TUCSON, ARIZONA--A salt marsh grass native to the East Coast has evolved new reproductive tactics since being introduced on the West Coast a century ago. The finding showcases how fast such traits can change when an organism moves into a new environment.
Coastal salt marshes along the eastern U.S. seaboard are packed full of the cordgrass Spartina alterniflora. It grows in dense mats, where it must struggle to survive in an ultra-competitive environment. New grass seedlings can establish themselves only after a hurricane or other major disturbance clears a swath of heavily vegetated beach. In contrast, wide-open beaches and mudflats were the rule on the West Coast. After appearing there a century ago, Spartina has now invaded numerous locales from Puget Sound to the San Francisco Bay.
That's because the western plants evolved traits that helped them exploit this wide-open environment, says Heather Davis, a graduate student in population biology at University of California, Davis. She collected cordgrass seeds from both coasts, grew them in a greenhouse, and monitored their reproduction. Western plants reached reproductive maturity at a smaller size and a much younger age than eastern plants, she found. Western plants also invested more in reproduction, producing many more flowers than eastern plants. And they tended to die earlier, particularly the ones that had reproduced the most. Finally, western plants were much more self-compatible (able to breed with themselves) than eastern plants. All these traits are typical strategies for quick and dirty reproduction.
To investigate whether natural selection had pushed the plants towards self-compatibility, Davis compared new and decades-old cordgrass, identified from historic aerial photos, in Willapa Bay, Washington. She found that younger plants at the edges of cordgrass clusters were more self-compatible than older ones in the center, indicating that self-compatibility had evolved over time. Davis presented her findings at the meeting of the Ecological Society of America and the Society for Ecological Restoration here last week.
Botanist C. Lynn Kinter of Washington State University, Pullman, says that aside from its evolutionary interest, the work could help land managers decide how to screen for or quarantine invasive plants, since "comparing native and introduced populations can give us valuable insights into traits that facilitate invasion."