|Scientific Name:||Reithrodontomys raviventris|
|Species Authority:||Dixon, 1908|
|Taxonomic Notes:||Two subspecies are recognized: R. r. raviventris and R. r. halicoetes, which some authors have proposed as distinct species. Neither Baker et al. (2003) nor Musser and Carleton (in Wilson and Reeder 1993, 2005) recognized halicoetes as a distinct species.|
|Red List Category & Criteria:||Endangered B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v) ver 3.1|
|Assessor/s:||Whitaker Jr., J.O. & NatureServe (Hammerson, G. & Williams, D.F.)|
|Reviewer/s:||Amori, G. (Small Nonvolant Mammal Red List Authority) & Chanson, J. (Global Mammal Assessment Team)|
Listed as Endangered, because its extent of occurrence is less than 5,000 km², its area of occupancy is less than 500 km², its range is severely fragmented, and there is a continuing decline in the extent and quality of its tidal marsh habitat, and the number of locations as well as the number of individuals.
|Range Description:||This species is restricted to habitats bordering San Francisco Bay in California in the United States. Occurrence within this small range is highly fragmented. The range encompasses salt marshes of the San Francisco Bay system (San Francisco, San Pablo, and Suisun bays), in central California (Shellhammer 1982). Subspecies halicoetes occurs in marshes of San Pablo and Suisun bays (and including Petaluma Marsh in Sonoma County) and along the northern Contra Costa County coast (east to Antioch dunes area), and subspecies raviventris occurs mostly in the southern part of South San Francisco Bay, with a few populations on the Marin Peninsula (west to the mouth of Gallinas Creek) and near Point Richmond (Shellhammer 1989). The remaining tidal marsh habitat in the San Francisco Bay Area is estimated to be 12,555 ha (=125 sq km). An unknown fraction of this habitat is actually occupied by this species.|
Native:United States (California)
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Shellhammer et al. (1983) estimated a total population of a "few thousand animals at the peak of their numbers each summer." They also mapped 24 "principal populations." Recent figures are unavailable. The former, probably more or less continuous historical distribution has been fragmented, leaving harvest mouse populations restricted to discontinuous patches of suitable habitat. About 84% of historical tidal marshes of the San Francisco Bay area have been destroyed. Over 3,600 acres of habitat have been filled or degraded since the 1970s. Since 1982, hundreds of acres of nontidal salt marsh in the South Bay have been disced and the damage continued into at least 1989 (California DF&G 1990).|
|Habitat and Ecology:||
The species' habitat consists of salt and brackish marshes, where plants provide a dense mat of cover, ideally around 30-50 cm high with a high percentage (e.g., 60%) of pickleweed (Salicornia) and complex structure of Atriplex and other species. The mouse needs access to refuge/cover on high ground, especially during highest tides in winter. It appears to be less dependent on cover in diked marshes than in tidal marshes (Geissel et al. 1988). In South San Francisco Bay, in diked marsh, harvest mice used more open and saltier pickleweed when vole populations were high, then moved into deeper and less salty pickleweed as vole populations declined (Geissel et al. 1988). In diked salt marshes, high densities of voles (Microtus) seem to force this species into marginal habitats or to become locally extirpated (Geissel et al. 1988).
Subspecies halicoetes builds ball-like nests of dry grasses/sedges on the ground. Subspecies raviventris does little nest building but may use accumulations of vegetation on the ground surface. Diet dominated by green vegetation. Will eat salt grass and pickleweed as well as some seeds. Diet influenced by availability of food plants. More green vegetation is eaten in winter. Primarily nocturnal but sometimes active during the day.
Average litter size is 3.7-4.0 young. There may be only a single litter/year in R. r. halicoetes (breeds May-November) but multiple litters in R. r. raviventris (breeds March-November).
This species is threatened by habitat destruction or disturbance due to development, residential encroachment, intrusion of fresh water into salt marshes, marsh subsidence, and predation (especially by housecats). In many marsh habitats, there is no higher ground to escape to during high tides. Habitat within the range of the southern subspecies (R. r. raviventris) has been more severely impacted than that of the northern subspecies (R. r. halicoetes).
Most of the historical tidal marsh habitat has been lost as a result of filling for urban development and conversion to agricultural uses and commercial salt production. Upland vegetative cover adjoining tidal marshes is important cover for harvest mice during high tides and storms, but much of this habitat is no longer available due to clearing and conversion to intensive human uses. Remaining populations are relatively small, isolated, and may lack the size and full range of resources needed for long-term persistence.
Threats include continued loss of habitat to development, pollution and other changes in water quality, and encroachment by exotic plant species (e.g., invasive cordgrasses, Spartina) that do not provide suitable habitat for salt marsh harvest mice. Most areas of remaining habitat support few to no mice because of backfilling, subsidence, or vegetation changes. Salt marshes are subject to subsidence and increased tidal coverage (flooding) as a result of groundwater pumping. This reduces the extent of the broad pickleweed zone favoured by harvest mice (Shellhammer 1998).
Most of the remaining habitat is diked. Much of the habitat is managed for waterfowl and such management (e.g., lowered salinities) may not be compatible with the needs of harvest mice. Excessive inputs of fresh water (e.g., from discharges of treated municipal sewage) have altered salinity levels, marsh vegetation, and mouse food resources. See Shellhammer (1989) for a discussion of threats to diked marshes.
Much of the East Bay shoreline from San Leandro to Calaveras Point is rapidly eroding. In addition, an estimated 600 acres of former salt marsh along Coyote Creek, Alviso Slough, and Guadalupe Slough has been converted to fresh- and brackish-water vegetation due to freshwater discharge from South Bay wastewater facilities; these areas likely no longer support populations of salt marsh harvest mice.
Some habitat has been lost as a result of discing of nontidal salt marsh. Discing of wetlands, an activity not regulated by the U.S. Army Corps of Engineers, has been employed by interests seeking to obscure the Corps' jurisdiction and circumvent the Clean Water Act and the Federal Endangered Species Act.
Some harvest mice are preyed upon by feral and free-ranging cats and non-native red foxes that roam salt-pond dikes or wander from residential areas now bordering many tidal marshes.
Habitat may be threatened over the long term by the rise in sea level that is predicted to occur within the next century. In combination with reduced sediment supply caused by upstream dams and water diversions, sea level rise could result in major losses of tidal marsh habitat that would be very difficult to prevent or mitigate.
Although populations occur in a number of protected areas, many are probably not large enough to ensure long term persistence. There was a recovery plan completed in 1984. However, the US Fish and Wildlife Service is developing a "Tidal March Ecosystem Recovery Plan," with R. raviventris to be included in this apparently updated plan.
This mouse is protected to some extent by federal and state listings as Endangered. Several small marshes are protected in national wildlife refuges and other preserves, but few provide refuge and cover from the highest tides.
Marshes and adjacent uplands need to be protected from development and degradation. Managed, diked marshes appear to be the key to the survival of the subspecies raviventris; future management prescriptions may include the possibility of a more patchy environment with some areas of high salinities plus the acceptance of some open areas within the managed areas (Geissel et al. 1988). Shellhammer (1989) questioned whether large enough areas of diked marsh can be acquired in the near future to protect this species in perpetuity.
Up-to-date data are needed on range-wide habitat occupancy and relative abundance. Research is needed on the effects of plant salinities on water balance and habitat use of mice.
|Citation:||Whitaker Jr., J.O. & NatureServe (Hammerson, G. & Williams, D.F.) 2008. Reithrodontomys raviventris. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>. Downloaded on 24 May 2013.|
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