argyritis). All three are under threatened status in Alberta, and the western silvery minnow was upgraded to endangered in Canada by COSEWIC in April 2008. Their greatest threats are habitat alteration or loss, changes in water flow or drought water levels, increased siltation, elevated water temperatures, surface water extraction, dam construction, and for the western silvery minnow, introduction of predatory fish species. Invertebrates Weidemeyer's admiral butterfly (Limenitis weidemeyerii) is only known to exist in Canada within the Milk River watershed, as well as the yucca moth mentioned previously. An estimated population of 3000 adult Weidemeyer's admiral adults exist in a total approximate total occupied area of less than one square kilometer, and are susceptible to extreme natural weather events or drought. They are ranked as a species of special concern provincially in Alberta and nationally in Canada but a knowledge gap exists and further research is needed to determine more accurate numbers and range. Environmental threats and issues Elements of concern to the area are fire, over- or under-grazing, native species reintroduction, human recreational (camping and hunting) and educational/scientific use, resource exploration, linear development, and invasive species. Invasive species, primarily weeds, are a significant threat that crowd out native plant species and reduce habitat and food sources for native animals, birds, fish, and insects. Weeds are quick colonizers that often move into an area disturbed by fire or human activity. Linear developments such as roads, pipelines, and powerlines often provide a vector for invasive species to enter new areas. Because of their rapid and opportunistic spread, weeds also often lack the deep root system of native plants and thus contribute to bank instability and soil erosion. The nodding thistle weed (Carduus nutans) was discovered in the area in 2006. Several other weed species have been found in the natural area, including the highly aggressive downy brome (Bromus tectorum), creeping meadow foxtail (Alopecurus arundinaceus), crested wheatgrass (Agropyron cristatum), and Russian olive (Elaeagnus angustifolia). Several of these species have invaded from across the U.S. border into Canada. Formerly utilized as a reclamation species, crested wheatgrass has had detrimental effects on soil and grazing. Russian olive, imported as a soil stabilizer, windbreak, and ornamental, reduces biodiversity of riparian areas. Management Once belonging to the Lost River Ranch lease, the land that now comprises the Milk River Natural Area was grazed infrequently and was therefore deleted from the grazing lease under the Public Lands