GEOGRAPHY 316-BIOGEOGRAPHY

The Biogeography of the Salt Marsh Harvest
Mouse (Reithrodonomys raviventris)

BY GALINA GOLOVANOVA, STUDENT IN BIOGEOGRAPHY 316

Thank you for visiting our site. This web pages was written by a student in Geography 316: Biogeography and edited by the instructor, Barbara Holzman, PhD.  All photos and maps are posted with specific copyright permission for the express use of education on these web pages. The students have tried to be as accurate as possible with the information provided and sources and references are cited at the end of each page.

PART I:

  
bulletName: Salt Marsh Harvest Mouse
bulletOther Common Names: Harvest Mouse & Salt-Marsh Mouse
bulletKingdom:  Animalia
bulletPhylum and Subphylum:  Chordata
bulletClass:  Mammalia
bulletOrder:  Rodentia
bulletFamily:  Muridae
bulletGenus: Reithrodontomys
bulletSpecies:  Reithrodontomys raviventris
bulletSubspecies: Reithrodontomys raviventris halicoetes (Northern subspecies)

 

Figure 1. Salt Marsh Harvest Mouse (Northern subspecies) Source: Dr. H. Shellhammer/courtesy of U.S.F.W.S. Pacific Region, 1998 (with permission)

 

 DESCRIPTION OF SPECIES:

         The scientific name Reithrodontomys raviventris means “grooved-toothed mouse with a red belly" (Shellhammer, 1998). Two species of Reithrodontomys raviventris are recognized.  The subspecies of study, R. raviventris halicoetes, the Northern Salt Marsh Harvest Mouse, is found in the marshes of northern and central San Francisco Bay, particularly in the marshes of San Pablo and Suisun bays.  The southern subspecies, Reithrodontomys raviventris raviventris, are found in the marshes of Corte Madera, Richmond and South San Francisco Bay . Both subspecies have grooved upper front teeth, but, generally, only the southern subspecies have a cinnamon or rufous-colored belly.   The northern subspecies is similar to the western harvest mouse, Reithrodontomys megalotis longicaudus, in that it has: a long bicolored tail, large ears, grooves in the outer surface of its upper incisors and other body parts which are buff or brown.  The backs and ears of the salt marsh harvest mice tend to be darker.  The northern mouse has a combined head and body length of around 3 inches (118-175 mm) and an average weight of less than half an ounce (about 8-12 grams).  The body of an adult mouse is about the size of one’s thumb and it weighs a bit less than a nickel.  Therefore, the salt marsh harvest mice are among the smallest rodents in the U.S. (Sacramento F.W.S., 2004).

 

Figure 2.  Salt Marsh Harvest Mouse ( Northern subspecies) Source: B. "Moose" Peterson/courtesy of C.D.F.G., 2005 (with permission)

  LIFE HISTORY:

    In the wild, on the average, the maximum age for the salt harvest mice, including R. raviventris halicoetes, is approximately twelve months, but most live less than eight months.  Females have a low reproductive potential:  they bear around 4 young per litter, and have only one litter per year.  Reproductive activity, for females, ranges from March to November.  Males are reproductively active from April to September.  Adults comprise the majority of the population.  The mice are density-dependent species: when the populations are too high, breeding is suppressed further into the spring.  If local densities are too high, populations can be reduced to the point of local extinctions.  During summer, when salinities of water and vegetation increase, the mice gain a competitive edge, since they can drink and survive on pure salt water (they can withstand high salinities in food and water intake).  Notably, most of the northern subspecies can survive on sea water, but prefer having fresh water (Suisun Eco Workgroup, 2004).

HABITAT REQUIREMENTS:

    Salt marsh harvest mice are cover-dependent species.  That is, they only live under thick vegetation.  They are dependent on thick cover of native halophytes (plants that thrive in salty environments) of the salt marsh environment, which is typified by salt marsh herbs, grasses and reeds.  Salt marsh harvest mice use pickleweed (Salicornia virginica) as their primary/preferred habitat as long as they have non-submerged, salt-tolerant vegetation for escape during the highest tides (Fisler, 1965). They eat leaves and stems of halophytes.  The mice prefer the deepest (60-75 cm tall), most dense pickleweed, which is intermixed with fat hen (Atriplex patula) and alkali heath (Frankenia grandifolia).  The mice are non-intra-aggressive; therefore, short durations of populations’ densities are sustainable (for the high tide period).  High tide’s refuge is taken in the upper zones of marshes, usually in the stands of fat hen and Australian salt brush (Atriplex semibaccata).  Marshlands with low salinities and sparse pickleweed are not utilized by the mice (this is important, since most dikes marshes exist within the range of northern subspecies, within the Suisun Marsh, where less saline conditions are encouraged to optimize the habitat for waterfowl (Suisun Eco Workgroup, 2004).

The mice’s diet consists of seeds, grasses, forbs and insects.  The northern and southern subspecies have longer intestines than their western counterpart, which is, primarily, a seed-eater.  Salt marsh harvest mice are, generally, nocturnal species, but may be active during the day as well (Daiber, 1982).  They are most active during the moonlit nights.  Salt marsh harvest mice are mobile in diked salt marshes.  The species is able to survive tidal or seasonal flooding due to their swimming, floating and climbing abilities.  The mice are quick re-colonizers of flood-disturbed areas (Pomeroy and Wiegert, 1981).  The mice exhibit long-distance dispersal abilities in young members.  However, no dispersal occurs to bare or human-developed adjacent areas (these areas constitute a dispersal filter for mice).  A narrow buffer zone of vegetation is probably needed for dispersal between adjacent preferable habitats, if they are isolated from each other (U.S.F.W.S., 1984).

SALINITY REQUIREMENTS:

    Salt marsh harvest mice are able to survive, for extensive periods of time, on salinities near that of salt water.  This constitutes a shared trait with desert-adapted animals (however, this is an adaptation based on salt tolerance, rather than on water conservation through the kidneys’ function, as seen in kangaroo rats).  The northern subspecies, R. raviventris halicoetes, is able to survive for more than one year on sea water, with a reduction in weight for the period, since food and water consumption are curtailed while drinking sea water.  The habitat for R. raviventris halicoetes experiences greater fluctuations in water salinities than that of R. raviventris raviventris (the southern subspecies) (Suisun Eco Workgroup, 2004).

STATUS:

    The U.S. Fish and Wildlife Service listed the salt marsh harvest mouse as endangered in 1970 (Suisun Eco Workgroup, 2004). The U.S.F.W.S. is responsible for the management/recovery, listing and law enforcement/protection of the species.  Department of Defense, D.O.D., is responsible for the law enforcement/protection of the species with applicable state and federal laws on public land under their control (U.S.F.W.S. Recovery Plan, 1984).  The State of California listed the mouse as endangered in 1971.  Therefore, the species has the endangered status wherever it is found, including the state of California.  Critical habitat for the species has not been designated.  A recovery plan for the species was prepared in 1984 and is currently under revision (Suisun Eco Workgroup, 2004).  The species is also protected by the Lacey Act, which makes it unlawful to import, export, transport, sell, receive, acquire or purchase any wild animal (alive or dead, including parts, products, eggs or offspring) (U.S.F.W.S., 1984).                 

The Salt marsh harvest mice are endemic to the S.F. Bay  salt marshes.  They are physiologically and behaviorally adapted to the salt marsh environment (Fisler, 1965).  The destruction and modification of the required habitat by human activities is responsible for the reduction of species to the endangered status/level.  Only 30,100 acres, out of 193,800 acres of tidal mars that bordered S.F. Bay in 1850, remain (Dedrick, 1983). This represents an 84% habitat reduction.  Backfilling and vegetation changes of most of remaining tidal marshes have made them unable to support harvest mice (Shellhammer et al., 1982).   Diking the marshes for salt production and landfilling have fragmented the native habitat into isolated pockets.  Eighty percent of the diked marshes exist in the range of the northern subspecies, in the Suisun Bay marshes, where managing waterfowl habitat is the primary goal.  About 30% of historic tidal marshes of the S.F. Bay remain as diked marshes.  Small, isolated populations of northern subspecies exist here (Harvey & Stanley Associates, 1980).  These populations are small and are separated by large areas of inappropriate habitat.  To insure survival of the species, human activity must be limited within the salt marsh habitat (Suisun Eco Workgroup, 2004).

PART II:

DISTRIBUTION OF REITHRODONTOMYS RAVIVENTRIS HALICOETES (Northern Subspecies) & REITHRODONTOMYS RAVIVENTRIS RAVIVENTRIS (Southern Subspecies):

    The San Francisco Bay is the largest estuarine ecosystem in California.  It is an extremely intricate “living” system which supports a very diverse and productive biota.  The Bay ecosystem, however, is being destroyed and a number of taxa that depend on it, including the salt mouse, are in danger of extinction (U.S.F.W.S., 1984).  By 1966, the saltmarsh harvest mouse has only occurred north and south of the San Francisco Bay, C. A., entirely within the narrow belt of wetlands surrounding the Bay.  By 1993, there were less than 2000 individuals.  Both the northern and the southern subspecies are currently listed as endangered within the U. S., as well as within the state of California (U.S.F.W.S., 1984).

The marshes of Delta and the Bay began to be diked off for salt-evaporating ponds as early as 1860.  By 1959, 581 square kilometers of marshlands and tidelands have been diked off or filled.  One of the most severely reduced habitats of the San Francisco Bay is the tidal marsh/salt marsh community (U.S.F.W.S., 1984).  The tidal marshes of today are fragments of the original marshes.  Some are narrow strips along outboard dikes.  Many have been back-filled so that the upland vegetation and most of the high marsh zones have been eliminated.  Only a few deep marshes remain like those on the northeastern shore of the San Pablo Bay (home to the northern subspecies of the mouse), Fagan Marsh or Petaluma Marsh (U.S.F.W.S., 1984).  The input of “freshwater” from some of the surrounding sewage treatment plants have shifted the salt balance, in portions of the Bay, from salt to brackish conditions.  Brackish conditions are of low value to the salt marsh harvest mouse (U.S.F.W.S., 1984).

In addition to tidal marshes, non-tidal (diked) marshes represent a second important wildlife habitat of the Bay.  Considerable difference exists between the diked marshes of the South San Francisco Bay and those of Suisun Bay.  Most of the diked marshes in the South and San Pablo Bays are highly saline and support monotypic stands of pickleweed.  Until recently, most of the diked marshes of Suisun Bay (75%) were managed brackish marshes with high waterfowl value, but with little plant or animal diversity.  Pickleweed was considered a “weed” of the more saline areas and of little value to the waterfowl; hence, waterfowl managers selected against it in favor of alkali bulrush.  This type of single-purpose management adversely affected many non-target species that inhabit the Suisun Marsh, especially salt marsh/tidal marsh dependent species (such as the salt marsh harvest mouse) (U.S.F.W.S., 1984).

Up until the last two hundred years, the salt marsh harvest mice were found in the most of the marshes throughout the S.F. Bay.  The wetlands and marshes of the original Sacramento-San Joaquin Delta were, probably, too water-fresh to support mice and, hence, the Collinsville-Antioch area was, and still is, the eastern limit of their distribution (U.S.F.W.S., 1984).

Distribution and abundance of the salt marsh harvest mice are dependent on the availability of dense pickleweed salt marsh.  Although this species makes some use of grassed and salt-tolerant forbs at the upper margins of salt and brackish marshes, it is closely tied to the cover of dense pickleweed, and it makes little use of pure alkali bulrush or Cordgrass stands (U.S.F.W.S., 1984).  Salt marsh harvest mice are critically dependent on dense cover and their preferred habitat is pickleweed.  Salt marsh harvest mice are seldom found in cordgrass or alkali bulrush.  In marshes, with an upper zone of peripheral halophytes, mice use this vegetation to escape the higher tides, and may even spend a considerable portion of their lives there.  Mice also move into the adjoining grasslands during the highest winter tides.  Throughout much of the range of the salt marsh harvest mouse, however, subsidence and diking have eliminated the important peripheral halophyte zone.  Few harvest mice survive in such marshes, even though other marsh conditions may be optimal, because there is little or no high tide escape.  Studies have shown that the best type of pickleweed association for harvest mice has the following characteristics: 100% cover; a cover depth of 30-50 cm at summer maximum; a high percentage cover of pickleweed, i.e., 60% or more; complexity in the form of fat hen and alkali heath or other halophytes.  The amount of salt grass, brass buttons, alkali bulrush, or other Scirpus or Typha species of plants, however, should be low.  The latter species may be present, but not in large continuous stands, as pure stands of them are avoided by mice.  Salt grass and brass buttons provide very poor habitat for the salt marsh harvest mice; they are low-growing, lack stratification and provide poor cover.  Fat hen provides good cover for mice during the summer, but cannot be used year-round because it is annual (U.S.F.W.S., 1984).

The northern subspecies (Reithrodontomys raviventris halicoetes) of the salt marsh harvest mouse are found in the marshes of San Pablo and Suisun Bays, from San Rafael Bridge to Collinsville, on the north, and from Martinez to Pittsburg, on the south.  Another way to put it is to say that the mouse is found on Marin Peninsula, through Petaluma, Napa and Suisun Bay Marshes, and in the northern Contra Costa County.  An important refuge for the mice is the marsh between Sonoma Creek and Mare Island.  Both the Reithrodontomys raviventris halicoetes and Reithrodontomys raviventris raviventris, the northern and the southern subspecies respectively, are found only in saline emergent wetlands of the San Francisco Bay and its tributaries (U.S.F.W.S., 1984).

The western limit of the northern subspecies is the marshes bordering the mouth of Gallinas Creek on the upper Marin Peninsula.  Narrow strips of marshes extend northward into and along the Petaluma River and connect to the large Petaluma Marsh.  Lower Tubbs Island, further east along the San Pablo Bay,  is being restored to tidal action by the U.S.F.W.S. and will provide a sizable marsh in the future.  Many of the marshes in the Napa Marsh are too narrow and too steep to support salt marsh harvest mice, although mice are present along Napa Slough and Sonoma Creek, on Coon Island, and in the Fagan Marsh.  The marsh along the San Pablo Bay, from Sonoma Creek to Mare Island, is naturally expanding from sediment accretion and is one of the major refugia for the species in the San Pablo Bay.  It is the principal marsh within the San Pablo Bay National Wildlife Refuge (U.S.F.W.S., 1984).

Young members of the northern subspecies have shown an ability to disperse over great distances; however, their dispersal depends on provision of a buffer zone between the salt marsh habitat and the adjoining habitat.  The mice, either northern or southern subspecies, are considered to be keystone species in tidal and brackish salt marsh habitats as the mice populations succeed best in complete, healthy ecosystems and decrease in numbers or are extirpated in human-altered marshes.  The populations are negatively affected by factors such as the elimination of upland marsh habitat areas that provide refugia during high tides (Padgett-Flohr et al., 2003).

The southern subspecies, Reithrodontomys raviventris raviventris, are found in San Mateo, Alameda and Santa Clara Counties (C.D.F.G., 2005).

 

Figure 3. Distribution of Salt Marsh Harvest Mouse (Northern & Southern subspecies)                                                                                                                   Source: C.D.F.G., 2005 (with permission)

Both of the subspecies occur with the closely-related, ubiquitous and abundant western harvest mouse, at upper edges of marshes, and in marginal areas.  Both may be found in pickleweed, though Reithrodontomys raviventris halicoetes and Reithrodontomys raviventris raviventris exclude or replace Reithrodontomys megalotis (the western cousin) in denser stands (U.S.F.W.S., 1984)

The endangered status of Reithrodontomys raviventris halicoetes, largely, results from commercial and residential development around San Francisco Bay, causing loss of pickleweed habitat.  Marsh loss is attributed mainly to filling, diking, subsidence and changes in salinity.  Both the southern and northern subspecies have been especially affected by habitat loss.  Vegetational changes, over the last three decades, especially the increase of bulrush and salt-grass and the decline in pickleweed, are attributed to changes in salinity of the marshes, brought about by increasing volumes of sewage water, as well as by subsidence-related causes (U.S.F.W.S., 1984) Although small marshes, separated by water, may be recolonized, after extinction, by swimming/rafting mice, those separated by open land or dikes have very low immigration.  Consequently, very few areas are likely to be recolonized by harvest mice, once the mice have been extirpated.  In summary, most of the remaining marshes are too small and too widely separated to support viable populations of the salt marsh harvest mouse.  Moreover, backfilling, subsidence and vegetational change continue to reduce the habitat value of the remaining marshes (U.S.F.W.S., 1984).

PART III:

EVOLUTION OF REITHRODONTOMYS RAVIVENTRIS:

   The earliest recorded mammals developed from a particular group of reptiles, the cynodonts, during the Triassic period, approximately 160 million years earlier.  Known as tricodonts, these were small creatures which probably laid eggs but had developed the typical mammalian pattern of dentition (Alderton, 1996).  In the subsequent Jurassic period, two new groups of mammals came into existence.  Firstly, there were the multituberculates, which show remarkable similarities to rodents in the pattern of their dentition.  They had characteristically sharp incisor teeth at the front of their mouth, with a gap behind.  The premolar and molar teeth had broad surfaces with cusps, which were used to crush the vegetation that these animals ate.  Their jaws moved up and down, rather than side to side (Alderton, 1996).  The largest multituberculates were about the size of a contemporary beaver, whereas most remained no bigger than mice.  It appears that this group of early mammals gradually died out, although some survived until the Eocene period, about 55 million years ago (Alderton, 1996).

The ancestors of today’s rodents evolved, about 60 million years ago during the Late Palaeocene epoch.  Their earliest fossilized remains have been unearthed in North America and indicate that the first rodents were probably similar in appearance to today’s squirrels (Alderton, 1996).  Squirrels were the first rodents to evolve and their earliest remains were uncovered in North America (Alderton, 1996).  Members of the ancient family Ischyromyidae were not unlike contemporary squirrels in appearance: they had more mouse-like heads, with the already evolved double incisor teeth in the upper jaw.  During the succeeding Oligocene epoch, rodents moved further south, pushing into South America.  These were members of the caviomorph group, such as Platypittamys (Alderton, 1996). 

 During the Miocene period, temperature across the globe continued to fall and this trend contributed to the decrease in movement of animals through the Bering Land Bridge, which connects present day Alaska with Asia.  The Pliocene epoch lasted for about 7 million years and it was a time of relative stability.  Fossil evidence reveals that many rodents, during this time, fell victim to hunting birds, notably owls, just as they do today (Alderton, 1996).  During the Pliocene, South America again became linked to the northern land mass.  This enabled mice to spread south but very little is known about them from the fossil record (Alderton, 1996).  South America was isolated from North America until about 3-2.5 million years ago when the Panamanian land-bridge emerged.  This resulted in migration of mammals in both directions, especially close to 2.5 million years ago,  comprising the famous Great American Interchange (Vrba, 1990).  The cooling climate, which resulted from the increasing development of mountains around the world, gave way to an ice age during the Pleistocene.  The effects of this were most severe in the northern hemisphere, where much of North America and northern Europe were buried under sheets of ice.  As cold periods and warmer intervals occurred throughout the Pleistocene, so opportunistic groups were able to exploit these climatic shifts.  Rats (Rattus species) developed rapidly, particularly during the latter part of this epoch, while voles also thrived.  Increasing signs of specialism were now seen, with members of the Microtinae, such as lemmings, becoming firmly established in tundra regions (Alderton, 1996).

The generic, and, to a certain extent, the species composition of a present muroid communities were established toward the early-middle Pleistocene (Ortiz et al., 2000).  The salt marsh harvest mice, both Reithrodontomys raviventris halicoetis and Reithrodontomys raviventris raviventris, the northern and southern subspecies, belong to the Myomorpha group.  Myomorphs have successfully colonized almost every type of ecosystem, from the Arctic to temperate woodlands, and deserts to tropical rainforest.  This suborder is split into five families, of which the largest is Muridae.  Members of this group first developed in North America, invading South America when the link between these two continents was forged for the second time about 5 million years ago.  All members of this group are quite small in size.  Reithrodontomys species occupy an evolutionary niche similar to that of the Old World Harvest mouse (Micromys minutus) (Alderton, 1996).

Figure 5. Skull Comparisons: Harvest Mouse vs. House Mouse)
 Source: C.D.F.G., 2005 (with permission)

Salt marsh harvest mice fall under the grouping of Cricetidae (Korth, 1994).  Cricetidae are small rodents.  The dental formula for cricetids is primitive for all.  The enamel surface of incisors was often ornamented by numerous ridges.  The primitive condition appears to have been numerous pinnate (radiating) ridges present on the earliest known cricetid, Pappocricetodon, from China, as well as Oligocene North American species and the genus Paracricetodon, from the Oligocene of Europe.  In late genera, the number of ridges on the incisors ranged from 1 or several to nearly 20, and they were aligned parallel to one another for the length of the incisor (Korth, 1994).  The skeleton of North American cricetids was relatively conservative in adaptations compared to the Old World muroids (Korth, 1994).  The skeleton is basically gracile, with slender limbs.  The hindlimb is longer than the forelimb and the fibula is fused to the tibia.  Within the Cricetidae, there were a number of advancements in the skull and dentition that appeared, in some cases, to have been attained independently in different subfamilies.  In North America, the major cusps on the cheek teeth showed the beginnings of alternation, the lingual cusps being anterior to the buccal cusps, on lower molars, and the buccal cusps, anterior to the lingual cusps, on upper molars (Korth, 1994).  The fossil record of cricetids, in North America , involves a number of immigration events.  The first was from Asia, in the late Eocene, which accounted for the first occurrence of cricetids in North America.  The second appears to have been in the Miocene, with the first occurrence of peromsycines.  After this, the number of events is dependent on the interpretation of the origin of different cricetid groups.

Based on a study of patterns of karyotypic megaevolution ((karyotypic megaevolution is described as a radical reorganization of the karyotype in which normally stable G-band patterns are disrupted or rearranged to a point that it makes it difficult to observe normal patterns typically shared among closely related species (Baker & Bickham, 1980)) in Reithrodontomys, it was determined that:

Analysis of restriction enzyme digestion with EcoRI revealed that the 350 base pair monomer repeat in R. montanus and R. megalotis also was in R. zacatecae and R. raviventris (Bell et al., 2001)

 

fulvescens

humulus

raviventris

montanus

sumichrasti

zacatecae

megalotis

 
Figure 6. Diagrammatical interpretation of times-since-divergence values (approximately 1 million years) relative to patterns of chromosomal evolution for 7 species of Reithrodontomys.  The tree branches read, from left to right: fulvescens, humulis, raviventris, montanus, sumichrasti, zacatecae and megalotis. Source: Bell et al., 2001 
bullet

The complete cytochrome b-gene (1,143 base pairs) was sequenced for seven species of Reithrodontomys. In all analyses, the two individuals representing R. megalotis, R. zacatecae, and R. raviventris formed sister taxa (Bell et al., 2001).

The analysis in which transitions & transversions were equally weighted resulted in a single most parsimonious tree.  The topology of that tree depicted two clades.  In the 1st clade, R. megalotis and R. zacatecae formed a sister taxon relationship and were then joined to R. sumichrasti.  R. montanus and R. raviventris formed a 2nd clade.  Those two clades were joined together, and R. humulis and R. fulvescens were added in a stepwise manner (Bell et al., 2001).

All analyses depicted R. raviventris and R. montanus as sister taxa (Bell et al., 2001).

R. megalotis, R. montanus, R. raviventris, R. humulis, R. sumichrasti and R. zacatecae have experienced substantial chromosomal evolution involving different chromosomal rearrangements (Bell et al., 2001).

R. raviventris and R. montanus have undergone karyotypic megaevolution and have returned to a mode of chromosomal stasis (Bell et al., 2001).

In general, studies done by Baker and Bickham (1980) led to the recognition of a high diploid, mostly arcocentric group (R. creeper, R. fulvescens, R. mexicanus, R. tenuirostris, and R. humulis) and a low diploid, entirely biarmed group (R. montanus, R. raviventris, R. megalotis, R. sumichrasti, and R. zacatecae).  These studies have documented extensive chromosomal evolution within these five species and provided evidence that radical eurochromatic rearrangements have occurred (Baker and Bickham, 1980).

Despite several attempts to determined systematic relationships among species of Reithrodontomys complete congruency among data sets has been rare (Arnold et al., 1983).

 

R. raviventris has a highly restricted geographic range and perhaps originated as a result of geographic isolation due to the formation of salt marshes in the San Francisco Bay region (Fisler, 1965). This information could be used to support the statement by the U.S.F.W.S. that the salt marsh harvest mice evolved with the creation of the S.F. Bay some 8,000-25,000 years ago (U.S.F.W.S., 2005). Before 1984, the specific status of R. raviventris was based on the assumption that its closest living relative was R. megalotis and that it was sympatric with R. megalotis (Fisler, 1965).  With the development of the hypothesis that R. raviventris was sister to R. montanus, the question became whether R. raviventris was an isolated subspecies of R. montanus or was it specifically distinct from R. montanus. Analyses of karyotypic data suggested that R. raviventris was a species distinct from R. montanus and was related more closely to this taxon than to R. megalotis (Bell et al., 2001).

Given the restricted distribution of R. raviventris to the highly populated San Francisco Bay region and the ever-increasing threat of loss of habitat, Bell et al. data indicates that this taxon is unique and contributes to the biodiversity of  the genus.  Conservation of this taxon is paramount (Bell et al., 2001).

 

For more information, visit:

California Department of Fish and Game   www.dfg.ca.gov/

California Department of Pesticide Regulation   www.cdpr.ca.gov/

Goals Project 1999    http://www.sfei.org/sfbaygoals/docs/goals1999/final031799/pdf

Sacramento Fish and Wildlife Office    http://sacramento.fws.gov/

U.S. Fish and Wildlife Service    http://www.fws.gov/

BIBLIOGRAPHY:

Alderton, David. 1996. Rodents of the World.  New York, N.Y.: Facts on File, Inc.

Arnold, L. J., Robbins, W., Chesser R. K., and Patton J.C. 1983. Phylogenetic relationships among six species of Reithrodontomys.  Journal of Mammalogy 64:128-132

Baker R. J., and Bickham, J. W. 1980.  Karyotypic evolution in bats: evidence of extensive and conservative chromosomal evolution in closely related taxa.  Systematic Zoology 33:339-341

Bell, D. M., Hamilton M. J., Edwards C. W., Wiggins L. E., Martinez R. M., Strauss R. E., Bradley R. D., and Baker R. J. 2001.  Patterns Of Karyotypic Megaevolution in Reithrodontomys: Evidence From A Cytochrome-b Phylogenetic Hypothesis.  Journal of Mammalogy 82(1):81-91

California Department of Fish and Game, California Department of Pesticide Regulation, Endangered Species Project. 2005.  Salt Marsh Harvest Mouse Field I.D. Card.   California Department of Fish and Game, California Department of Pesticide Regulation, Endangered Species Project. 2005. Salt Marsh Harvest Mouse Field I.D. Card. [Online] Available: http://www.cdpr.ca.gov/docs/es/espdfs/smhm1.pdf ( Accessed April, 2005)

California E.P.A., Department of Pesticide Regulation. 2005. Salt Marsh Harvest Mouse (Reithrodontomys raviventris). [Online] Available: http://www.cdpr.ca.gov/docs/es/espdfs/smhmall.pdf ( Accessed April, 2005)

Daiber, F. C. 1982. Animals of the Tidal Marsh.  New York, N.Y: Van Nostrand Reinhold Company

Dedrick, K. 1993. San Francisco Bay tidal marshland acreages: recent and historic values. In: O.T. Magoon, ed. Proceedings of the Sixth Symposium on Coastal and Ocean Management (Coastal Zone '89). Charleston, South Carolina, July 11-14, 1989. Published by the American Society of Civil Engineers.

Fisler, G.F. 1965. Adaptations and speciation in harvest mice of the marshes of San Francisco Bay. University of California Publications in Zoology, Volume 77. Berkeley, C.A.: University of California Press

Harvey and Stanley Associates, Inc., 1980. Status of the salt marsh harvest mouse (Reithrodontomys raviventris) in the Suisun Marsh. Report to Water and Power Resources Service. Sacramento, California

Korth, William W. 1994. The Tertiary Record of Rodents in North America : New York , N.Y: Plenum Press

Ortiz, P. E., Pardinas U. F. J., and Steppan S. J. 2000. A New Fossil Phyllotine (Rodentia: Muridae) From Northwestern Argentina And Relationships Of The Reithrodon Group. Journal of Mammalogy 81(1):37-51

Padgett-Flohr, G. E., and Isakson, L. 2003.  A Random Sampling of Salt Marsh Harvest Mice in a Muted Tidal Marsh. Journal of Wildlife Management 67(3):646-653

Pomeroy L. R., and Wiegert R. G. 1981.  The Ecology of the Salt Marsh. New York, N.Y.: Springer-Verlag

Sacramento Fish and Wildlife Service Office. 2004.  Salt Marsh Harvest Mouse (Reithrodontomys raviventris). Species Account. [Online] Available: http://sacramento.fws.gov/es/animal_spp_acct/salt_marsh_harvest_mouse.htm ( Accessed April, 2005)

Shellhammer, H. S., Jackson R., Davilla W., Gilroy A. M., Harvey H. T., and Simons L. 1982. Habitat preferences of salt marsh harvest mice (Reithrodontomys raviventris). The Wasmann Journal of Biology 40(1-2):102-114

Suisun Eco Workgroup. 2004.  Wildlife of the Suisun Marsh, Salt Marsh Harvest Mouse. [Online] Available:  http://www.iep.water.ca.gov/suisun_eco_workgroup/workplan/report/wildlife/shmouse.html (Accessed April, 2005)

Thelander, C. ed. 1994. Life on the edge: a guide to California's endangered natural resources. Santa Cruz, C.A.: Biosystems Books. p 80-81

U.S. Fish and Wildlife Service. 1984. Salt Marsh Harvest Mouse and California Clapper Rail Recovery Plan. Portland, Oregon. [Online] Available: http://ecos.fws.gov/docs/recovery_plans/1984/841116.pdf ( Accessed April, 2005)

Vrba, E. S. 1992.  Mammals As A Key To Evolutionary Theory. Keynote Address, Presented at the 70th Annual Meeting of The American Society of Mammalogists, Frostburg, MD, June 1990. Journal of Mammalogy 73(1):1-28

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