Effects of cattle grazing on terrestrial invertebrate subsidies to trout in Central Rocky Mountain rangeland streams

Research worldwide indicates that input of terrestrial invertebrate prey to streams provides about 50% of annual energy for stream fish.  We tested whether terrestrial invertebrate input to rangeland streams is as great in western Wyoming and northern Colorado, and how it is modified by cattle grazing.  During summers 2004 through 2008 we sampled falling invertebrate input and trout diets in streams that had riparian zones under four different grazing systems including season-long, intensive-rotation, and wildlife-only grazing regimes.  Input of terrestrial invertebrates in both regions was similar to estimates for other semi-arid regions and was consistently greater in streams with riparian zones under rotational systems versus season-long grazing.  In turn, trout inhabiting streams managed under rotational grazing consistently consumed more invertebrate biomass (both aquatic and terrestrial) than fish in sites under season-long grazing.  Moreover, >50% of biomass in trout diets in all streams consisted of terrestrial invertebrates, regardless of grazing management.  Total trout biomass also was greater in streams under rotational grazing systems in the more arid region.  These results indicate that inputs of terrestrial prey provide important resources that sustain trout in western rangeland streams, and that intensive rotational grazing may increase trout biomass, especially in more arid regions.