Leaf litter in a tropical stream: food or substrate for macroinvertebrates?

The densities of macroinvertebrates colonizing three types of leaf litter in a shaded and an unshaded reach of a Hong Kong stream were compared in order to determine whether the leaves served as a food source or a substrate for colonizers. Macroinvertebrate densities on leaf packs made of Ficu fistulosa (Moraceae) - a palatable leaf were compared with those on Castopsis fissa (Fagaceae) - a less palatable species rich in polyphenols - and on artificial and inedible polyester leaves. Colonizer densities (total individuals and abundance of major taxa) were highest on Ficus leaves and lowest on artificial leaves. Densities increased with time and most of the variation in colonizer densities was explained by leaf type and time. Numbers per leaf pack increased with time on Castanopsis and artificial leaves, but abundance on Ficus declined at the end of the study (day 35) when little of the leaf packs remained. The densities of Baetis mayflies (Baetidae) and Orthocladiinae (Diptera: Chironomidae) were apparently affected by the amounts of algae that developed on leaf surfaces, and these taxa were more numerous on leaf packs placed in the unshaded stream reach. However, the effects of algae did not override the influence of leaf type. Despite a paucity of shredders in this tropical stream, we believe that greater densities of macroinvertebrates (mainly collectors) on palatable leaves reflected their importance as a food source rather than a substrate.