Yasushi Harada | Search USGS

USGS2011

A nematomorph parasite explains variation in terrestrial subsidies to trout streams in Japan

Nematomorph parasites alter the behavior of their orthopteran hosts, driving them to water and creating a source of food for stream salmonids. We investigated whether nematomorphs could explain variation in terrestrial subsidies across several streams. In nine study streams, orthopterans comprise much of the stomach contents of trout (46 ± 31% on average). Total mass of ingested prey per trout biomass positively correlated with the mass of orthopterans ingested, suggesting that the orthopterans enhanced absolute mass of prey consumption by the trout population. The orthopterans ingested per trout biomass positively correlated with the abundance of nematomorphs in the stream, but not with the abundance of camel crickets (the dominant hosts) around the streams. Streams in conifer plantations had fewer nematomorphs than streams in natural deciduous forests. These results provide the first quantitative evidence that a manipulative parasite can explain variation in the allochthonous energy flow through and across ecosystems.

Oikos↗

USGS2011

Nematomorph parasites drive energy flow through a riparian ecosystem

Parasites are ubiquitous in natural systems and ecosystem‐level effects should be proportional to the amount of biomass or energy flow altered by the parasites. Here we quantified the extent to which a manipulative parasite altered the flow of energy through a forest‐stream ecosystem. In a Japanese headwater stream, camel crickets and grasshoppers (Orthoptera) were 20 times more likely to enter a stream if infected by a nematomorph parasite (Gordionus spp.), corroborating evidence that nematomorphs manipulate their hosts to seek water where the parasites emerge as free‐living adults. Endangered Japanese trout ( Salvelinus leucomaenis japonicus ) readily ate these infected orthopterans, which due to their abundance, accounted for 60% of the annual energy intake of the trout population. Trout grew fastest in the fall, when nematomorphs were driving energy‐rich orthopterans into the stream. When infected orthopterans were available, trout did not eat benthic invertebrates in proportion to their abundance, leading to the potential for cascading, indirect effects through the forest‐stream ecosystem. These results provide the first quantitative evidence that a manipulative parasite can dramatically alter the flow of energy through and across ecosystems.

Honshu↗