The interactions between native and exotic species occur on a continuum from facilitative to competitive. A growing thrust in invasive species science is differentiating where particular native species occur along this continuum, with practical implications for identifying species that might reduce the invasibility of ecosystems. We used a greenhouse experiment to develop a competitive hierarchy of 27 native species with red brome, an invasive annual grass in the arid lands of the southwestern United States, and a field study to assess in situ responses of brome to native perennial species in the Mojave Desert. Native species most competitive with brome in the competition experiment included the annuals Esteve's pincushion and western fiddleneck and the perennials eastern Mojave buckwheat, sweetbush, and brittlebush, which reduced brome biomass to 49 to 70% of its grown-alone amount. There was no clear difference in competitive abilities with brome between annual and perennial natives, and competiveness was not strongly correlated (r = 0.15) with the biomass of the native species. In the field, sweetbush and brittlebush supported among the least cover of brome, suggesting congruence of the strong early competitive abilities of these species with in situ patterns of brome distribution. At the other extreme, brome attained its highest average cover (19%) below littleleaf ratany, significantly greater than all but 3 of the 16 species evaluated. Cover by brome was only weakly related (r = 0.19) to the area of the perennial canopy, suggesting that factors other than the sizes of perennial plants were linked to differences in brome cover among species. Results suggest that (1) interactions with brome differ substantially among native species, (2) these interactions are not as closely linked to biomass production as in more temperate regions, and (3) there is potential for identifying native species that can reduce invasion of desert ecosystems.
Nomenclature: Red brome, Bromus rubens L.; brittlebush, Encelia farinosa A. Gray ex Torr.; eastern Mojave buckwheat, Eriogonum fasciculatum Benth.; Esteve's pincushion, Chaenactis stevioides Hook. & Arn.; littleleaf ratany, Krameria erecta Willd. ex Schult.; sweetbush, Bebbia juncea (Benth.) Greene; western fiddleneck, Amsinckia tessellata Gray
Interpretive Summary: Identifying native species, for use in revegetation and plant-community augmentation projects, capable of reducing the fitness of invasive species is a major thrust in invasive species science and management. We sought to identify native species competitive with Bromus rubens (hereafter Bromus), an exotic annual grass increasing fuel loads and facilitating fires devastating to resources in the arid lands of the southwestern United States. In a greenhouse experiment screening the competitive abilities of 27 native species, we found that the native annuals Chaenactis stevioides and Amsinckia tessellata and the perennials Eriogonum fasciculatum, Bebbia juncea, and Encelia farinosa mostly strongly competed with Bromus. In a companion field assessment of Bromus below different native perennial plants in the eastern Mojave Desert, Bromus cover varied more than ninefold among 16 species. Species such as Thamnosma montana, B. juncea, E. farinosa, and E. fasciculatum showed promise in their ability to support l