Theharvest mouse is generally restricted to saline or subsaline marsh habitats around the San Francisco Bay Estuary and, with some exception, mixed saline or brackish areas in the Suisun Bay area, as documented by the U.S. Fish and Wildlife Service in 2013 and more recently in 2021. Their distribution in tidal and diked marshes closely corresponds with the abundance of pickleweed (Sarcocornia), a dominant plant species of tidal marshes and a common component of brackish marsh vegetation.
The salt marsh harvest mouse is morphologically similar to the more widespread western harvest mouse (Reithrodontomys megalotis), which co-occurs in some habitats. Accurate field identification of mice in tidal marsh habitats requires special expertise, as some populations of the salt marsh harvest mouse may exhibit morphological characteristics similar to those of the western harvest mouse. Several researchers have noted that this is especially the case in the northern reaches of the estuary.
There are two subspecies: the northern salt marsh harvest mouse (Reithrodontomys raviventris halicoetes), which lives in the marshes of the San Pablo and Suisun bays, and the southern salt marsh harvest mouse (R. r. raviventris), which is found in the marshes of Corte Madera, Richmond and South San Francisco Bay, as was documented in 2013 by the U.S. Fish and Wildlife Service.
Habitat loss due to human actions is the greatest threat to the salt marsh harvest mouse. This includes habitat loss due to filling, diking, subsidence, changes in water salinity, non-native species invasions, sea level rise associated with globaland pollution. In addition, habitat suitability of many marshes is further limited by small size, fragmentation and lack of other vital features like sufficient escape habitat. Larger tracts of high quality habitat are needed to maintain stable populations over time. Viable populations of salt marsh harvest mice also appear to be limited by the distribution of high tide cover and escape habitat. The 2013 recovery plan noted that recurrent, but shallow, flooding by saline water is probably needed to maintain habitat that favors the salt marsh harvest mouse over its potential competitors. Additionally, researchers anticipate that sea level rise presents a severe threat in the long-term, especially in the central and South San Francisco Bay where opportunities for landward migration of habitat are absent.
The basic habitat of theharvest mouse is pickleweed-dominated vegetation (Sarcocornia), as J. Dixon documented in 1908 and G. F. Fisler later confirmed in 1965. While the species was previously considered to be restricted to saline or subsaline marsh habitats around the San Francisco Bay Estuary, it is now known that mice have been found in high numbers in brackish diked marshes in the Suisun Bay, as K.R. Smith and others documented in 2014 and later in 2019. Habitats include primarily mature stands of chairmaker's bulrush or Olney's three-square bulrush (Schoenoplectus americanus), with deep masses of thatch within them, as was documented in 2011 by D. Sustaita and others. Habitats also include mature and heavily thatch-filled alkali bulrush (Bolboschoenus maritimus), in the South San Francisco Bay as H.T. Harvey and Associates documented in 2007. Other highly important habitat considerations include high tide or flood refugia of emergent gumplant (Grindelia) - both at the upper edge of the marsh and within mature marshes and even at the highest high tides. H.T. Harvey and Associates noted in 2007 that other habitat considerations include the seasonal use of terrestrial grassland, exploitation of suboptimal habitats and habitat selection in brackish marsh vegetation where pickleweed is a relatively minor component next to alkali bulrush, as often is the case in Suisun and South Bay marshes.
Within pickleweed marshes the taller, denser stands tend to support the most salt marsh harvest mice, although they may also be abundant in tidal marshes with relatively short pickleweed canopies. A pickleweed canopy height of approximately 15 centimeters (6 inches) appears to be the lowest commonly used by salt marsh harvest mice, as documented by G.F. Fisler in 1965 and H.S. Shellhammer and others in 1982. In 2013, U.S. Fish and Wildlife Service researchers noted that the relationship between pickleweed height and salt marsh harvest mice abundance may depend on degree of canopy submergence rather than height alone.
Salinity may influence salt marsh harvest mouse habitat independent of its correlation with pickleweed. In 1978, D. Zetterquist found that salt marsh harvest mice were most abundant in portions of diked tidal marshes where salinity was extremely high. A high physiological tolerance for salt in food and water, as was documented by G.F. Fisler in 1965 and later by H.N. Coulombe in 1970. This may confer a competitive advantage to salt marsh harvest mouse in harshly saline marsh habitats, particularly where competition with the more aggressive, but less salt-tolerant, California vole occurs, as was documented by A.R. Blaustein in the early 1980s and later by W. H. Geissel and others in 1988.
Areas such as marshes or swamps that are covered often intermittently with shallow water or have soil saturated with moisture.
Theharvest mouse is active during daylight hours, especially on warmer days, but is considered mostly active at night. They begin their activity at about half an hour after sunset and stop about half an hour before sunrise. G.F. Fisler documented in 1965 that the species is considered a strong swimmer. In 2014, K.R. Smith and others also documented in Suisun Bay that the majority of the time mice remain in vegetation above the water during high tides, rather than moving to transition zone or upland areas, and further research noted that the species demonstrate behavioral flexibility which allows them to adapt as food availability changes. This behavioral flexibility also extends to the species using diked wetlands.
Little is known about predation impacts to the species, although R.F. Johnston documented in 1957 and was later confirmed in 1965 by G.F. Fisler that predation-related to flooding has been viewed as important. Mice in flooded areas or at the upper edges of marshes are vulnerable to avian predators such as herons, egrets, gulls and raptors. Mice moving into transition or upland areas are also vulnerable to terrestrial predators such as foxes, feral or otherwise free-roaming cats, skunks and raccoons, as was noted in the 2013 recovery plan.
Telemetry studies of the northern subspecies found an average home range size of 0.21 hectare (0.52 acre) and linear distance moved of 11.9 meters (13 yards) in two hours, as M.A. Bias and M.L. Morrison documented in 1999. Most movements occurred in June, and least in November. Geissel and others in 1988 noted that smaller home ranges were found for the southern subspecies, which were no greater than 0.15 hectare (0.37 acre).
Salt marsh harvest mice are known to eat leaves, seeds and stems of a variety of plant species and are considered to prefer pickleweed and saltgrass, as was noted in the 2013 recovery plan. In 2019, K.R. Smith and D.A. Kelt recordedharvest mice in Suisun Marsh eating insects, including beetles and amphipods, as well as leaves, seeds and stems from a variety of both native and non-native plant species. This suggests a more flexible diet than previously thought. The northern subspecies can drink sea water, but prefers fresh water. The southern subspecies can't live completely on sea water, but it prefers moderately salty water over fresh, as was noted in the 2013 recovery plan.
Length combined head and body: Approximately 3 inches (7.6 centimeters)
Less than 0.353 ounces (10 grams)
Theharvest mouse is a rodent in the Order Rodentia) and the family Muridae (subfamily Sigmodontinae). The scientific name Reithrodontomys raviventris means grooved-toothed mouse with a red belly. Both subspecies of salt marsh harvest mouse have grooved upper front teeth, but only a few populations of the southern subspecies have animals with a cinnamon or rufous-colored belly. Both subspecies have rich dorsal brown hair and a unicolored to moderately bicolored tail, as noted in the recovery plan.
Salt marsh harvest mice typically live less than one year according to G. F. Fisler documented in 1965.
Salt marsh harvest mice are generally sexually active from May through November for the northern subspecies, and March through November for the southern subspecies, as G.F. Fisler documented in 1965. Bias and Morrison, in 1993, suggested that the breeding season of the Mare Island population (northern subspecies) extends from August through November and that more than 30 percent of the females trapped were pregnant during September and October. Females have one to two litters each year. Nests are built from dry grasses and sedges on the ground surface or within and upon the hummocks of vegetation, as G.F. Fisler documented in 1965. Based on data from laboratory and field studies, Fisler estimated litter size at 3.96 to 4.21 for the northern subspecies and at 3.67 to 3.85 for the southern subspecies. Compared with environmentally determined mortality factors, reproduction does not appear to be a limiting factor for the species, as noted in the recovery plan.
Theharvest mouse is morphologically similar to the more widespread western harvest mouse (Reithrodontomys megalotis), which co-occurs in some habitats. Accurate field identification of mice in tidal marsh habitats requires special expertise as some populations of the salt marsh harvest mouse may exhibit morphological characteristics similar to those of the western harvest mouse, especially in the northern reaches of the estuary.
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