Changes in the population genetics of an invasive Spartina after 10 years of management

Spartina foliosa is native to the San Francisco, California, Bay and Estuary. Spartina alterniflora was introduced to the Bay in the 1970s and subsequently hybridized with S. foliosa. Backcrossing created an invasive hybrid swarm able to outcompete S. foliosa within its tidal range and spread into higher and lower intertidal zones, drastically altering ecological communities. The San Francisco Estuary Invasive Spartina Project (ISP) has implemented a treatment program that has reduced the net area of invasive Spartina by 96 % from its peak of 323 hectares. An endangered bird, the California Ridgway’s Rail, takes advantage of habitat characteristics supplied by hybrid Spartina, prompting treatment restrictions in some marshes to preserve hybrid Spartina for the benefit of rails. We investigated changes in the population genetics of the invasion after 10 years of eradication efforts and after three subsequent years without eradication in some locations. Our investigation covered three areas: sites surrounding and including the restricted treatment marshes; sites in the southern Bay similar to proposed salt pond restoration projects; and samples collected Baywide. Our results support previous descriptions of a system of self-fertile hybrid plants that supply seed for mostly localized recruitment into available habitat. Compared to published work from 2003 to 2004, before large-scale eradication efforts began, genetic diversity has generally declined and inbreeding within hybrid populations has increased during and after the period of treatment by the ISP. These observations are perhaps due to pollen limitation as a result of the elimination of most hybrid plants by the ISP. We did not detect strong population genetic boundaries Baywide, but a genetically distinguishable localized introduction was detected in 10 of 11 Bay regions, suggesting leptokurtic dispersal. The restricted treatment areas may allow for continued introgression with S. foliosa, recolonization, and longer persistence of these invasive plants in the San Francisco Bay. Proximity to hybrid Spartina threatens the success of major restoration projects.