Effects of fish predators and litter pack size on leaf breakdown in a subtropical stream

Abstract

Many studies have examined trophic cascades in allochthonous lotic systems of temperate streams, but few have examined those in subtropical and tropical streams. Many subtropical and tropical streams are characterised as detritivore-poor stream systems, which might result in a fundamental difference in the trophic cascades of brown food webs. We performed an in situ experiment to test the effects of the presence or absence of a small-bodied endemic benthic fish and of the mass of litter packs (five levels), arranged in a full factorial design (10 treatments), on macroinvertebrate detritivore assemblages and invertebrate-mediated breakdown rates and to identify possible trophic cascades. Fish slightly reduced the invertebrate-mediated breakdown rates by depressing detritivore abundance. Effects of fish predators and litter pack size on detritivore abundance were species dependent. Nemouridae (Plecoptera), a non-camouflage detritivore (without a casing), was highly sensitive to the presence of fish predators. By contrast, camouflaged detritivores, such as Stenochironomus spp. (Diptera), which mine decayed leaves, and Anisocentropus sp. (Trichoptera), a case-building caddisfly, were able to avoid being preyed upon and thereby buffered the effect of predation on litter processing. These results suggest that ecosystem functioning of detritus-based stream systems could be regulated by the traits of dominant detritivores.