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Mouse, cont.

in particular have been affected by a triple whammy of human-made changes. First, almost all of the upper edges of most marshes have been filled in, covered over, or converted to salt ponds reducing the peripheral halophyte zone to narrow, near vertical bands along leeve banks with no connections to adjacent grasslands (if any exist); without this zone the salt marsh harvest mouse no longer has cover from predators when high tides push them out of the middle zone. Second, salt marsh zones are subject to subsidence and increased tidal coverage resulting from ground water pumping hence the broad pickleweed zone, favored by the salt marsh harvest mouse for habitat, diminishes. Lastly, non-saline sewage effluent being discharged into the Bay changes marsh vegetation by benefiting plant species that prefer fresh or brackish water (like cattail and bulrush) and are not used by the mouse. If the cordgrass zone increases due to one or more of the above factors, salt marsh harvest mice do not reap any reward as cordgrass is too often covered by tides and inaccessible.

With years of habitat loss and modification new predators have been introduced into the salt marsh harvest mouse’s environment. Given that most remaining marshes share an upper side with a leveed salt pond, business park, or subdivision, access to the marshes is much easier for feral cats, house cats and non-native red foxes. These animals prey on the mice in addition to native predators like hawks, owls, herons, and clapper rails. The California clapper rail, interestingly enough, is one endangered species that eats another endangered species...I guess they don’t read government bulletins.

While the salt marsh harvest mouse continues to fight for survival, a few marsh areas within the boundary of Don Edwards San Francisco Bay National Wildlife Refuge, like Calaveras and Dumbarton Points, Greco Island, and New Chicago marsh, provide hope for the species. Research indicates that mouse populations may be sustaining themselves in some of these areas. How many mice, you might ask, are necessary for the long term survival of the species?