Species Spotlight: Salt Marsh Harvest Mouse

Endangered salt marsh harvest mouse, San Pablo Baylands. Judy Irving © Pelican Media

The story of the salt marsh harvest mouse is the story of the San Francisco Bay wetlands – once abundant, now extremely scarce due to human development and increasingly at risk from sea level rise. But we can save both the salt marsh harvest mouse and our Bay wetland ecosystem if we act now.

Meet the “salty”

Salt marsh harvest mice, known as “salties” to their many fans, live only in the tidal marshlands of the San Francisco Bay. They are highly adapted to this challenging environment – they can drink salt water, and their diet consists primarily of salty marsh vegetation like pickleweed (seen in the photo shown here) and saltgrass. Salties are tiny compared to most rodents – only about three inches long – and weigh a fraction of an ounce. Their thick brown fur keeps them warm during wet winters in the marshlands, and when high tides flood their marshy habitat, they are adept at climbing up to the top of a patch of pickleweed to escape the rising waters.

Salties were one of the first species to be listed as endangered in the early 1970’s, and they remain endangered today. Habitat loss is the biggest problem. San Francisco Bay has lost over 90% of its historic wetlands due to filling in for development and diking to create salt ponds. This species, like many others, can’t thrive on isolated patches of habitat – they need large areas that are connected to each other so they don’t inbreed.

Sea level rise poses a particularly acute problem for wetland species. As tidal marshes become flooded more frequently, salties will be forced to spend more and more time avoiding or escaping from these rising waters. Salties construct nests on the ground for their young, and when those nests are flooded, the babies may not survive. Rising seas also mean marshland habitat will gradually shrink, except in the rare locations where marshland vegetation can migrate further inland.

How can we save the salty?

The top priority for salt marsh harvest mouse survival is to restore as many acres of Bay tidal marshland as possible, while protecting existing marshland from development and other threats. The South Bay Salt Pond Restoration Project, which is restoring large areas of salt ponds (former wetlands that were diked off for salt production), is a big step towards this goal, but much more is needed.

Green Foothills, together with many other environmental organizations, for years has been calling for the protection and restoration of the Cargill salt ponds in Redwood City. The 1400+ acres of former wetlands owned by Cargill at this site would be a significant addition to the Don Edwards Wildlife Refuge and other Bay wetlands. Restoration of the Cargill salt ponds would not only benefit the salt marsh harvest mouse and other wildlife, it would help protect the residential areas next to the Cargill site from sea level rise.

The salty is a survivor. Despite losing over 90% of its former habitat and facing increasing threats today, this species is still here. When you go for a walk on a trail in the Don Edwards Wildlife Refuge, Bair Island, Ravenswood Open Space Preserve, or somewhere else with plenty of healthy tidal marsh habitat, there may be a salty hiding underneath the pickleweed not too far from you. If we take action now, we can still save the salty.

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