Because the wetlands of California's Central Valley probably are noted more fortheir wintering waterfowl, mammals typically have been overlooked as an elementof these wetland ecosystems. However, habitat use and diet of many"terrestrial" mammals suggest that wetlands play a vital role for severalspecies.

Mammals are an easily recognizable and definable group of animals. Theypossess hair and, along with birds, are homeothermic, that is, having aconstant body temperature. They possess a 4-chambered heart and non-nucleatedred blood cells which allows the cell to carry more oxygen. Mammals possess avaried and complex dentition allowing them to exploit a wide array of foodresources. Being the only animals that produce milk for their young,reproduction of mammals is unique. The brain and sensory structures generallyare well developed, especially the sense of smell and hearing in some mammalgroups.

Mammals are considered a very successful group of animals occurring on everycontinent and every ocean. However, the number of mammal species (4,000) iscomparatively less than birds (8,500), fish (11,500), and insects (over750,000). There are about 190 species of mammals which occur in California.

This bulletin discusses the more important mammal species related to themanagement of wetlands in the Central Valley of California. Somewetland-associated species are immediately obvious, such as the muskrat andbeaver. However, others, such as kangaroo rats, bats, and some threatened andendangered mammals, also play a role in the management of wetlands.


Historically, the vast wetlands, grasslands, and riparian woodlands of theCentral Valley were occupied by many large mammalian species, including muledeer, pronghorn, tule elk, grizzly bear, and gray wolf. The historic CentralValley mammalian assemblage also included large numbers of rabbits, hares,ground squirrels, mice, and kangaroo rats. However, due to vast changes in thelandscape, some mammals, such as kangaroo rats and kit foxes, have becomeendangered. Others, such as the California ground squirrel, probably haveincreased because of their ability to adapt to new conditions. Further,introduced species, such as house mice, Norway and black rats, pigs, and goats,can often outcompete or adversely effect some native mammalian species.

Early accounts of high numbers of large herbivores in the Valley are ofinterest. Apparently, the numbers of deer have diminished greatly. Pronghornsonce occurred throughout the Central Valley, but were rare in the Valley by1875 and have since been extirpated. Efforts now are underway to reintroducethe pronghorn to different parts of their former range.

Similar to pronghorn, the population of tule elk also was much reduced by 1875.Although it's life history is similar to that of the Roosevelt elk of the NorthCoast, tule elk are much smaller and endemic to California. By 1904, the lastremaining population of 145 animals occurred near former Lake Buena Vista inthe southern San Joaquin Valley. A number of transfer attempts were tried, butthe only successful one was in Yosemite National Park. In 1933, the remainingYosemite population of 26 animals was relocated to the Owens Valley. By 1960,the number of tule elk in the Owens Valley reached 300. To alleviate habitatstress from too many animals, the State started relocating animals to otherareas. By 1985 there were about 1,800 tule elk in wildlife areas and refugesin the state.

Besides large grazing mammals, there were populations of large carnivores whichoccurred in the Central Valley prior to the 1850s. These predators includedmountain lions, grizzly bears, and gray wolves. Today, the largest predatorsin the Valley are coyotes, foxes, and occasionally mountain lions.


Four semi-aquatic mammalian species occur in the Central Valley: muskrat,beaver, river otter, and mink (Appendix). Routinely, beaver and muskrat areconsidered in wetland management practices in the Valley. However, practicesconcerning river otters and mink probably are limited to predator management(see CARNIVORES, below).

Beavers occur throughout continental North America from coast to coastincluding the Alaskan peninsula. In California, they are found primarily inthe northern two-thirds of the state. Optimum habitat includes riparian,riverine, lacustrine, and emergent wetlands. In spring and summer, beavers eatgrasses, leaves, and aquatic vegetation, such as tules and cattails. Inwinter, they eat the bark and cambium of trees, primarily willow, aspen, alder,and cottonwood.

Muskrats are common and widely distributed throughout the U.S. and Canada.Muskrats occur sporadically in California, but are continuously distributedthroughout the Central Valley. Muskrats are mainly herbivorous, eating aquaticplants, especially roots and basal stems of cattails and tule bulrush.

Because beavers and muskrats will den in the banks of rivers and lakes, andbecause of the beaver's dam building activity, wetland management for beaversand muskrats is primarily concerned with damage to, and alteration of, leveesand water control structures. To a lesser extent, management concerns alsoinclude vegetation changes from herbivory. For example, beavers can easilyruin a newly restored riparian area.

Beaver damage varies by type, magnitude, and region. In the West, beaverdamage mostly effects gravity flow water delivery, drain systems, andstructures, such as chewing through stoplogs or plugging culverts. Pasturesand agricultural fields that are adjacent to riparian areas which containwillows or other preferred foods can be flooded from beaver dams.

In areas where beavers are causing damage, a number of beaver control measurescan be employed. Poisons have been shown to be unproven on wild beaverpopulations. Trapping or shooting have both been shown to be highly effectivein controlling nuisance beavers. Because beaver are a furbearing animal,finding a local trapper may be beneficial to both the trapper and land managerfor ridding nuisance beavers. Relocating nuisance beavers through livetrappinghas been conducted successfully. Local wildlife officials, such as gamewardens or biologists, should be contacted before any beaver control techniquesare implemented.

Muskrat damage arises most commonly from their burrowing into levees or aroundwater control structures. Muskrats, also fur-bearers, can be effectivelymanaged through trapping and shooting. Other management includes muskratdeterrence, such as: levees constructed of well-packed clay soils, leveeshaving shallow side slopes of 4:1 or 5:1 (steeper slopes encourage muskratburrowing), wide (12-foot) levee crowns, or even 3-foot levee freeboard toallow for denning and burrowing.

Although beavers and muskrats are probably known more for the damage theyinflict on water control structures or the nuisance they cause to humans, theyare an integral component of many wetland ecosystems. Managed populations ofbeaver and muskrat can actually improve marsh conditions. For example, beaverponds can increase plant diversity thereby increasing the diversity of habitatsavailable to various wildlife species. Muskrats can be beneficial in marshmanagement by opening up dense tule stands. These vegetation "eat-outs" mayopen up an otherwise choked-out marsh, benefitting shorebirds, wading birds,and certain waterfowl. Beaver and muskrat activity also may disrupt climaxstages of marsh vegetation, setting successional stages back to more open(early) stages (see Valley Habitats 2 & 7).


Small mammals, although not generally thought of as importantwetland-associated species, fill several important niches in the wetlandecosystem. For the purpose of this paper, small mammals are thosesquirrel-size or smaller. This section discusses some small mammal speciesimportant to wetland management, such as California ground squirrels, bats,mice and voles, and threatened or endangered kangaroo rats.

Like the muskrat, the California ground squirrel and other ground squirrelswill burrow into dikes and levees of water control systems. Because of theseburrowing activities, necessary deterrence should be taken to avoid groundsquirrel damage to levees. As with muskrats, levees should have shallow (4:1or 5:1) side slopes and wide (12-foot) levee crowns. Ground squirrels also areknown to predate waterbird nests. In addition, because ground squirrels preferopen areas, well-vegetated levees and dikes will greatly deter their presence.

Twenty-three bat species are found in California, of these, 9 occur throughoutthe Valley (Appendix). All bats that occur in the Valley are insectivorous andrequire some form of water which they use as foraging sites and sources ofdrinking water. Bats occur among a wide variety of wetland habitat typesincluding riparian, permanent and semi-permanent marsh, and seasonal wetlands.In autumn, many bats migrate to warmer climates of favorable areas tohibernate. Bats require two different roost types, day and night. Day roostsallow for resting or nursing of young; whereas, night roosts provide for restbetween feeding bouts. Because roosts are required features of bat habitat,they are often a limiting factor of bat distribution. Bats will roost incaves, trees, under tree bark, and in buildings. Because of their over-waterforaging, role as insect predators, and importance of adequate roost sites,wetland management can play an important role in the ecology of many batspecies.

The most numerous small mammal species which use Central Valley wetlandsprobably would be the California vole, deer mouse, and western harvest mouse,with shrews occurring in lesser numbers. The introduced house mouse also occursregularly throughout the Central Valley's wetlands. Of these, all are rodentsexcept for shrews. Rodents are primarily herbivores; whereas, shrews areprimarily insectivorous. Vole, deer mice, and harvest mice primarily occur ingrasslands, but occur in seasonal marshes when not flooded.

The role of small mammals in the energy and nutrient dynamics of wetlands ispoorly known. However, the production and abundance of small rodents are ofgreat importance to secondary and tertiary consumers, that is predators. Volescan make up a great portion of the diet of red foxes, coyotes, mink, hawks,owls, and other predators. With small mammals composing such a portion ofpredator diets, large populations of small mammals may buffer ground nestingbirds from nest and brood predation.

The threatened or endangered small mammals of the San Joaquin Valley can be ofgreat importance to wetland management of the area. These include: the SanJoaquin antelope squirrel (state threatened), giant kangaroo rat (state andfederal endangered), and the Fresno and Tipton kangaroo rats (state and federalendangered) which are subspecies of the San Joaquin kangaroo rat. Thesespecies once ranged over vast areas of the San Joaquin Valley, and now occurprimarily in saltbush scrub, alkali sinks, arid grassland, and other lowrainfall areas. These species all burrow and are herbivores.

Because so little of their native habitat remains, any wetland restoration orenhancement, including the conversion of alkali flats to wetlands, can haveadverse impacts. Extreme caution must be exercised in areas where these smallmammals may occur and a wetland restoration is planned. With the great loss ofwetlands in the San Joaquin Valley and the need for wetland restoration, thepossibility exists that these small mammals can be displaced if wetlands arerestored. Therefore, any alteration of wetland management or plannedrestoration should include extensive communication and consultation with theU.S. Fish and Wildlife Service and the California Department of Fish andGame.


Carnivores are primarily meat eaters, although many will eat berries, nuts,fruits, and grasses. Although many carnivores are not consideredwetland-associated species, they are important to consider in wetlandmanagement because of their predation on waterbirds. Carnivores known topredate on waterbirds or waterbird nests include: coyote, red fox, raccoon,skunks, mink, and badger. Although different mammalian carnivores have varyinglife histories and ecologies, they are all terrestrial predators. Therefore,wetland management strategies for these carnivores are similar.

Because high predation rates are generally a symptom of an imbalance in theecosystem, the solution to predator problems often can be alleviated bymodifying land use practices. However, private land areas are seldom largeenough to overcome imbalances in the ecosystem. Therefore, predator managementoften needs to be conducted on relatively small "islands" of suitable waterbirdhabitat.

Because adult waterbirds often can escape predators, the primary managementconcern is predation of waterbird nests, nesting hens, and broods. Twomanagement strategies can be used to reduce predation: alter the behavior ordensity of predators, or reduce the vulnerability of nests and broods topredators.

Although locally effective, management techniques used to reduce predatordensities, such as shooting and trapping, are controversial (e.g., destroyingfox or coyote dens) and may require special permits (e.g., poisoning).Trapping furbearers can be effective, but is legal only during fall and winterand nest predation occurs in the spring. Further, these techniques aretypically short term, and predators from surrounding areas will fill the voidleft by the killed predators.

The ideal situation is to reduce the vulnerability of nests and hens topredation. This can be accomplished in a number of ways, such as increasingthe area of suitable nesting habitat, altering the shape of suitable nestinghabitat, or increasing the vegetation density of nesting habitat. Creatingmore suitable nesting area and changing the shape of suitable nesting areasreduce the probability that predators can find nests. For example, largeblocks of dense nesting habitat offer better predator protection than narrow,linear areas of nesting habitat. Increasing vegetation density makes predatortravel more difficult. Restoring native vegetation (creating ecosystembalance) can stabilize or increase the abundance of the small mammals, therebyproviding an alternate prey source. Providing suitable nesting islands oradequate nest structures also are effective means to separating waterbirds frompredators.

Another method to separate nesting waterbirds from predators is to enclosesuitable nesting areas with fence. Contemporary designs use a 7-strandelectrified fence with poultry netting at the base. These designs effectivelyreduces predation, but require regular maintenance. However, recent findingssuggest that some hens will not bring their broods through the fence.Therefore, land managers should consult recent literature or wildlife agencypersonnel for fence designs and alternative methods of reducing waterbirdpredation.


Common Mammal Species of California's Central Valley:

Virginia Opossum - Didelphis virginiana (exotic)

Ornate Shrew - Sorex ornatus

Broad-footed Mole - Scapanus latimanus

Yuma Myotis - Myotis yumanensis

California Myotis - Myotis californicus

Western Pipistrelle - Pipistrellus hesperus

Big Brown Bat - Eptesicus fuscus

Red Bat - Lasiurus borealis

Hoary Bat - Lasiurus cinereus

Townsend's Big-eared Bat - Plecotus townsendii

Pallid Bat - Antrozous pallidus

Brazilian Free-tailed Bat - Tadarida brasilliensis

Brush Rabbit - Sylvilagus bachmani

Desert Cottontail - Sylvilagus audubonii

Black-tailed Hare - Lepus californicus

San Joaquin Antelope Ground Squirrel - Ammospermophilus nelsoni

California Ground Squirrel - Spermophilus beecheyi

Eastern Gray Squirrel - Sciurus carolinensis (exotic)

Western Gray Squirrel - Sciurus griseus

Fox Squirrel - Sciurus niger (exotic)

Botta's Pocket Gopher - Thomomys bottae

California Pocket Mouse - Chaetodipus californicus

San Joaquin Pocket Mouse - Perognathus inornatus

Heermann's Kangaroo Rat - Dipodomys heermanni

California Kangaroo Rat - Dipodomys californicus

Giant Kangaroo Rat - Dipodomys ingens

San Joaquin Kangaroo Rat - Dipodomys nitratoides

Beaver - Castor canadensis

Western Harvest Mouse - Reithrodontomys megalotis

Salt Marsh Harvest Mouse - Reithrodontomys raviventris

California Mouse - Peromyscus californicus

Deer Mouse - Peromyscus maniculatus

Brush Mouse - Peromyscus boylii

Pinion Mouse - Peromyscus truei

Southern Grasshopper Mouse - Orychomys torridus

Desert Woodrat - Neotoma lepida

Dusky-footed Woodrat - Neotoma fuscipes

California Vole - Microtus californicus

Muskrat - Ondatra zibethicus (exotic)

Black Rat - Rattus rattus (exotic)

Norway Rat - Rattus norvegicus (exotic)

House Mouse - Mus musculus (exotic)

Porcupine - Erethizon dorsatum

Coyote - Canis latrans

Kit Fox - Vulpes macrotis

Gray Fox - Urocyon cinereoargenteus

Red Fox - Vulpes vulpes (exotic)

Ringtail - Bassariscus astutus

Raccoon - Procyon lotor

Long-tailed Weasel - Mustela frenata

Mink - Mustela vison

Badger - Taxidea taxus

Western Spotted Skunk - Spilogale gracilis

Striped Skunk - Mephitis mephitis

River Otter - Lutra canadensis

Mountain Lion - Felis concolor

Bobcat - Felis rufus

Wild Pig - Sus scrofa

Elk - Cervus elaphus

Mule Deer - Odocoileus hemionus


Burt, W. H. and R.P Grossenheider. 1976. A Field Guide to the Mammals. Thirded. Houghton Mifflin Co. Boston, Mass. 289pp.

Chapman, J.A. and G.A. Feldhamer, eds. 1983. Wild Mammals of North America.Johns Hopkins Univ. Press. Baltimore, Maryland. 1147pp.

Delany, M.J. 1982. Mammal Ecology. Chapman and Hall, New York, NY. 162pp.

Fritzell, E.K. Mammals in prairie wetlands. 268-301. in: A.G. van der Valk,ed. Northern Prairie Wetlands. Iowa State Univ. Press. Ames, Iowa.

Jameson, E.W., Jr. and H.J. Peeters. 1988. California Mammals. Univ. ofCalif. Press. Berkeley, Calif. 403pp.

Ringelman, J.K. 1991. Evaluating and Managing Waterfowl Habitat. Div. Rpt.16. Colorado Div. Wildl. Fort Collins, Co. 46pp.

Schoenherr, A.A. 1992. A Natural History of California. Univ. of Calif.Press. Berkeley, Calif. 552pp.

Steinhart, P. 1990. California's Wild Heritage: Threatened and EndangeredAnimals in the Golden State. Calif. Dept. Fish and Game. Sacramento, Calif.108pp.

Zeiner, D.C., W.F. Laudenslayer, Jr., K.E. Mayer, and M. White. 1990.California's Wildlife: Mammals, Volume III. Calif. Dept. Fish and Game.Sacramento, Calif. 407pp.

Prepared by: Michael A. Bias, Ducks Unlimited, Inc.

Valley Habitats is published as part of Ducks Unlimited's VALLEY CARE Programto provide information to private land managers who wish to integrate wildlifemanagement into their existing operations.

For more information regarding conservation related land management practicescontact: Ducks Unlimited, Western Regional Office, 9823 Old Winery Place,#16, Sacramento, CA 95827

TELEPHONE: (916) 363-8257 FAX: (916) 363-9849

The National Fish and Wildlife Foundation, Hofmann Foundation, WildlifeConservation Board, and David and Lucile Packard Foundation provided thegenerous funding for this issue of Valley Habitats.

Valley Habitats is produced by Ducks Unlimited's Western Regional Office.Items contained herein may be reproduced with permission. Copyright, DucksUnlimited, Inc., 1995.