Patterns and Influences of Exotic Species Invasion into the Grassland Habitat of the Threatened Plant Silene spaldingii

摘要

The threatened plant Silene spaldingii occurs in the canyon grasslands at Garden Creek Ranch Preserve in Idaho, an area rapidly being invaded by Centaurea solstitialis and Bromus tectorum. We examined environmental and community patterns of site invasion at two spatial scales, and evaluated the apparent influence of invasion on Silene population vigor. We found that Silene-supporting sites on gentle slopes receiving more incident radiation were most often invaded by exotics, which may relate to light requirements of C. solstitialis. Invaded sites were also typically at higher elevations. The plant communities in invaded and uninvaded Silene-supporting sites were similar, although invaded sites tended to have greater cover of several exotic annual grass species and Lupinus sericeus. Similarly, portions of invaded sites that supported C. solstitialis and B. tectorum were characterized by greater frequency and abundance of several other exotic species than were the portions lacking these two species. Exotic species invasion did not appear to negatively influence Silene populations, as indicated by similar plant height and comparable reproductive output in invaded and uninvaded populations. Similarities in Silene vigor between invaded and uninvaded sites may reflect a moderating influence of invaded site environment or may indicate that Silene and the exotic species partition resources differently in space or time. Additional studies, including assessments of recruitment and establishment of Silene in close proximity to C. solstitialis and B. tectorum, are needed to assess whether these exotics threaten the continued existence of Silene in this preserve.