Assessments of stream ecosystems often include an evaluation of riparian condition; a key stressor in riparian ecosystems is the presence of invasive plants. We analyzed the distribution of 12 invasive taxa (common burdock [Arctium minus], giant reed [Arundo donax], cheatgrass [Bromus tectorum], musk thistle [Carduus nutans], Canada thistle [Cirsium arvense], teasel [Dipsacus fullonum], Russian olive [Elaeagnus angustifolia], leafy spurge [Euphorbia esula], English ivy [Hedera helix], reed canarygrass [Phalaris arundinacea], Himalayan blackberry [Rubus armeniacus], and saltcedar [Tamarix spp.]) to characterize a portion of that stressor. Observations from 961 probability survey reaches and 355 additional reaches distributed across 12 western US states provided a statistically defensible foundation for trend monitoring, risk assessments, or economic evaluation of these 12 taxa over a large area. We estimate that ≥1 of these taxa are present in riparian areas on 47 ± 3.6% of the perennial stream length in the western US. One or more of these taxa were present in >⅓ of the reaches identified as least-disturbed (reaches that define reference condition and are used to quantify instream biotic integrity). Association between target invasive presence and instream biotic integrity varied, particularly as a function of ecoregion. Relationships were often statistically significant in the Mountain climatic region, sometimes significant in the Xeric climatic region, and never significant in the Plains climatic region. Regional variations in associations and confounding relationships between multiple factors suggest that multiple variables should be examined to explain or predict the presence of invasive species or their associations. Our survey illustrates strengths and limitations of collecting information on a limited number of invasive plants in riparian vegetation as part of general probability surveys of aquatic ecosystems. Our survey of only 12 somewhat arbitrarily selected invasive plants provided much information for a limited cost. We recommend including similar efforts in future surveys.
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