Habitats: Tidal Flats

Julian Wood and Gary Page (PRBO Conservation Science)

Status: Tidal flats birds overall stable, with a geographic shift from south to north.

Exposed twice a day by the Bay’s low tides, tidal flats are teeming with life. Small clams, marine worms, and crustaceans feed more than one million shorebirds each year. Today, 42% of the Bay’s tidal flats have been lost compared to historic levels.
In addition, shorebirds in San Francisco Bay are also dependent on salt ponds, many of which are now managed to maximize their value as shorebird breeding and foraging habitat. (See Managed Ponds for more information.)
San Francisco Bay is so critical to the health of shorebird populations that it has been designated a site of Hemispheric Importance for Shorebirds (www.whsrn.org). Today, 42% of the Bay’s tidal flats have been lost from historical levels.


November high tide roost counts of shorebirds throughout San Francisco and San Pablo Bays.

Willet

Trends — Overall, the shorebird population in San Francisco Bay has remained stable since the 1990s, but an increases in the North Bay and apparent decreases in the Central and South Bays indicate a shift from south to north.

One of the more common species, the Western Sandpiper, appears to have declined Bay-wide.

Two other common species — Least Sandpiper and Willet — have increased greatly.

Keeping one million shorebirds in San Francisco Bay will require maintaining sufficient tidal flat habitat as well as other shallow water habitats, especially as sea level rises. The amount of tidal flat and other habitats needed by shorebirds should be determined.

Threats

Primary threat: Loss of tidal flat feeding habitat due to sea level rise, erosion from storm surges, and invasive plants (e.g., hybrid Spartina).

Loss of shallow water feeding habitat, as former salt ponds transition to tidal marsh through active restoration or through levee failure due to impacts of sea level rise.

Reduction of food (invertebrates) caused by invasions of non-native invertebrates, pollution, and climate change impacts.

Human-caused disturbance to feeding and resting shorebirds, resulting in birds having less energy for migration and survival.

Loss of high tide roosting habitat such as levees, islands, structures, and high ground as sea level rises, levees are removed or deteriorate, and islands within restored ponds are submerged.

Greater Yellowlegs

Greater Yellowlegs

Western Sandpipers

Western Sandpipers

Actions

Planning, Management, and Restoration

Plan for mudflat creation and sustainability by conducting physical modeling exercises and assessing those results over time to ensure that current and future coastal defense (e.g., levees, rip-rap, and seawalls), salt pond restoration, and development does not reduce mudflats. Future restoration should focus on increasing both tidal flat and tidal marsh habitats.

Maintain shallow pond feeding and roosting habitat, especially when tidal flats are inaccessible during high tides.

Control non-native plants that colonize mudflats (e.g., hybrid Spartina).

Remove non-essential barriers such as dams, culverts, levees, and other structures that inhibit natural flow and settling of sediment.

Restore watersheds to facilitate movement of tidal flats to higher areas as sea level rises and to promote movement of sediment downstream to feed tidal flats.

Minimize pollution from runoff on paved surfaces, allowing rainfall to soak into the ground; convey and treat storm water runoff using landscape features Western Sandpipers such as rain gardens and other water conservation systems.

Manage for a mix of pond conditions with depths ranging from 2 to 5 cm and salinities from 120 to 200 ppt for optimum shorebird use.

Provide and protect roosting habitat away from areas of frequent human use.

Reduce human-caused disturbance (e.g., hiking, dog walking, boating) in areas where shorebirds feed in high densities (e.g., Napa River tidal flats, San Leandro Bay, and Hayward southward to southern San Francisco Bay). Collaborative planning between the San Francisco Bay Water Trail and the Bay Trail can consider actions to minimize disturbance.

Scientists

Determine the amount of ponds, other shallow water habitat, and tidal flats needed to support the Bay’s breeding and migratory shorebirds.

Conduct early winter Bay-wide shorebird surveys annually to spot potential declines quickly. Participate as a citizen scientist in the Pacific Flyway Shorebird Survey (www.prbo.org/pfss).

Monitor site-specific shorebird response to restoration, and study how mudflat characteristics influence habitat quality for shorebirds.

Conduct research to better understand and predict changes in tidal flat habitat in the context of sea level rise and potentially decreasing sediment supply.


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