The Suisun Thistle (Cirsium hydrophilum var. hydrophilum) was once widespread in brackish tidal marshes that frame the northern edge of Suisun Bay. Over time, 87% of these marshes were transformed into managed ponds. The few remaining tidal marshes in the region are relatively small and fragmented. The Suisun Thistle declined over time and was considered extinct until the discovery of a population near Rush Ranch Open Space Preserve in 1989. It was then listed as Endangered under the Federal Endangered Species Act in 1997. It is found today at Rush Ranch and Hill Slough, and potentially may be at Peytonia Slough, its former type locality. Despite its highly restricted range, a 2003 survey at Rush Ranch found it to be relatively widespread and abundant, particularly along shallow channels created by previous mosquito ditching to “drain” standing water on the marsh plain. Since 2003, populations have continued to persist at Rush Ranch but numbers appear to have declined significantly. This decline has occurred despite protection from disturbance by the designation of Rush Ranch as part of the SF Bay NERR. A study by Schneider suggested that physical disturbance might be needed to create opportunities for the Suisun Thistle to recruit. Unlike most long-lived rhizomatous species that dominate tidal marsh habitats, the Suisun Thistle is short-lived and light dependent. Further, recent studies have demonstrated that its seed output is heavily impacted by an introduced weevil. It appears that the Suisun Thistle is a “gap opportunist” that potentially may be threatened with too little disturbance over time! A current focus of the USFWS Tidal Marsh Recovery Team is to explore the potential of pilot reintroduction projects associated with tidal wetland habitat restoration opportunities to address this problem.
Speaker: Michael Vasey, UC Santa Cruz
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