Conserving fluxes of terrestrial invertebrates to trout in streams: A first field experiment on the effects of cattle grazing

Abstract

Abstract Conserving fluxes of terrestrial invertebrates to streams, which supply about half of the diet of stream fish, is an important goal for aquatic conservation. Riparian land uses such as forestry and grazing could have strong indirect effects on fish, by reducing these prey inputs. A large‐scale replicated field experiment was conducted in four western US rangeland catchments to test the effects of three levels of cattle grazing in riparian pastures on vegetation, fluxes of invertebrates, trout diets, and trout abundance and biomass. The treatments mimicked common grazing systems. The grazing treatments reduced riparian vegetation biomass to levels as low as or lower than rotational grazing systems. The most intense treatment also removed two‐thirds of riparian shrubs, thereby reducing overhead cover. All grazing treatments reduced the inputs of terrestrial and adult aquatic insects to streams, compared with controls, but only reductions caused by the most intense treatment were statistically significant, owing to the high levels of natural variation among control sites. Significant reductions of invertebrate inputs by this treatment during key seasons confirmed that riparian shrubs are an important component in the supply of invertebrates to streams. All treatments reduced the levels of terrestrial invertebrate prey found in trout diets, by half or more at the end of the experiment. Some reductions appeared to be compensated for by greater foraging of trout on larvae and adults of aquatic invertebrates. Trout did not respond to reduced prey by adjusting their density or biomass, probably because they were in good body condition, prey inputs were the highest yet recorded to rangeland streams, and trout foraged selectively to buffer the loss. Conserving fluxes of terrestrial prey to trout in rangeland streams will require the careful management of cattle grazing, especially to sustain riparian shrubs that are likely to serve as sources and vectors for invertebrate inputs to streams.