea level. As expected, highland sites supported higher abundance and diversity of shredders (9-15 species per site) than did lowland sites (3-8 species per site). Shredder densities were similar among lowland sites, but species composition was variable. Large snails (Brotia spp.) were the dominant shredders in nonacidic streams in Taman Negara, but they were absent from acidic streams and from streams elsewhere in peninsular Malaysia. Shredder biomass was generally high because of the large body size of most species (e.g., crabs, snails, semiaquatic cockroaches, calamoceratids, and tipulids). Large mouthparts might allow these species to shred tough leaves efficiently, and large body size might enable them to tolerate a wide variety of toxic secondary compounds. Our results suggest that shredder diversity might depend on elevation, water temperature, characteristics of the riparian vegetation along altitudinal gradients, and variation in water chemistry at local scales.