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ALIENS IN WESTERN STREAM ECOSYSTEMS
RINGOLD, P. L. ALIENS IN WESTERN STREAM ECOSYSTEMS. Presented at California Bioassessment Workshop, Davis, CA, November 29 - 30, 2006.
To determine the extent of invasive plants, alien aquatic vertebrates, and some alien macroinvertebrates species in western streams
The USEPA's Environmental Monitoring and Assessment Program conducted a five year probability sample of permanent mapped streams in 12 western US states. The study design enables us to determine the extent of selected riparian invasive plants, alien aquatic vertebrates, and some alien macroinvertebrates species. For example, cheatgrass (Bromus tectorum) was found in 11% of the stream length - 33,700 km of western streams; its presence varied from less than 0.1% in the Pacific Northwest ecoregion to 42% in the Xeric Northern Basin ecoregion. Landscape disturbance was typically higher and stream biotic condition lower at sites where each of the selected invasive plants was found as compared to sites from which they were absent. Alien plants were found to be present at reference sites, underscoring the difference in ecological condition between minimally disturbed and least-disturbed conditions.
Over half of the stream length sampled for aquatic vertebrates contained nonnative vertebrates - 109,400 km. Their presence varies from a high of 60% in the Plains to a low of 35% in the Mountains. Rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta) and brook trout (Salvelinus fontinalis), deliberately introduced, were the most common nonnative aquatic vertebrates found in the western states. The richness or abundance of alien fish are negatively correlated with the richness or abundance of native fish within some trophic classes at the ecoregion scale.
Asian clam (Corbicula fluminea), the most extensively reported alien stream macroinvertebrate, was found in 2% of western streams - 7,400 km; it was present in over 20% of the stream length in Xeric California and the Mountain Southwest ecoregions. Clearly nonnative species are extensively distributed throughout western US stream ecosystems.
These are implications of these results for using clean protocols by field crews moving from one site to another, for the manner in which laboratories identify macroinvertebrates, and for insights into the difference between least-disturbed and minimally disturbed conditions in evaluating reference conditions.