Rank Method Rank calculation - Biotics v2
Review Date2020-09-10
Change Date2005-04-05
Edition Date2020-09-10
Edition AuthorsTreher (2020)
Threat ImpactUnknown
Range Extent<100 square km (less than about 40 square miles)
Number of Occurrences1 - 5
Rank ReasonsCirsium eatonii var. viperinum is endemic to Nevada and found on mountain tops of White Pine County at 2 known occurrences. This taxon may be unintentionally targeted for removal, because it is a thistle and perceived to be a weed. Otherwise, threats are not well known. There are no repetitive surveys of occurrences to inform trends.
Range Extent CommentsCirsium eatonii var. viperinum is endemic to Nevada and restricted to the Snake Range of White Pine County (FNA 2006a). There is a herbarium specimen loosely assigned to this taxon that was collected in the Schell Creek Range; there may be other specimens identified as the species Cirsium eatonii that belong to this variety. Range extent, including the Schell Creek specimens, is calculated at 72 sq km.
Threat Impact CommentsThistles, broadly speaking, are often targeted weeds for removal. The native species can be difficult to differentiate from weedy species, or just be targeted because they are thistles, and thus subject to herbicide treatments, mechanical removal, or biological control agents that are used to control the non-native species. Native Cirsium are increasingly at risk from a Eurasian flowerhead weevil (Rhinocyllus conicus Froeh.), which was first introduced in 1968 for biological control of Eurasian Carduus thistles such as the weedy alien Carduus nutans (musk thistle). The weevil is extending its range to new host species, as well as geographically; it already was released in southern Canada (British Columbia to Quebec) and the western and eastern U.S. (in many states, such as from Washington, California, Arizona and Texas to Idaho, Minnesota, New Jersey, Virginia and Tennessee). Some (but not all) native Cirsium species in Colorado, Nebraska and South Dakota - including populations in protected areas - have been shown to be damaged, with quantified decrease in seed production being "severe", over 70-85% loss. Furthermore, Cirsium species are fugitives which generally depend upon current seed production for their establishment and persistence (Carol Spurrier, pers. com., 1999, Louda et al. 1997, Strong 1997). A related species, Cirsium ownbeyi, was found to remain relatively stable despite the drop in reproductive output. However, the combination of decreased seed production, a small population, and stochastic events could put the species at risk of extirpations (DePrenger-Levin et al. 2010).