Salt Marsh Harvest Mouse

Reithrodontomys raviventris

Description

The tiny salt marsh harvest mouse, Reithrodontomys raviventris, has an average body length of 2.75 in (7 cm). Upper parts are various shades of buff mixed with brown, depending on geographic location. It often has a dark stripe down the back. Most northern subspecies have whitish bellies. Members of the southern subspecies have cinnamon-colored bellies and are sometimes called "red-bellied" harvest mice. In appearance these mice resemble the much more widely distributed western harvest mouse (R. megalotis ), from which they may have evolved. Originally described as two separate species, the salt marsh harvest mouse is now considered a single species with two subspecies: the northern (R. raviventris halicoetes ) and the southern (R. r. raviventris ).

Behavior

Less active than other harvest mice, the salt marsh harvest mouse feeds placidly on green vegetation and seeds. Also in contrast to other harvest mice, it swims well, often floating on the surface of the water. It does not burrow but builds its nest from a loose ball of grasses on the surface of the ground. Consequently, vegetative cover is a necessity. Females have a long breeding season that extends from March to November, but reproductive potential is low. The average litter size is about four, with females having the capacity to bear two or three litters per year but usually producing only one.

Habitat

The salt marsh harvest mouse inhabits saline or brackish marshes. It requires dense ground cover and prefers stands of pickleweed as long as it has non-submerged, salt-tolerant vegetation for escape during high tides. Harvest mice move into higher grasslands during the highest winter tides. Studies indicate that the salt marsh harvest mouse can adapt to a variety of locations and plant diversity.

Distribution

This species once ranged along the central coast of California and was particularly concentrated around the San Francisco Bay. As of the late 1990s, it inhabited wetlands that ring the San Pablo-Suisun-San Francisco Bay region. Marshes extend north from San Pablo Bay along the Petaluma River and connect to the large Petaluma Marsh (in Sonoma County), which supports a sizable population. The Napa River marshes for the most part are too narrow to support harvest mice. The eastern limit of its distribution extends through Suisun Bay to the mouth of the Sacramento River at Antioch Dunes (in Solano and Contra Costa counties), with the western limit stretching to the marshes at the mouth of Gallinas Creek on the upper Marin Peninsula (Marin County). To the south, the distribution reaches from San Mateo Bridge to include marshes in the San Francisco Bay National Wildlife Refuge (San Mateo, Santa Clara, and Alameda counties). Established in 1971, the San Pablo Bay National Wildlife Refuge encompasses marshlands that extend from the mouth of the Petaluma River to the Naval Shipyard on Mare Island and provide one of the major refuges for northern subspecies of the harvest mouse. No population figures are available, but the total population is thought to be a few thousand.

Threats

Snakes, owls, and hawks inhabit most marshes and are potential predators, but the major reasons for the decline of the salt marsh harvest mouse are loss of wetlands, habitat fragmentation, and vegetational changes. Groundwater pumping has diminished some marshes while sewage discharges have polluted others. Many marshes have been diked or drained, and most remaining marshes around South San Francisco Bay are too small and too widely separated to support large populations. Some populations occur near or adjacent to highways, urban industrial and residential lands, aqueducts, and flood control channels, which make them vulnerable to continual disturbance, especially habitat alteration.

Conservation and Recovery

Established in 1972, the San Francisco Bay National Wildlife Refuge has protected marshes in the South Bay, the largest being Greco Island. In the 1970s the California Department of Fish and Game acquired marshes on Coon Island. Enactment of the Suisun Marsh Protection Plan and the establishment of the Suisun Resource Conservation District by the California legislature largely eliminated marsh destruction around the Suisun Bay. The state of California has acquired several areas that provide mouse habitat: Grizzly Island and Joice Island Wildlife Areas, Hill Slough Wildlife Area, and Peytonia Slough Ecological Reserve.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Reference

U. S. Fish and Wildlife Service. 1984. "Salt Marsh Harvest Mouse and California Clapper Rail Recovery Plan." U. S. Fish and Wildlife Service, Portland.