The invasion potential of Bromus tectorum results from its ability to colonize variable environments via phenotypic plasticity, standing genetic variation, or adaptive evolution. Genetic data and historical records indicate that B. tectorum was introduced to North America repeatedly starting in 1790. Its current distribution is the result of direct introduction from Eurasia and multi-directional range expansion. Overall B. tectorum genetic diversity (e.g., allelic richness and number of polymorphic loci) is lower in the invaded range than in native populations. Conversely, invasive populations of B. tectorum exhibit higher within-population genetic diversity than that found in native populations, suggesting that multiple introductions have partially offset founder effects. Invasive populations are mostly genetic admixtures composed of two or more independently derived native genotypes that are often less genetically differentiated than native populations. Understanding the genetic basis of the invasion can help shed light on how to manage this species across the large expanse where it is problematic and predict where it is likely to become disruptive in the future as climates change. Common garden studies reveal outcrossing frequency, microsite effects on establishment and growth, and variation in germination, morphology, and physiology. Deciphering the underpinnings of invasion potential can help shed light on how to manage this species. Extrapolating lessons learned about B. tectorum to congeners, we developed a database with traits of Bromus species and their interactions with biotic and abiotic features of their environments to look for correlations among introduction potential, weediness, a suite of life history traits, polyploidy, human use, cultivar availability, and climate factors.
Invasiveness in Bromus species was associated with the ability to grow at high and low temperature and precipitation levels; and with human activities. B. tectorum appears to tolerate a wide range of habitat conditions as the result of genetic variation among populations, a range of locally adapted ecotypes, and phenotypic plasticity. The invasive ability of B. tectorum may result from its ability to maintain fitness in both high-quality and marginal environments, as well as its ability to rapidly evolve. Indeed, experimental climate manipulation studies have shown that B. tectorum can respond quickly and there is evidence of an evolutionary response. The genetic consequences of multiple introductions likely complicate the management of this invader, especially in western NA. Information about Bromus species in a centralized database facilitates comparisons among species and provides insights for modeling, prediction, management, and eventual control of brome-grass invasions.