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

Abstract

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.

Nematomorph parasites indirectly alter the food web and ecosystem function of streams through behavioural manipulation of their cricket hosts

“…Because cricket subsidies occur over large areas (Sato ; Sato et al . ), these results can apply to whole streams, not just reaches. Furthermore, although terrestrial‐aquatic linkages are most obvious in the headwater regions of streams (Vannote et al .…”

“…Nematomorph parasites create a resource subsidy for trout in the form of manipulated crickets (Sato et al . ,b) that might have broad implications. Our experiment suggests that this subsidy was enough to alter a trophic cascade that indirectly increased grazers and shredders, which, in turn, decreased benthic algae and, to a lesser extent, increased leaf break‐down rate.…”

“…75%) natural inputs known from previous studies of Kirikuchi char (Sato et al . ,b). This treatment was the ‘control’ we used to approximate what we think happens in nature.…”

“…The experiment coincided with the season when nematomorphs manipulate their cricket hosts (Sato et al . ,b). The two study streams (Hachiman and Ninomata streams) have similar in‐stream characteristics and riparian vegetation (see Table S1 in Appendix S1).…”

Terrestrial prey inputs to streams bordered by deciduous broadleaved forests, conifer plantations and clear‐cut sites in southwestern Japan: effects on the abundance of red‐spotted masu salmon

“…Although lateral input of ground‐surface invertebrates may be important as well (Sato et al. ), falling input is the main component of prey inputs from outside of the stream (Nakano et al. ; Kawaguchi et al.…”