Decomposition of allochthonous organic matter that enters shaded headwater streams during a short autumn leaf fall period provides much of the energy the streams receive throughout the year. As such, alterations of riparian communities, including those resulting from invasive species, should have a significant impact on these energy inputs and potentially alter microbial communities and the behavior of shredders that process leaf litter. We compared consumption of leaves of an abundant native species (green ash Fraxinus pennsylvanica) and nonnative species (common buckthorn Rhamnus cathartica) by the northern spring amphipod Gammarus pseudolimnaeus, with and without periods of stream conditioning. Amphipods consumed a very small proportion of the unconditioned leaves of common buckthorn and green ash and demonstrated no significant preference for the unconditioned leaves of either species. When leaves were stream-conditioned, there was a significant interaction between the effects of leaf species and days of stream conditioning on both the mass and area of leaf disks consumed by the amphipods. Overall there was a greater consumption of buckthorn leaves compared to ash, with peak consumption occurring during a narrower range of conditioning times for buckthorn. Common buckthorn's higher overall consumption level is expected to provide rapid and short-lived input of energy into the stream, in marked contrast to the slower and more sustained input from native green ash. These results suggest there may be significant changes in the organic matter dynamics of stream ecosystems throughout the Midwest as common buckthorn continues its expansion and green ash declines with the invasion of emerald ash borers (Agrilus planipennis).
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