Habitat Mosaic Needed
PRBO research to understand the effects on bird populations of converting salt ponds to tidal marsh habitats in San Francisco Bay provides quantifiable support for the idea that tidal marsh restoration will benefit tidal marsh-dependent bird species. But our analyses and on-the-ground experience also show that, to ensure no net loss of bird diversity or abundance in the bay, restoring a mixture of different habitat types will be necessary. A variety of habitats--higher-salinity salt ponds (both shallow and deep water), tidal flats, tidal marsh with large amounts of open water--all will be critical to maintaining San Francisco Bay's international importance to birds. Creating this mixture of habitats will require sound, scientifically based planning and active adaptive management.--Nils Warnock, PhD, Co-Director, PRBO Wetlands Ecology Division
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.|
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). .|
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.