Salt Marsh Harvest Mouse

Wed, Mar 9th, 2016

The salt marshes, both tidal and diked, which encompass the San Francisco
Bay and neighboring waterways are home to a small mouse species named
salt marsh harvest mouse (Reithrodontomys ravinventris). The scientific
name identifies this mouse as a groove toothed mouse with a red belly. There
are two known subspecies of this mouse. A northern species (R. r.halicoetes)
which lives in the marshes of Suisun Bay and San Pablo Bay and a southern
species (R. r. ravinventris) which reside in the marshes around the San
Francisco Bay (Shellhammer, 1982, 1989). These mice are closely related
to the western harvest mouse (R. megalotis). It is difficult to identify the salt
marsh harvest mouse from the western harvest mouse in the field. Salt marsh
harvest mice are relatively placid creatures when compared to the more
active nature of the western harvest mouse. This behaviour difference
may aid field identifcation. Reithrodontomys ravinventris is approximately
2.75 - 3.0 inches (69-75 mm) in length and weigh 0.3 - 0.5 ounces (9 - 14 g).
Their body color is dark brown on top with the underside a lighter shade
of brown. This mouse is mostly a nocturnal animal and noted to be a
good swimmer. Breeding season is from spring to fall and litters contain 3 - 4
offspring. The salt marsh harvest mouse is listed as an endangered species
at both state and federal levels. Habitat changes from diking, flooding, and
filling of the marsh land around San Francisco Bay and connected waterways
for urban development are the primary reason numbers of this mouse species
has declined (Sustaita et al., 2011). Since the salt marsh harvest mouse is so
closely related to the salt marsh in which it lives, preservation and restora-
tion of salt marsh regions will help ensure their existence as a species.

The preferred habitat of the salt marsh harvest mouse is no longer as vast
as it once was. Major changes with the San Francisco Bay wetlands started
with the onset of the California gold rush. From 1850 to present day, the
shores of the bay have been developed to suit human needs such as for hous-
ing and manufacturing. Large areas of north bay tidal marshes were diked
and drained to support agriculture. The salt marshes around the eastern
south bay were transformed to support salt production. Of the 190,000 acres
of original tidal marsh only 40,000 acres remain (Goals Report, 1999). There
are no longer large contiguous regions of salt marsh along the shores of San
Francisco Bay and San Pablo Bay, but instead small scattered areas remain
many in proximity to urban development. The north shore of San Pablo Bay
has one of the largest remaining sections of salt marsh (Thorne el al., 2012).

The San Francisco Bay wetlands are home to a vast number of plants and
animals. The salt marsh harvest mouse is not the only endangered species
in and around the bay. Today there are 51 species of plants and animals
around the bay wetlands that are listed as threatened or endangered at a
state or federal level (Goals Report, 1999). Salt marsh, when compared to
other ecosystems is a relatively harsh environment for such reasons as threat
of flooding and the high salinity level of the water, plants, and soil. The ani-
mals that do live in salt marshes have adapted over time to the conditions this
type of environment o ers. Fewer animal species live in salt marshes when
compared to other ecosystems because of the relatively harsh conditions.
However, for those animals that do live in the salt marshes, a greater num-
ber of them are endemic to this type of ecosystem. The salt marsh harvest
mouse was identified as endemic to the tidal salt marshes of San Francisco
in 1908 (Greenberg el al., 2006). Other animals that are endemic to the salt
marsh include, the San Pablo song sparrow (Melodia melodia samuelis), salt
marsh common yellowthroat (Geothlypis trichas sinuosa), and the California
black rail (Laterallus jamaicensis corturniulus) which is listed as a threat-
ened species by the state of California.

The Salt marsh harvest mouse is closely tied to the plant pickleweed (Salicor-
nia spp) also called glasswort. This mouse species prefer a salt marsh with
an abundant, 100% cover of pickleweed (Shellhammer, 1982, 1989). The
mice rely on the pickleweed cover to avoid predators and rarely stray from
its shelter. Salt marsh harvest mice have been found living in areas of less
than 100% pickleweed cover when meadow mice (Microtus californicus) are
occupying their preferred habitat. One reason for this change is likely due to
the fact that meadow mice have been known to kill nestlings of other mice
(Geissel et al., 1988). Other types of micro habitats around the bay are suit-
able for salt marsh harvest mice. Areas which include a mix of pickleweed
and other types of vegetation such as fat hen and alkali heath (Frankenia
salina) are also home to R.ravinventris. Salt marsh harvest mice have also
been spotted living in diked salt marshes that have little or no pickleweed but
have annual grasses instead (Sustaita et al., 2011). Scientists have noted that
pickleweed height and not only density are factors in habitat selection for the
salt marsh harvest mouse. They have been noted to prefer taller pickleweed.
This may be due to the level of salinity in the taller plants (Padgett-Flohr
and Isakson, 2003). While the micro-habitat of the salt marsh harvest mouse
may vary slightly, the one constant is a thick plant cover (Shellhammer,
1982, 1989). Below the pickleweed cover, neither mouse subspecies burrow.
Instead the northern subspecies builds ball-like nests from dry grass directly
on the ground. The southern subspecies nests are less organized and crudely
built (Shellhammer, 1982). These mice have also been noted nesting in aban-
doned birds nests. It is interesting to note that pickleweed cover is also the
prefered habitat of the California black rail.

Not only does pickleweed serve as shelter for the salt marsh harvest mouse,
but it is also a food source for them most of the year. In winter it prefers
to eat green grasses. Seeds, salt grass (Distichilis spicata) and forbs are also
part of their diet. Salt marsh harvest mice are able to drink saline water.
In fact, the northern subspecies prefers saline water even when fresh water is
available (Padgett-Flohr and Isakson, 2003).

The rich diversity of life in and around the San Francisco bay area is
due to habitat that is conducive to a complex food web. A change in the
numbers of one species directly a ects the numbers of another. Salt marsh
harvest mice are active all year (Bias and Morrison, 2006) and therefore pro-
vide a food source for predators throughout the year. Habitat protection
for these mice and other animal species around the bay is vital in keeping a
balanced food web. Some of the predators that feed on the bay area wetland
mice including the salt marsh harvest mouse are striped skunks (Ackerman,
2002), feral cats (Felis silvestris), non-native red foxes (Vulpes vulpes regalis;
Meckstroth et al., 2007 ), and predatory birds such as hawks and owls. Non-
native predators pose a threat on native wildlife. In recent years, programs
have been implemented to remove non-native predators, such as the red fox
and feral cats (Meckstroth et al., 2007 ). A population of rodents such as
mice play an important role in ensuring the success of other animals in the
bay wetlands. Waterfowl nests are more likely to produce successful o spring
if there is a population of rodents to buffer them from predators (Ackerman,
2002).

Key to the survival of the salt marsh harvest mouse is protection and
restoration of its salt marsh habitat. If the remaining salt marshes around
the San Francisco Bay, San Pablo Bay and Suisun Bay ceased to exist due to
urbanization for instance, then so would the salt marsh harvest mouse and
other animals that rely on the salt marsh habitat. Furthermore, the aesthetic
qualities of the bay area would su er which is likely to have a negative impact
on the local economy since tourism is big business for the San Francisco Bay
area (Goals Report, 1999). Protection and restoration of the salt marshes
has already begun. One of the largest restoration projects is the South Bay
Salt Pond Restoration Project. This project plans to restore up to 54 square
kilometers of diked salt ponds back to tidal habitats (Brew and Williams,
2010). Over time these areas will ll with sediment allowing vegetation to
grow and o er suitable habitat for creatures such as the salt marsh harvest
mouse. These changes are not necessarily slow. At Pond B-1 in the south bay,
pickleweed and cordgrass are establishing themselves quicker than expected
since sedimentation rates are occurring faster than rates required by U.S.
Army Corps of Engineers (Goals Report, 1999). Other habitat conservation
projects have also been implemented. 30 miles east of San Francisco is Suisun
Marsh, which consists of wetlands which have been set aside for salt marsh
harvest mouse conservation (Susaita el al., 2011). Increasing the amount
of suitable habitat for the salt marsh harvest mouse does not guarantee its
survival as a species, but is a logical rst step in an attempt to do so.

Literature Cited

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