No selection for greater size in an introduced grass invading semiarid grassland

Abstract

Exotic species are often planted for restoration or to enhance production. Populations that expand from plantings may undergo selection in directions that differ from those produced by artificial selection prior to planting. Here, we test whether a highly productive grass, Agropyron cristatum (L.) Gaertn., introduced to the semiarid Great Plains of North America, evolves during invasion from seeded fields into native grassland. We grew individuals of A. cristatum from six seeded populations and 12 invading populations in 12 common gardens separated by 0.5–12 km for two growing seasons. Contrary to expectations, individuals from invading populations did not have significantly greater tiller number or growth (increase in tiller number) than did individuals from seeded populations, suggesting that they have not undergone evolution towards increased invasion ability through increased size or growth. Instead, there was a general trend for individuals from invading populations to have lower growth rates and fewer tillers when grown without neighbours. Large size or high growth rates arising from artificial selection prior to introduction may be disadvantageous to populations invading semiarid grassland on dry, nutrient-poor soils. In this case, evolution may have resulted in convergence between the relatively large planted species and the smaller native species.