Impact of invasive smooth brome on a native prairie plant, Spartina pectinata

Habitat destruction and fragmentation have been implicated as the main causes for the reduction of biodiversity worldwide. Disturbances to native habitats have made many ecosystems vulnerable to the invasion of non-native plant species. Within the prairie pothole region of the United States and Canada, the grass, Bromus inermis (smooth brome) has become established by invading disturbed prairies, and through repeated introductions for soil retention and animal graze. The purpose of this study was to examine the impact of brome on a dominant native grass species, Spartina pectinata (prairie cordgrass) that grows in discrete clonal patches in hydric prairie landscapes. In three large prairie fragments, we mapped the distribution of cordgrass and brome in 1999, 2004 and 2006. Using GIS spatial analyses, we determined that cordgrass patches embedded in >50% brome matrix declined 51% in total area from 2000 to 2006. The probability of establishment of a new patch of cordgrass is 2.5 times more likely in areas of low brome coverage (< 25%) than areas of high brome coverage (> 75%).  It is also 2 times more likely for a patch of cordgrass to disappear in areas of high than low brome coverage. Field experiments that we conducted between 2005 and 2007 indicated that brome seeds and seedlings are capable of germinating and establishing under a wide variety of habitats and environmental conditions. However, brome establishment success is greater in high elevation (with low moisture) than low elevation sites, irrespective of the presence of cordgrass. The reverse pattern was true for cordgrass.  Results from this study indicate that much of the tallgrass prairies remnants of the Great Plains are vulnerable to invasion of smooth brome. In particular upland areas may allow for initial invasions and act as a source population for future expansion into low-lying areas.