Parasite-Mediated Energy Subsidy: Terrestrial Hosts Dominates the Energy Intake of Land-Locked Japanese Charr

In the southern part of their distribution, charrs often spend all of their life in small streams with low in situ productivity. Many literatures have suggested the importance of trophic subsidy, terrestrial invertebrate input into stream, to maintain charr population. However, ecological mechanisms underlying the terrestrial input have completely ignored.

In this presentation, we show that parasites with unusual parasitic life-history strategy enhance the trophic subsidy for charr population through riparian ecosystems. In a Japanese headwater stream, camel crickets and grasshoppers (Orthoptera) were 20 times more likely to enter a stream if infected by nematomorph parasites (Gordionus spp.), corroborating evidence that nematomorphs manipulate their hosts to seek water where the parasites emerge as free-living adults. Endangered Japanese charr (Salvelinus leucomaenis japonicus) readily ate these infected orthopterans, which, due to their abundance, accounted for 60% of the annual energy intake of the charr population. Charr grew fastest in the fall, when nematomorphs were driving energy-rich orthopterans into the stream.  When infected orthopterans were available, charr did not eat benthic invertebrates in proportion to their abundance, which let us to speculate the potential for cascading, indirect effects through riparian ecosystem. This hypothesis was demonstrated by our recent large-scale field manipulation experiment. Furthermore, we determined the phenomenon has commonly occurred at least through Japanese archipelago, although the amount and seasonality varied across streams and/or regions. Thus, the parasite-mediated terrestrial subsidy may partly explain why land-locked charrs can persist in small streams with low in situ productivity.