Detritus processing by endemic and non-native Hawaiian stream invertebrates: a microcosm study of species-specific effects

Abstract

In studies of detritus processing in streams, invertebrate detritivores are often pooled into functional groups of species that have similar feeding modes. Differ- ences among functional groups in detritus processing are well-known, but differences among species within functional groups are rarely considered. Introductions of non- native species and losses of native populations are changing the composition of detriti- vore assemblages in many streams, and a species-specific approach is needed to under- stand the effects of these changes on detritus processing. In this study, we compared the rates at which the native and non-native invertebrate detritivores from streams on the Island of Hawai' i processed detrital leaves. Experimental treatments consisted of single detritivore species and species-pairs. Results from species-pairs were used to determine whether interspecific interactions increased (facilitated) or decreased (inhib- ited) rates of detritus processing. Results of the single-species treatments indicated that the endemic shrimp Atyoida bisulcata and the non-native crayfish Procambarus clarkii significantly increased rates of leaf processing compared with controls with no ani- mals; two other non-native and one other endemic species did not have significant ef- fects. For all species-pairs, observed processing rates were not significantly different from rates predicted in the absence of interaction, i.e., the sum of the effects of each species alone. At present, P. clarkii is the only leaf-shredding stream invertebrate in Hawai'i. P. clarkii processed detrital leaves >10 times faster than the other species, and any facilitory or inhibitory interactions between P. clarkii and other species are likely to be small relative to the effects of P. clarkii alone. Results of the experiments suggest