STREAM MACROINVERTEBRATES AND HABITAT BELOW AND ABOVE TWO WILDERNESS FORDS USED BY MULES, HORSES, AND HIKERS IN YOSEMITE NATIONAL PARK

摘要

Wilderness stream crossings used by mules, horses, and hikers are localized disturbances that may affect habitat immediately downstream, but the potential influence of fords on streams has received little investigation, particularly in terms of possible effects on fauna. Our overall null hypothesis was absence of below-above differences for either benthic macroinvertebrate assemblages or habitat characteristics at such fords. We further sought to determine (1) whether any such differences were present prior to annual use, suggesting longer-term effects, and (2) whether differences were present in late season, after annual use. We examined macroinvertebrates and habitat immediately below and above 2 fords crossing subalpine streams in Yosemite National Park in the Sierra Nevada (California, USA) in early and late season and over 2 years. There were both longer-term below-above differences, as well as differences that became apparent in late season, both of which were indicative of below-ford effects. Below fords there was evidence, either as main effects or interactions, of higher silt, sand, and gravel cover; a thicker periphyton layer; a greater Hilsenhoff biotic index; a higher proportion of tolerant taxa; higher chironomid midge and total densities; and greater species richness, largely a function of chironomid richness. There was also a lower expected number of species, a smaller proportion of sensitive taxa and predators, and lower densities of some sensitive Ephemeroptera (mayflies) and Plecoptera (stoneflies) below fords. Both hikers and stock may contribute to the apparent effects, but management interventions targeting stock may be particularly achievable. Among other approaches, simply halting stock strings briefly before reaching fords should reduce the volume of urine and feces directly entering streams, and handlers can expedite crossings if watering is not necessary.