Grazing management influences the subsidy of terrestrial prey to trout in central Rocky Mountain streams (USA)

Abstract

Summary 1. Research in forest and grassland ecosystems indicates that terrestrial invertebrates that fall into streams can be an important prey resource for fish, providing about 50% of their annual energy and having strong effects on growth and abundance. However, the indirect effects of land uses like cattle grazing on this important prey subsidy for stream salmonids are unclear. 2. During summer 2007, we compared the effects of three commonly used grazing systems on terrestrial invertebrate inputs to streams in northern Colorado and their use by trout. Cattle graze individual pastures for about 120 days under traditional season‐long grazing (SLG), about 35–45 days under simple rotational grazing and 10–20 days under intensive rotational grazing in this region. We also compared these effects to a fourth group of sites grazed only by wildlife (i.e. no livestock use). 3. Overall, rotational grazing management (either simple or intensive), resulted in more riparian vegetation, greater inputs of terrestrial invertebrates, greater biomass of terrestrial invertebrate prey in trout diets, a higher input compared to trout metabolic demand and more trout biomass than SLG. However, these differences were frequently not statistically significant owing to high variability, especially for trout diets and biomass. 4. Despite the inherent variability, riparian vegetation and terrestrial invertebrates entering streams and in trout diets at sites managed for rotational grazing were similar to sites managed for wildlife grazing only. 5. These results indicate that rotational grazing systems can be effective for maintaining levels of terrestrial invertebrate subsidies to streams necessary to support robust trout populations. However, factors influencing the effect of riparian grazing on stream subsidies are both spatially variable and complex, owing to differences in microclimate, invertebrate and plant populations and the efforts of ranchers to tailor grazing systems to specific riparian pastures.

Aquatic pollution increases use of terrestrial prey subsidies by stream fish

“…Age‐0 fish [< 50 mm fork length (FL) for Brook Trout Salvelinus fontinalis and < 60 mm FL for Brown Trout Salmo trutta ] are not sampled effectively using this method and were excluded from all analyses (Saunders, Fausch & White ). Estimates of capture probability and abundance for fish larger than age 0 were calculated in Program MARK (White & Burnham ; Saunders, Fausch & White ) using the Huggins closed‐population estimator, incorporating species identity and FL as individual covariates and pooling data across the nine sites with two‐pass data to improve capture estimates (Saunders & Fausch ).…”

Hierarchical multi‐population viability analysis

“…We expected this effect to be particularly strong for low elevation sites where there is greater riparian disturbance from cattle grazing (Li et al. , Saunders and Fausch , ). We calculated NDVI based on surface reflectance Landsat images (Landsat 5, 7, and 8) using Google Earth Engine (Gorelick et al.…”

Importance of Riparian Zone: Effects of Resource Availability at Land-water Interface

“…Fortunately, researchers have started to value the importance of crosssystem subsidies while restoring or managing aquatic and terrestrial ecosystems [12,170]. Saunders and Fausch (2012) compared the impacts of three general grazing systems on terrestrial invertebrate subsidies to adjacent streams and their consumption by trout in northern Colorado [171]. They found that rotational grazing management (either intensive or simple) led to more riparian vegetation, higher input of terrestrial invertebrates, greater terrestrial invertebrate consumption by trout.…”

“…They found that rotational grazing management (either intensive or simple) led to more riparian vegetation, higher input of terrestrial invertebrates, greater terrestrial invertebrate consumption by trout. Rotational grazing systems can effectively maintain terrestrial invertebrate subsidies, which were needed to sustain robust trout populations [171].…”