Quantifying the environmental factors that determine benthic macroinvertebrate assemblages in streams by analyzing stressors associated with a gradient of cattle grazing

Abstract

Relationships between macroinvertebrate assemblages and environmental stressors were assessed from fall 2002 through spring 2004 in five small streams that represented a study design that involved a gradient of increasing stress (increased cattle density). Macroinvertebrate assemblages were related to environmental factors that were quantified at the sample scale. Environmental factors and macroinvertebrates were concurrently collected so that assemblage structure could be directly related to environmental variables and so that the relative importance of stressors associated with cattle grazing in structuring assemblages could be assessed. Macroinvertebrate metrics showed significant and strong responses to cattle density during most sampling periods. The majority of metrics responded negatively to the grazing gradient, while a few (total taxa richness, number of sensitive taxa, and % collector filterers) increased along the gradient before declining at the most heavily grazed sites. Total number of sensitive taxa and % Coleoptera had the strongest relationship with cattle density throughout the study period. Based on sample-scale, quantitative measures of environmental variables, measures of physical habitat (% fines and substrate homogeneity) were most important in structuring assemblages. Detrital food variables (coarse benthic and fine benthic organic matter) were secondarily important while autochthonous food variables (chlorophyll a and epilithic biomass) were not as important in influencing assemblage structure. Based on a comparative analysis of reach-scale habitat measures and estimates, quantitative measures of % fines, collected from within an enclosed sampler concurrently with macroinvertebrates, were the best predictor of macroinvertebrate assemblages. Quantitative measures and visual estimates of riparian and channel characteristics had strong relationships with macroinvertebrate metrics but the relationships were never as strong as those detected with instream measurements of % fines. The channel characteristic, bank height, was the best predictor of % fines.