Leaf-litter breakdown in streams of East Malaysia (Borneo) along an altitudinal gradient: initial nitrogen content of litter limits shredder feeding

Breakdown of litter in streams is an important terrestrial-aquatic C pathway in forests. Macroinvertebrate detritivore shredders that feed on litter may be more abundant in temperate than tropical streams because the feeding guild is generally restricted to cool waters and because temperate streams may have more high-quality resources. However, rich shredder communities have been found in some high-altitude tropical streams. We investigated litter breakdown and the variation in resource quality of native riparian vegetation in tropical streams at different altitudes (86-3393 m) in eastern Malaysia (Borneo). At each stream, we collected 3 native local leaf species, analyzed their initial resource quality, and incubated them for 56 d in coarse- (shredder and microbial colonization) and fine- (only microbial colonization) mesh bags. We incubated Macaranga tanarius litter in all streams to compare breakdown rates directly. Shredder-mediated breakdown of native litter was detected in only 5 of 24 cases and was not accompanied by altitudinal trends, whereas Macaranga shredder-mediated breakdown rates increased with elevation when rates were normalized for temperature (/degree-day). Regressions of resource quality and breakdown rates revealed consistent effects of initial N concentration in predicting breakdown rates that transcended decomposer and temperature effects. Shredder abundance, but not diversity, increased with altitude. The shredder guild may be important in tropical streams, particularly streams at higher altitudes and when litter N is not limiting.