Effects of riparian grazing on terrestrial invertebrate subsidies that feed trout in central Rocky Mountain streams
Date
2010
Authors
Saunders, William Carl, author
Fausch, Kurt D., advisor
Kondratieff, Boris, committee member
Clements, Williams H., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Habitat degradation is the leading cause of biodiversity loss worldwide, affecting plant and animal populations directly through habitat loss, but also indirectly by decoupling important linkages among habitats. Linkages between streams and the terrestrial environments they drain are likely to be especially important because streams have small habitat area but long boundaries with the adjacent riparian zone. Riparian livestock grazing reduces riparian vegetation, altering the stream-riparian interface, and so may reduce the flux of terrestrial invertebrates to streams. To evaluate the potential for riparian grazing to affect trout populations by reducing this flux, I conducted two large-scale field studies. In the first, a study of three commonly used grazing systems at sites on 16 streams in northern Colorado, I compared invertebrate resources and salmonid populations among stream reaches managed for season-long (i.e., continuous) or two types of rotational livestock grazing, as well as streams grazed by wildlife only. Rotational grazing generally supported greater inputs of terrestrial invertebrates to streams (2-5 times more), and trout at rotational grazing sites consumed 2 - 4 times the biomass of these prey as trout at sites grazed season-long. However, factors influencing the flux of invertebrates to streams were complex and resulted in variable responses by trout populations. In the second field study, a large-scale grazing experiment conducted in four streams in western Wyoming, I evaluated whether two levels of grazing intensity (i.e., the amount of vegetation removed) and manual removal of streamside woody vegetation influenced terrestrial prey resources for trout when compared to controls with wildlife grazing only. Two grazing treatments, designed to reduce vegetation to 10-15-cm stubble height (moderate intensity grazing) or 5-7.5-cm stubble height (high intensity grazing) within a few days, had no detectable effect on terrestrial invertebrates entering streams, whereas high intensity grazing combined with manual removal of two-thirds of streamside woody vegetation reduced inputs of terrestrial invertebrates to streams by 45%. In contrast, all treatments reduced the biomass of these prey in tout diets by 50 - 75%, relative to control sites. However, neither grazing nor removal of woody vegetation affected the biomass of fish that remained in treatment reaches. Finally, I conducted field research and computer simulations to validate removal estimates of trout abundance, based on night-time electrofishing, to address recent concerns over the accuracy of these types of estimators. I found that night-time electrofishing was highly effective for estimating abundance of trout in small streams like those where I studied the effects of cattle grazing in Colorado and Wyoming. Furthermore, I show that modern analytical methods provide powerful tools to account for heterogeneity in capture probability among individual fish.
Description
Rights Access
Subject
Conservation
Ecology
Range management