Background/Question/Methods For invasive species, introduction to a new range results in exposure to new biotic and abiotic environments. Therefore, it is not surprising that invasive species often adapt to conditions in their introduced range by responding to novel enemies, evolving to exploit novel resources, or adjusting to novel abiotic conditions. However, it may be somewhat more surprising that native species adapt to the environmental changes caused by invasive species. Most studies investigating adaptation by native species in response to invasion focus on a single species. Furthermore, adaptation by invaders to one another is rarely considered, despite the facts that multiple invaders are often present in invaded ecosystems and that these invaders are often from different parts of the world and mixed for the first time in non-native ranges. Here we investigate whether Bromus japnonicum (Japanese brome), an invasive annual grass, adapts to the presence of Euphorbia esula (leafy spurge), a long-lived clonal invasive forb that forms dense, discrete near-monocultures in the grasslands of the Northern Great Plains and the Mountain West. Euphorbia esula invasions often co-occur with invasions by annual Bromus species that occupy the spaces between Euphorbia stems.

Results/Conclusions We found evidence that the offspring of B. japonicum growing within E. esula stands often performed better in single-pot competition trials with E. esula than those outside of E. esula stands in a site-specific manner, suggesting that B. japonicum may adapt to the presence of E. esula at some of our research sites. We also found that adaptation to the presence of E. esula was sometimes associated with increased competitive ability of B. japonicum against Festuca idahoensis, a native perennial bunchgrass. This may have important consequences for remnant native populations growing within E. esula patches. If E. esula cultures variants of B. japonicum that are superior competitors against native plants, the native plants that persist within E. esula stands may experience elevated competition from the invasive bromes that commonly occur in E. esula stands. Furthermore, our research suggests that, when investigating the evolutionary consequences of association with a particular invader, it may be important to examine traits other than just competitive ability against that species, as adaptation to compete with one species may influence other aspects of a plant's ecology.