Post-introduction evolution in invasive Bromus tectorum
Evidence for rapid evolutionary change in plants in response to changing environmental conditions is widespread in the literature. The first objective of this research was to estimate rates of evolution for previously published studies to begin a foundation for comparison and to examine trends and rates of microevolution in plants and to demonstrate how estimated rates could be used to address specific microevolutionary questions.
The remainder of the research in this paper focused on invasion as a stimulus for microevolution using a major invasive grass, Bromus tectorum . Identification of pairs of European, source and North American, introduced population pairs of B. tectorum provided an ideal opportunity to test for post-introduction increase in invasiveness because parallel change involving multiple introduction events suggests a non-random process. This research tested to see if B. tectorum evolved to be more invasive by becoming larger and more fecund, more competitive, and more phenotypically plastic in its invasive range than its source range.
All B. tectorum experiments were done in common garden or greenhouse settings to control environmental differences, and comparisons were made between morphological characteristics of introduced and source B. tectorum populations. The experiments showed that introduced B. tectorum was taller and had higher reproductive output than source B. tectorum, but the populations did not differ in aboveground biomass. Contrary to predictions, North American B. tectorum had less competitive effect than European B. tectorum on one of the native bunchgrasses, with no other differences in competitive ability. All of the B. tectorum populations show plasticity for the traits measured in this study, however there was no strong trend to suggest that there has been a change in the level of plasticity in introduced populations.
Overall, these results suggest that B. tectorum may have evolved to be more ruderal to capitalize on characteristics that increase its ability to spread in its new environment while not focusing as many resources on competitive ability. As for plasticity, it is likely that the same strategy of phenotypic plasticity from its native range could be a key to the success of B. tectorum in its invaded range with similar levels of selection for plasticity in both ranges.