Research Spotlight: Dabbling Ducks Prefer Managed Wetlands and Pond-Like Features in Suisun Marsh

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A recent study by USGS scientists found that dabbling ducks in Suisun Marsh, California, spend about 98% percent of their time in managed wetlands, consistently selecting managed wetlands over tidal marsh habitat.

Photo of a hen pintail with a GPS transmitter.

Photo of a hen pintail equipped with a camouflage GPS transmitter. USGS Western Ecological Research Center scientists based out of Dixon, CA are marking and tagging waterfowl in Suisun Marsh with GPS transmitters as part of an ongoing study.

(Credit: Jeffrey Kohl, U.S. Geological Survey. Public domain.)

In the western United States, substantial tracts of freshwater, brackish and tidal wetlands have been lost due to agricultural and urban development. One of the largest remaining wetlands in the region is Suisun Marsh, on the eastern edge of the San Francisco Bay Estuary, which includes areas of both managed wetland and tidal marsh. Local management and conservation plans focus on the restoration of tidal marshes for the benefit of numerous endemic and threatened species that rely on them.

Given extensive development in the region, wetland restoration plans in the San Francisco Bay Estuary call for the conversion of existing habitat types rather than adding new wetlands. These restorations can cause conflicts among stakeholders and species that win or lose depending on identified restoration priorities. For example, waterfowl rely on managed wetlands that are currently slated for conversion to tidal marsh restoration.

To understand how converting managed wetlands to tidal marsh would impact waterfowl populations and whether future tidal marsh restorations could provide suitable habitat for dabbling ducks, USGS scientists examined wetland use with a robust tracking dataset (442,017 locations) from six dabbling duck species. Using location data from GPS-GSM transmitters attached to 315 ducks, the researchers calculated the proportion of time the ducks spent in different types of habitat on a daily basis.

Map of Suisun showing different habitat types and duck locations, with most duck locations in managed wetland (47% of area).

Habitat types (SFEI 2019) and associated duck GPS locations in Suisun Marsh, CA. Percentages represent the proportion of Suisun Marsh that the habitat constitutes.

From Casazza et al. 2021.

As expected, managed wetlands, which comprise 47% of Suisun Marsh, were consistently selected by dabbling ducks over tidal marshes, used about 98% of the time across seasons and species. Although overall use of tidal marsh (only 14% of Suisun Marsh habitat) was generally very low, used less than 2% of the time, almost half of the individual ducks (~44%) spent at least some time in this habitat. When using tidal marshes, ducks showed a preference for features that are similar to managed wetland ponds, including depth, bottom topography and vegetative cover. Ponds within tidal marsh habitat were used by ducks at 4.5 times the rate that would be expected based on availability.

The study will inform wetland restorations that provide benefits to a variety of species and minimize the risk of population declines for waterfowl. The results indicate that managed wetlands are essential for dabbling ducks. However, habitat losses from the planned conversion of these habitats to tidal marshes may be partially mitigated by incorporating pond features that are more attractive to waterfowl and likely to offer multi-species benefits into tidal marsh restoration designs. For example, saltmarsh harvest mouse (Reithrodontomys raviventris) is known to be successful in managed wetlands as well as tidal marsh, and managed floodplains can provide valuable habitat for Sacramento splittail (Pogonichthys macrolepidotus) and juvenile chinook salmon (Onchorhynchus tshawytscha), both federally listed fish species.

Most dabbling duck species and other waterfowl species are presently common, but population declines can be avoided through informed ecosystem-based management that promotes species richness, biodiversity and helps “keep common species common”.

MANAGEMENT IMPLICATIONS

  • Dabbling ducks spent ~98% of their time in managed wetlands and <2% in tidal marshes, though about half of ducks spent some time in tidal marsh habitat.
  • In tidal marsh habitat, ducks preferred pond-like features over vegetation and channels.
  • Incorporating pond-like features into tidal marsh restorations may help mitigate waterfowl habitat losses while providing benefits for other tidal marsh species.
  • This strategy could increase stakeholders’ value and involvement and provide economic, social, cultural and environmental benefits by enhancing ecosystem services and biodiversity.

This research spotlight refers to the following publication and data release:

Casazza, M.L., McDuie, F., Jones, S., Lorenz, A.A., Overton, C.T., Yee, J., Feldheim, C.L., Ackerman, J.T. and Thorne, K.M., 2021, Waterfowl use of wetland habitats informs wetland restoration designs for multi‐species benefits. Journal of Applied Ecology, https://doi.org/10.1111/1365-2664.13845.

Overton, C.T., Casazza, M.L., McDuie, F.P., and Jones, S.F., 2021, Suisun tidal marsh duck use dataset: U.S. Geological Survey data release, https://doi.org/10.5066/P94B0WUV.

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