Leaf breakdown in tropical streams: the role of different species in ecosystem functioning

Forested headwater streams are detritus-based systems where shredder activity is usually crucial, but information on the role of different species on ecosystem function is very limited, especially in the tropics. We investigated the roles of the four most common shredder species on leaf breakdown in two tropical Australian streams by (1) studying variation in their abundance between seasons and habitat types, (2) relating their abundance to in situ leaf breakdown rates, (3) determining their leaf breakdown rates in the laboratory, and (4) comparing their niche characteristics. The species differed in their abundance, seasonal and spatial variation, leaf breakdown rates and other characteristics, suggesting different roles in the ecosystem: Lectrides varians was the most abundant species in pools in both the dry and wet season, and also the fastest processor, suggesting a particularly strong influence of this species on leaf breakdown; Anisocentropus kirramus appeared to have an important role in riffles, being the only shredder species found in that habitat; Triplectides gonetalus predominated in patches of small woody material, and used wood as food; and Atalophlebia sp. was important as both a collector and a shredder in pools. Our results suggest each species has a different role in the ecosystem, having an important function in a specific habitat or microhabitat, or showing some differentiation in resource use. This implies that the effect of species loss on ecosystem functioning would be more dependent on the identity, rather than the number, of the species that are lost.