Can replacement of native by non‐native trout alter stream‐riparian food webs?

Abstract

Summary Non‐native species can affect food web and community structure, including the flow of resources from one habitat to another. In many streams of western North America, non‐native brook trout (Salvelinus fontinalis) have replaced native cutthroat trout (Oncorhynchus clarkii). Because brook trout naturally occur at higher densities and exhibit different feeding habits, this replacement may have consequences for a range of organisms in stream‐riparian food webs. We conducted a large‐scale, 2‐month field experiment to test whether, compared with cutthroat trout, brook trout reduce benthic insects, cause an increase in stream algae, and reduce emerging adult aquatic insects as well as riparian spiders that rely on emergence as prey. Twenty enclosed reaches from which trout were removed were treated by adding age‐1 and older: (i) cutthroat trout at natural density (0.15 fish m−2); (ii) brook trout at a natural density (0.40 fish m−2); (iii) brook trout at a low density equal to the cutthroat trout treatment (0.15 fish m−2) or (iv) no trout added. Brook trout reduced the flux of emerging insects by 55% compared with cutthroat trout, but, surprisingly, only at the lower density. This reduction in emergent insects was predicted to reduce spider abundance by 20%, provided that sufficient branches were available to support riparian spider webs. We also detected an effect of trout on large‐bodied benthic and emerging insects, but not on the entire assemblage of benthic insects, or periphyton. We hypothesise that our results were influenced by trout foraging behaviour mediated by their density and size structure. Regardless of the mechanisms responsible, our results indicate that non‐native brook trout can reduce the flux of emerging insects, and further analysis showed that this could reduce riparian spiders and birds that prey on them. Similar effects of non‐native fish are likely to occur in lotic and lentic waters beyond the western United States.