POPULATION NUMBERS AND TRENDS 



It occurs throughout the salt marshes of the 

 San Francisco Bay area, except for very small 

 marshes and those extensively modified by 

 human activity (Fisler 1968). The southern sub- 

 species (R. r. raviventris) seems the most immedi- 

 ately threatened, for its range includes marshes 

 surrounded by heavily populated areas. It has 

 already been eliminated from the Corte Madera 

 marshes in Marin County (Fisler, 1965). 



Density estimates are unavailable for this spe- 

 cies, partly because of the rapidly changing pat- 

 tern of marsh occupation by harvest mouse popu- 

 lations. The few remaining large marshes support 

 moderately high populations (Schaub 1971), 

 especially in San Pablo and Suisun Bays. The rate 

 of population turnover is high; few mice reach the 

 age of 12 months, and most survive less than 6 

 months (Fisler 1971, Wondolleck et al. 1976). 



The main influence on the decreasing num- 

 bers of these mice is habitat destruction rather 

 than any direct removal or direct mortality result- 

 ing from human activities. Seasonal changes in 

 population distribution occur in these mice. Dur- 

 ing the winter months there is a movement 

 toward the upper edge of the marsh to escape the 

 highest tides, whereas the population spreads 

 throughout the marsh during the summer. In 

 some high tides, the populations move to higher 

 ground. 



REPRODUCTION 



Some males are reproductively active during 

 most of the year, with the low point in the winter 

 months. The season for pregnancy and lactation 

 in females is March through November (Fisler 

 1965). Females may produce only one litter per 

 year in the field (Fisler 1965). Average litter size 

 is 3.7 in R. r. raviventris and 4.2 in R. r. halico- 

 etes (Fisler 1965). 



MANAGEMENT AND CONSERVATION 



This small, secretive mouse is little noticed by 

 humans. It poses no economic problems. Habitat 

 protection is the critical factor in its management. 

 The dense cover available in the preferred salt 

 marsh habitat would seem to effectively protect 

 the species from predation by human commensals 

 during most of the yearly cycle. Occasionally, 



mice forced onto levees during high tides may be 

 captured by feral cats and birds of prey. Increas- 

 ing pressure for land in the San Francisco Bay 

 area has resulted in outright destruction of much 

 of the original range of this species. Its continued 

 survival depends directly on the persistence of the 

 remaining salt-marsh habitat. 



AUTHORITIES 



George F. Fisler 



Department of Biology 



California State University, Northridge 



Northridge, California 91330 



Howard S. Shellhammer 

 Department of Biological Sciences 

 San Jose State University 

 San Jose, California 95192 



PREPARER'S COMMENTS 



The two subspecies of this mouse appear to 

 have evolved different physiological and genetic 

 traits and they may be considered incipient spe- 

 cies (Fisler 1965, Shellhammer 1967, 1977). 

 While some populations of the northern subspe- 

 cies (R, r. halicoetes) appear to occupy secure 

 habitat, others are restricted to small areas. The 

 remaining populations of the southern subspecies 

 are all restricted to small refugia of native salt 

 marsh and need careful management. 



Habitat occupied by the salt marsh harvest 

 mouse is much diminished and fragmented from 

 its original condtion. Remaining populations are 

 isolated genetically, living on natural islands in a 

 sea of human-altered land. Under such circum- 

 stances, populations can be predicted to become 

 extinct (McArthur and Wilson 1967). Since these 

 mice avoid areas without cover, a single levee or 

 wide space may effectively isolate two adjoining 

 marshes, reducing the effective genetic pool. At- 

 tention should be given to the maintenance of 

 larger refugia of continuous salt-marsh habitat, 

 rather than equal areas of habitat dissected by 

 access roads, walkways, or barren ground. 



LITERATURE CITED/SELECTED 

 REFERENCES 



Dixon, J. 1908. A new harvest mouse from the 

 salt marshes of San Francisco Bay, California. 

 Proc. Biol. Soc. Washington 21:197-198. 



