Food webs in tropical Australian streams: shredders are not scarce KIM CHESHIRE, LUZ BOYERO AND RICHARD G. PEARSON School of Tropical Biology, James Cook University, Townsville, Queensland, Australia SUMMARY 1. Macroinvertebrates were collected in dry and wet seasons from riffles and pools in two streams in tropical north Queensland. Total biomass, abundance and species richness were higher in riffles than in pools but did not differ between streams or seasons. 2. Gut contents of all species were identified. Cluster analysis based on gut contents identified five dietary groups: I, generalist collectors; II, generalist shredders and generalist predators; III, generalist scrapers; IV, specialist shredders; and V, specialist predators. Species were allocated to functional feeding groups (FFGs) based on these dietary groups. 3. Many species were generalist in their diets, but specialist predators and shredders were particularly prominent components of the invertebrate assemblages in terms of biomass and species richness. 4. Community composition (proportions of biomass, abundance and species richness of the different FFGs) varied between habitat types, but not between streams or seasons, although differences between riffles and pools varied with season. 5. Comparison of the fauna of 20 streams showed that our study sites were similar to, or not atypical of, low-order streams in the Queensland wet tropics. Keywords: functional feeding groups, gut content analysis, latitude, macroinvertebrates Introduction Food webs in forest stream ecosystems are typically driven by allochthonous organic inputs, mainly leaf litter, as the main energy source (Kaushik & Hynes, 1971; Vannote et al., 1980). In streams, leaf litter is readily leached, colonized and decomposed by micro- organisms, and consumed by macroinvertebrate shredders (Gessner, Chauvet & Dobson, 1999). These processes lead to production of fine particulate organic matter (FPOM), which is consumed by a suite of collector organisms. The shredders and collectors are thus the major primary consumers in forest streams, providing the main link between the organic inputs and the predatory invertebrates and vertebrates. The phenomenon of leaf processing by macroinver- tebrate shredders has been extensively studied in temperate streams, where a numerically important shredder guild, comprising especially stoneflies and caddisflies, exists. Leaf decomposition rates have been shown to depend on a range of factors, including physicochemical characteristics of the water, the conditioning of leaves by microbial colonization, and feeding preferences of shredders (Grac ¸a, 2001). Much of this activity takes place in the cooler seasons, following major leaf fall in the autumn and continuing processing of accumulated litter through winter. These patterns have been particularly well des- cribed in the temperate zone but, as is often the case, there are fewer published studies on tropical systems. Tropical rainforest streams may receive comparable leaf litter inputs to their temperate counterparts (Benson & Pearson, 1993; Dobson et al., 2002), but some differences exist. For example, in the Australian wet tropical rainforests, the peak of litter fall occurs in Correspondence: Luz Boyero, School of Tropical Biology, James Cook University, Townsville, Queensland 4810, Australia. E-mail: luz.boyero@jcu.edu.au Freshwater Biology (2005) 50, 748–769 doi:10.1111/j.1365-2427.2005.01355.x 748 Ó 2005 Blackwell Publishing Ltd