The contemporary flora and fauna of North America represent the survivors of repeated waves of emigration through geologic time mixed with local evolutionary processes.

The rate of intercontinental species exchange has increased exponentially during the last 500 years due to intentional and accidental transport by humans.

Altered ecosystem composition, structure, and functionality are an inevitable consequence of species migration and naturalization.

Highly successful newcomers, such as cheat-grass, should be viewed as permanent additions to North American flora.

Researchers, land owners and managers, and policy makers would do well to acknowledge the new realities created by introduced species and focus efforts on 1) limiting new introductions, 2) assessing the variability of impacts across affected ecosystems, and 3) developing reasonable expectations and practices for mitigating effects while preserving core ecosystem functionality.