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Tidal Marsh in Transition

Hildie Spautz and Nadav Nur, PhD


San Francisco Bay is one of the most important estuaries in the western hemisphere for many kinds of birds. After 150 years of habitat loss and threats to wildlife, there is growing interest today in restoration on San Francisco Bay. Its complex of habitats includes tidal marshes, tidal flats, riparian corridors, and artificial salt evaporation ponds that provide foraging and roosting sites for large numbers of migratory waterbirds.

Hundreds of millions of public and private dollars are being spent on enhancing the estuary, as exemplified by the recent purchase of over 15,000 acres of South Bay salt ponds. Restoration of these salt ponds is about to commence, with the likely conversion of a majority of them to tidal marshes. Yet the science of tidal marsh restoration is new and evolving: studies of tidal marsh restorations are few, especially with respect to bird and other vertebrate responses. PRBO is conducting research on San Francisco Bay as part of a cooperative effort to provide the needed information.

Tidal marsh habitat edging the bay has been heavily compromised, with over 85% of its original extent lost. The remnant brackish and salt marsh still present has been impacted by changes such as the presence of dikes, loss of adjacent upland vegetation, invasions by non-native plants and animals, contaminants, and changes in salinity.

Part of Napa Sonoma Marsh, in northern San Francisco Bay, has been undergoing restoration since 1995. Photo © Michelle Orr.
A number of unique subspecies of birds, whose characteristics reflect their dependence on this diminished and altered habitat, have suffered severe population losses and are classified as endangered, threatened, or species of special concern.* To prevent extinction of depleted species and improve the health of the San Francisco Bay ecosystem, a number of areas have been restored to tidal marsh over the last ten years; new restoration plans are in the works. This is a propitious time for researchers to assess the projects already under way and to use the knowledge gained to improve restoration and management planning methods.

Typically, when a marsh is diked and dries up, the land subsides (decreases in elevation). Restoration requires a return to tidal action, so that marsh elevation can build up with sediments deposited from the bay water, channels can re-form, and natural marsh vegetation can become reestablished. In some cases, merely breaking the levees that hold back bay water can start the process. Planners have also experimented with jump starting elevation buildup with dredge materials from the bay. Several issues complicate restoration plans, including the presence of potentially harmful non-native plants in surrounding marshes, whose seeds will wash in with the tides and become established.

However the rebuilding process occurs, questions arise: Do the resulting young marshes function as well as natural marshes? Do the plant and animal species we expected return? And do they thrive, or have we merely created a haven for non-native plants and a tasty smorgasbord for predators?

Study sites (click for larger map). .
PRBO is currently active in several large-scale, multidisciplinary regional efforts to assess the status of restoration marshes, comparing them with natural marshes. We also model the evolution of marsh habitat undergoing restoration, describing patterns of change in the physical and biological components. At more than 10 restoration sites and 40 natural sites (see map), we conduct bird surveys alongside a group of fisheries and invertebrate biologists, hydrologists, and sediment specialists.

Our preliminary findings show that while each restoration and natural site in the bay has a unique set of characteristics, based on its history and on influences from the surrounding landscape, there are general differences in bird uses between early restoration sites and older vegetated sites.

Young sites have no vegetation, are primarily bare mud exposed at low tide, and are submerged during high tides. They are often used by vast numbers of waterbirds-swimmers such as diving ducks and dabbling ducks when the tidal water is deep; waders such as shorebirds, egrets, and herons when the tide is out and mud exposed. The tidal marsh-breeding birds (passerines and rails) don't begin to use the habitat until the elevation is high enough for vegetation to come in, which usually takes several years. As the marsh becomes vegetated, shorebirds and other waterbirds find less in the way of food in the marsh, but particular waterbird species may still forage in channels and ponded areas or roost in the marsh vegetation.

When a restoration site reaches the elevation of a typical natural site, the two marshes may look very similar, with the same plant and bird species present. Birds, though, respond to subtle differences in the habitat, such as the degree of channel formation and presence of many layers of vegetation. We are examining the extent to which abundance of tidal marsh birds depends on features of restored marsh. One of our large projects has as a goal to compare nest success rates at restoration sites with those at nearby natural sites.

With our growing expertise, PRBO biologists continue to monitor tidal marsh restoration sites. Now, in addition, we are serving on teams charged with effective project design. This is an exciting time for PRBO to participate in efforts to restore San Francisco Bay for the benefit of the wildlife populations that depend on it.