Estuary News

December 2019

Not So Picky Marsh Mouse

The endangered salt marsh harvest mouse (informally “Salty”) is a poster child for tidal marsh restoration in San Francisco Bay. But recent research, presented by University of California at Davis postdoc Katie Smith in a State of the Estuary conference session on tidal wetlands, suggests we’ve misinterpreted what the mouse needs. “It’s been managed as a habitat specialist,” she said, based on assumptions that it requires tidal wetlands and a diet of pickleweed.

However, hours of mouse-tracking around the Bay show that it also thrives in managed wetlands and eats a variety of plants, including non-native species. Although restoration projects have created high-tide refuges for the mice, Smith’s preliminary data suggest other rodent species exclude them from those sites. That was in a wet year, though; a dry year could shift the dynamic, favoring salties. “We’ve thought about the species in isolation, ignoring the community context,” said Smith. “We need to look at it more holistically.”

This, as she observes, is no ordinary mouse. For starters, it’s the world’s only mammal restricted to coastal marshes. Genetic studies show that the species originated four million years ago, long before the Bay came into existence. Salt marsh harvest mice can drink salt water and swim for hours. Females build crude nests in tall marsh vegetation, sometimes appropriating marsh wren nests. The mice are highly vocal, especially on warm summer nights; if Smith can distinguish their squeaks from those of co-occurring western harvest mice, monitoring calls may provide a non-invasive way of recording populations.

Smith credits her research interests to her family’s pet rats and the wildlife at a pond near her childhood home. After fish-tagging work, she joined ongoing salt marsh harvest mouse field studies and made the rodent her thesis and dissertation subject (only the third PhD on the species).

Previous work had detected the mice in managed wetlands, including waterfowl areas in Suisun Marsh where water levels are managed to favor the birds. For her post-graduate research, Smith trapped and radio-tagged mice, spending nights in the marsh tracking their movements. “I was surprised at how diverse their habitat use was,” she recalled. “We had mice running all over the place. There were variations in individual behavior: some crossed levees, some never approached them.” They used high ground for foraging, not for nesting. Reproductive and survival rates and home range sizes of mice in managed wetlands proved comparable to those in tidal marshes.

Smith offered captive mice a buffet of native and non-native plants and watched them chow down on non-natives like rabbitsfoot grass, in addition to native pickleweed. The salt marsh harvest mouse’s food preferences overlap those of waterfowl; the duck-hunting clubs in Suisun Marsh have been inadvertently farming mice.

Mouse buffet. Photo: Katie Smith

Her current study looks at interactions with other rodent species and how this affects the mouse’s habitat use, especially during king tides. Even with high water, winds, and waves, the mice didn’t seek high ground this year; they stayed out in the marsh, chugging along like little Diana Nyads or clinging to tall vegetation. Voles in particular may exclude the mice from potential high-tide refugia, at least in wet years: “It’s like kids that play in the mud instead of using the nice new playground we built for them because that’s where the bullies hang out.” The mice might benefit from more vertical structure in the low marsh; even woody debris can be a life raft.

Food choices salt marsh harvest mouse. Source: Smith & Kelt, 2019

In the same session, Nadav Nur (Point Blue) and Levi Lewis (UC Davis) discussed other tidal marsh creatures. Lewis found unexpected fish abundance and diversity in tidal marshes previously unmonitored, including spawning aggregations of the threatened longfin smelt. Nur dealt with special-status tidal marsh birds like rails and song sparrows and their response to features of the transition zones they use as high-tide refuges, with implications for design parameters in future restoration projects.

Tidal wetlands are caught between the rising deep blue sea and the devil of shoreline development. “We’re losing habitat on both ends,” Smith said. “The marshes have no chance to migrate.” Industrial and residential construction is pushing right up to the edge of the marsh all around the Bay.

Mouse on pickleweed. Photo: Katie Smith

Why invest scarce resources in these obscure creatures? Smith’s take on the salt marsh harvest mouse would apply to smelt and rails as well: “It should be a point of pride for the Bay Area to have this cool and unique species — a very specially adapted and unique mouse. Beyond that, a natural ecosystem is like a Jenga tower. If you pull out some number of species, eventually it will collapse.” When marsh habitat changes, fish and birds can move elsewhere, but the mice, with their fragmented populations, lack that option: “If we’ve lost our mice, we know something bad is happening.”

All photos and video accompanying this are the property of Katie Smith and Chris Rudolph and may not be replicated without permission.

Top Photo: Katie Smith and marsh mouse. Photo: Chris Rudolph

Toward Mouse Recovery, SFEWS 2018

Waterfowl Management & Marsh Mouse Diet, Journal of Wildlife Management 2019

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Special coverage 2019 State of the Estuary Conference