Invasion of Bromus tectorum (L.) into California and the American Southwest: rapid, multi-directional and genetically diverse

The arrival and subsequent spread of the invasive annual grass Bromus tectorum into California and much of the American Southwest was documented by assembling the grass’s early regional collection history in conjunction with analysis of its genetic variation at 25 allozyme loci within 60 widely separated populations. Herbarium records collected ca. 1900, coupled with the distribution of multilocus genotypes, substantiate the view that the invasion of B. tectorum into our study area was rapid, multi-directional, and genetically diverse. The invasion in California and the American Southwest appears to have followed three distinct pathways: (1) direct introduction from native range populations (e.g., the Pgi-2b multilocus genotype), (2) range expansion as descendants of populations in Eastern North America migrated westward (e.g., Pgm-1a and Pgm-2a), and (3) separate introduction(s) into Northern California and Southern Arizona from the Pacific Northwest (e.g., Got-4c). Populations in California and the American Southwest have, on average, lower genetic diversity (A = 1.03, %P = 3.27 and H_exp = 0.007) compared with populations from other regions in North America. Most of this genetic diversity however is partitioned within populations rather than among populations (G_ST = 0.277). For an introduced species with a broad invasive range, such as B. tectorum in North America, our results indicate that multiple pathways likely contributed to its current (and potentially expanding) geographic distribution.