Cooperative Fish and Wildlife Research Units Program: Alaska
Education, Research and Technical Assistance for Managing Our Natural Resources

Resource subsidies often have major consequences on recipient species and food webs. While invasive species are known to negatively affect some species and ecosystems, their influences on prey subsidies and effects on recipient consumers are unknown. The green alder sawfly (Monsoma pulveratum) is a recently introduced invasive wasp responsible for defoliating riparian thin-leaf alder (Alnus tenuifolia) stands across southcentral Alaska. To examine the effect of riparian alder defoliation by the larval green alder sawfly on the flow of terrestrial invertebrate prey subsidies to stream salmonids, we sampled 1) terrestrial invertebrate communities on riparian alder foliage, 2) their subsidies to streams, and 3) prey consumption by juvenile Coho Salmon (Oncorhynchus kisutch) along stream reaches with low and high densities of this invasive Hymenopteran. Sawfly biomass peaked mid-summer in all three categories—on riparian alder foliage, as terrestrial subsidies to streams, and ingested by juvenile Coho Salmon. Broader terrestrial invertebrate communities did not decline from sawfly defoliation; instead, invasive sawflies were supplemental prey for juvenile Coho Salmon mid-summer, on top of the background invertebrate subsidy. Our study showed that invasive green alder sawfly defoliation changed terrestrial prey subsidies for these stream fishes, as predicted. Considering that this study was conducted at the early stage of sawfly invasion, the loss of riparian alder is expected to have other ecological consequences that merit further investigation. However, in this case, invasive sawflies did not affect terrestrial subsidies to stream consumers in these study streams in southcentral Alaska.