UC Davis

Wetland conservation in California faces a multitude of challenges, this is especially true in the Suisun Marsh, one of the largest tidal marshes on the west coast of North America. The marsh has historically supported large numbers of non-breeding waterfowl but the capacity of the marsh to continue to do so remains uncertain, due to drought, habitat loss, future sea level rise, and conflicting conservation needs for many species of fish and wildlife. A guiding principle for the conservation of non-breeding waterfowl is to manage for food resources, under the working hypothesis that food resources may be limiting in winter, and the decline in the abundance of waterfowl within Suisun Marsh may be due to reduced food production or availability. Non-breeding waterfowl depend on plant seeds as a primary source of food throughout winter. However, we lack information concerning what food resources may be in limited supply because there has never been a direct assessment of total seed abundance in Suisun Marsh. Efforts to recover threatened and endangered species within the marsh – through the conversion of managed wetlands to tidal marsh ¬– could further impact the carrying capacity of the marsh for waterfowl by reducing the abundance or availability of waterfowl food resources. To evaluate these concerns, I undertook a multifaceted study to evaluate the abundance, production, and depletion of food resources for wintering waterfowl in both tidal and managed wetlands. I found that seed abundance in both wetland types was much lower than previously predicted. Seed abundance in Fall was similar in tidal (172.9 kg/ha) and managed (163.8 kg/ha) wetlands, but there was considerable spatial variation. The species composition of seeds differed substantially according to wetland type; over 75% of aggregate seed mass in managed wetlands was comprised of food species that waterfowl prefer, while a non-preferred food, Schoenoplecutus sp., was the dominant seed in tidal marshes. The occurrence of several plant species within managed wetlands that produce waterfowl-preferred seeds was correlated with salinity, wetland management intensity, and disking. The best predictor of total seed abundance in managed wetlands was habitat zone, which is defined by shared physical factors known to impact vegetation growth and production in different regions in the marsh. These regional differences in seed abundance have important implications for tidal restoration efforts, as the loss of seasonal wetland acreage within more productive regions could have a disproportionate impact on total food availability. To evaluate seed dynamics at a finer scale, I used small-scale sampling sites and foraging exclosures in a managed wetland within the marsh and stratified these sampling locations according to high and low waterfowl use areas. Contrary to expectations of a “table-is-set” hypothesis, I found seed that abundance increased over three weeks post-flooding, and over seven to ten weeks in foraging exclosures. Observed declines in the abundance of seeds were substantially greater than rates of decomposition, suggesting seed removal by waterfowl was a major source of seed depletion. However, the amount of seed consumption that could be attributed to waterfowl was spatially variable and correlated with waterfowl locations – high use areas exhibited significantly higher rates of seed loss. Hydrochory (movement of seed due to water flow) was likely responsible for the changes in seed abundance observed for several weeks after the wetlands were flooded, as abundances within foraging exclosures nearly tripled over ten weeks in high waterfowl use areas. These results suggest that seed removal by waterfowl and hydrochory can impact our ability to accurately estimate peak seed abundance. Without accounting for seed removal by waterfowl, and hydrochory, bioenergetic models may produce inaccurate estimates of habitat carrying capacity. My results provide new insight and greater refinement of estimates of seed abundance to more accurately determine the carrying capacity of the Suisun Marsh for wintering waterfowl, and to assess the potential impacts of future changes.