The Life and Times of the Salt Marsh Harvest Mouse

Salt Marsh Harvest Mouse 4

This article originally appeared in the 1991 Fall issue of Tideline, the newsletter of the San Francisco Bay National Wildlife Refuge Complex.

A two-dimensional battle is raging along the edge of San Francisco Bay.  In salt marsh areas, in close association with pickleweed and in diked, non-tidal marshes, the salt marsh harvest mouse is fighting for survival.

Found only along the edges of San Francisco, San Pablo, and Suisun Bays, the salt marsh harvest mouse exists no where else in the world.  Existence here has not been easy.  Hawks, owls, snakes, skunks, and other predators consume many hapless mice.  Recently, the red fox has appeared in bayshore wetlands, consuming untold numbers of mie and other endangered spceices.  In addition, other rodents compete with the endangered mice for prime habitat.


The only way that the salt marsh harvest mouse can survive is to stay hidden from the many predators that find and eat any mouse that shows itself above the pickleweed.  It also has to remain in areas where other rodents, such as the meadow mouse, cannot compete effectively for food and cover.

Harvest mice are known to avoid meadow mice when possible.  This makes sense, since meadow mice are superior competitors, and are even known to dine on smaller mice of other species that fail to remove themselves from the meadow-mice vicinity rapidly enough.  During the rainy season each year, when meadow mice are numerous, salt marsh harvest mice are restricted to the places that do no attract meadow mice.  These are the most saline places in the marsh, where meadow mice cannot survive.  A fugitive in the marsh's saltiest habitats, the salt marsh harvest mouse is the high-salinity-habitat champion of the rodent world.  It prevails in these super-salty areas because of its ability to drink sea water.

Studies of salt marsh harvest mice from the north bay have demonstrated that these mammals can drink sea water indefinitely, without loss of weight.  They are also able to eat pickleweed, which has very salty sap.  This tolerance for high salt intake allows them to prevail in the salt marsh and compete successfully for food and cover against non-saline rodents.

Food is abundant in the salt marsh.  Pickleweed is everywhere and high-protein spiders, bugs, and flies are within reach.  To compensate for their short lives (each mouse lives for up to six months or so) salt marsh harvest mice begin reproducing at less than 2 months of age and produce two litters.  Each litter consists of about four baby mice (3.7 baby mice per litter, exactly).  Protective cover, in the form of pickleweed and other plants, provides sheltered mouse runways.  Conditions in the marsh, with ample food and ample shelter, are perfect for the salt marsh harvest mouse.  So why is this species endangered?

The other dimension of the salt marsh harvest mouse's battle for survival is the encroachment of urban development.  The mouse can only live in certain places, which lately have become attractive as sites for industrial parks and subdivisions.  These sites have become hotly contested.

The reason that the salt marsh harvest mouse is nearly extinct is that its habitat is being destroyed.  In South San Francisco Bay 95% of the historic salt marsh is gone.  Individual mice nowadays do as well as individual salt marsh harvest mice have ever done, but the population is declining as habitat disappears.  Since most salt marsh habitat is but a memory, there are so few mice left that even minor fluctuations have major impacts on the remaining mice.  An example is wintertime flooding.

Salt Marsh Harvest Mouse by Sonoma Creek BridgeDuring wintertime flood tides, most of the natural salt marsh is inundated.  Salt marsh harvest mice that live there must climb the levees, swim or drown.  There is evidence that they do all three.  Fortunately for them, they do not all live in tidal marshes.  Much of the historic salt marsh was diked off and isolated from tidal action years ago.  Salt marsh harvest mice still survive in some of these, non-tidal, seasonal wetlands.  These areas are important because they provide pickleweed and other plants necessary for the survival of the mouse.  They also flood with pools of rainwater, but they are protected from the highest tides of the year and many of their harvest mouse residents survive.

Many don't survive.  Flooding in seasonal wetlands or in tidal salt marshes is catastrophic for the mice.  Climbing the levee banks and swimming exposes them to predators, who gorge on mouse after mouse.  The survivors are those mice in non-flooded seasonal wetlands who can stay hidden beneath the pickleweed and stay alive.  These mice, as well as a few lucky survivors from flooded tidal marshes, can serve as the progenitors of the post-flood mouse population.

Nontidal marshes are botanically reminiscent of tidal marshes that once occurred in the area.  For years, developers have disced these places, plowing under the wetlands vegetation and transforming the identity of these habitats into something not protected by wetlands protection laws.  The mice on hundreds of acres have been plowed under along with the pickleweed.  Recently, since the abundance of mice in these areas has become better known, discing has been suspended and developments have been stopped.  The innocuous salt marsh harvest mouse, invisible to all but the most acute observers as it scuttles back and forth beneath the pickleweed, has become the focal point in repeated skirmishes between environmentalists and developers.

 The salt marsh harvest mouse was listed as an endangered species in 1970.  As such, it is protected by the Endangered Species Act, which makes it immune to incidental "take" if such mouse mortality is likely to jeopardize the continued existence of the species.  This means that in areas where salt marsh harvest mice occur, development usually doesn't.

In the San Pablo Bay region, more than one major development has been canceled.  Plans for a new P.G.&E. power plant, scheduled for construction in Solano County, were scrapped.  The Cullinan Ranch, the proposed site of a housing development, became part of San Pablo Bay National Wildlife Refuge instead.In South San Francisco Bay, where only 5% of the suitable habitat for this species remains, a number of proposed subdivisions and other projects along the east shore have been abandoned.  San Francisco Bay National Wildlife Refuge is actively pursuing the purchase of many of these areas.

Whether the Fish and Wildlife Service is successful or not in its attempts to save the salt marsh harvest mouse will probably depend upon our ability to acquire enough suitable habitat.  The salt marsh harvest mouse is able to wage its battle against nature all by itself; it has faced severe predation and competition always, and has come out on top.  However, it cannot fight the other dimension of its battle for survival.  Habitat conservation is something that humans must do.  If enough habitat is conserved, the salt marsh harvest mouse will take care of the rest of its needs by itself.