PRBO Conservation Science
Quarterly Journal of PRBO Conservation Science, Number 136, Spring 2004: San Francisco Bay


What is the optimal mix of salt ponds and tidal marshes for birds?

Guiding Effective Wetland Restoration

Diana Stralberg

Salt Ponds and Tidal Marshes
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South San Francisco Bay salt ponds and tidal marshes. Click on image for larger map.

The recent acquisition of more than 16,000 acres of commercial salt evaporation ponds by state and federal wildlife agencies offers an unprecedented opportunity to restore large areas of tidal wetlands in South San Francisco Bay (see map). The conversion of existing salt ponds to tidal marsh will create new habitat for marsh-dependent bird species, such as the federally-listed California Clapper Rail and endemic Alameda Song Sparrow (a California Species of Special Concern), greatly increasing their chances for long-term survival.

At the same time, many waterbird species, including the threatened Western Snowy Plover, have come to depend on salt ponds as a replacement for natural shallow-water habitats, greatly reduced since European settlement. Artificial salt ponds now provide valuable feeding and roosting habitat for a wide range of breeding, over-wintering, and migrating waterbirds.

In light of these trade-offs, PRBO has completed the first phase of a long-term effort to evaluate potential effects on bird communities of restoring salt ponds to tidal marsh. Using bird survey data from salt pond and tidal marsh habitats, we have produced decision-making tools that predict the impacts of specific restoration scenarios on South Bay bird diversity and numbers.

Key findings thus far:
The American Avocet depends on salt ponds, while the Clapper Rail (below) dwells in tidal marsh habitat. Photos Peter LaTourrette

  • While songbirds and rails could benefit greatly from creation of new tidal marsh habitat, the loss of salt ponds may cause substantial reductions in waterbird numbers in those areas, especially of diving ducks and shorebirds.
  • The amount of open-water habitat in a tidal marsh strongly affects waterbird use of this habitat. In newly created tidal marshes, ponded areas and tidal channels can help offset the loss of salt pond habitat. Numbers of some species, especially dabbling ducks, may even increase with appropriate marsh management.
  • Due to differences in the prey they support, lower-salinity salt ponds (< 60 parts per thousand or ppt) support the highest densities of diving ducks and fish-eaters, while high-salinity ponds (120-200 ppt) support the highest shorebird densities. Retaining many low-salinity ponds along with some of higher salinity would be most beneficial to a large number of species.
  • Landscape context can influence bird numbers and species diversity. For example: shorebird numbers are higher in salt ponds than in tidal marshes; but in salt ponds surrounded by tidal marsh, shorebird numbers (and overall species diversity) are even higher.

These findings have led us to recommend maintaining a South Bay wetland mosaic that includes carefully designed and managed tidal marsh habitat in various successional stages, interspersed with salt ponds managed for appropriate depths and salinities.

In this process, trade-offs among different bird groups will be inevitable. What is the optimal mix of salt ponds and tidal marshes? Future PRBO research will address this question under a range of assumptions and management objectives. Given the known trade-offs for birds, and the many remaining uncertainties, we recommend that restoration proceed with caution, with the South Bay's important bird populations monitored and evaluated throughout the process.

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