Upstream Deforestation and Nonnative Crops Affect Leaf Breakdown and Aquatic Invertebrate Assemblages in Streams

ABSTRACT Stream ecosystem functioning and community structure are influenced by local and catchment land use. In mosaic landscapes, agricultural activities can also affect leaf breakdown and detritivore assemblages in downstream forest reaches. We evaluated the combined effects of upstream deforestation and the input of a nonnative crop species on leaf breakdown and associated invertebrate assemblages within forest reaches. We incubated the leaves of banana trees ( Musa spp.) and the native tree species Miconia chartacea in eight streams under both single‐species and mixed‐species treatments. The studied reaches were located upstream of the deforested areas in half of the streams and downstream of the deforested areas in the second half. We tested whether (i) crop leaves influence the leaf breakdown of native leaves and leaf colonisation by aquatic invertebrates and (ii) whether upstream deforestation exacerbates the effects of crop leaves on these processes. In both reach types, M. chartacea monocultures had the highest breakdown rate, followed by M. chartacea exposed in mixtures, Musa spp. exposed in mixtures, and Musa spp. monocultures. All leaf treatments decomposed more slowly in downstream forest reaches, with mass losses 17.8%–8.7% lower than the mass loss of M. chartacea monocultures in upstream forest reaches. The invertebrate richness and density, shredder density, and scraper density did not differ among leaf treatments or reach types. The abundances of collector‐gatherers were positively correlated with the mass loss of M. chartacea and Musa spp. In the downstream forest reaches, the calamoceratid shredder Phylloicus and the hydropsychid collector‐filterer Smicridea were positively correlated with overall leaf mass loss. Our study showed that upstream deforestation may cause alterations in the taxa distributions of invertebrate assemblages with consequences on leaf breakdown rates in forest reaches. These effects however strongly depend on the characteristics of the leaves and of the detritivores (i.e., their identity).