Endangered Species Recovery Program
U.S.A. Endangered and California Species of Concern
Although some species have more specialized diets than others (e.g., Stephen's woodrat, N. stephensi feeds almost exclusively on juniper), woodrats are, for the most part, generalist herbivores. They consume a wide variety of nuts and fruits, fungi, foliage and some forbs (Linsdale and Tevis 1951).
Dusky-footed woodrats live in loosely-cooperative societies and have a matrilineal (mother-offspring associations; through the maternal line) social structure (Kelly 1990). Unlike males, adjacent females are usually closely related and, unlike females, males disperse away from their birth den and are highly territorial and aggressive, especially during the breeding season. Consequently, populations are typically female-biased and, because of pronounced polygyny (mating pattern in which a male mates with more than one female in a single breeding season), the effective population size (i.e., successful breeders) is generally much smaller than the actual population size. This breeding system in combination with the small size of the only known extant population means that the riparian woodrat is at an increased risk of extinction because of inbreeding depression. Small, isolated populations are more susceptible to genetic, demographic, and environmental stochasticity than large widely distributed populations..
Dusky-footed woodrats are highly arboreal (Kelly 1990). Evergreen or live oaks and other thick-leaved trees and shrubs are important habitat components for this species (Kelly 1990, Williams et al. 1992). Riparian woodrats are common, however, where there are deciduous valley oaks, but few live oaks. They are most numerous where shrub cover is dense and least abundant in open areas. In riparian areas, highest densities of woodrats and their houses are often encountered in willow thickets with an oak overstory.
Dusky-footed woodrats are well known for their large terrestrial stick houses, some of which can last for 20 or more years after being abandoned (Linsdale and Tevis 1951). At Caswell Memorial State Park, riparian woodrats also make houses of sticks and other litter (Williams in litt. 1993). At the Hastings Reserve, Monterey County, dusky-footed woodrat houses range from 60 cm (2 ft) to 150 cm (5 ft) in height, and can be 120 cm (4 ft) to 240 cm (8 ft) in basal diameter. Houses typically are placed on the ground against or straddling a log or exposed roots of a standing tree and are often located in dense brush. Nests also are placed in the crotches and cavities of trees and in hollow logs. Sometimes arboreal nests are constructed but this behavior seems to be more common in habitat with evergreen trees such as live oak. With their general dependence on terrestrial stick houses, riparian woodrats at Caswell MSP are vulnerable to flooding of the Stanislaus River. But, because of its well-developed arboreality, the woodrat itself is not as sensitive to flooding as some other brush-dwelling species (e.g., the riparian brush rabbit). However, since woodrat houses are essential for survival, severe flooding will affect population viability.
The riparian woodrat is one of eleven recognized subspecies of the dusky-footed woodrat. The species range extends from the Columbia River and the Willamette Valley in Oregon to north-western Baja California. It is generally found in dense chaparral, oak and riparian woodland, and in mixed coniferous forest that has a well developed understory. Neotoma fuscipes prefers fairly moist habitat but it is also found in drier communities such as pinyon-juniper woodland. In all locations, it seems to favor brushy habitat or woodland that has an oak component.
A striking 'hole' in the species' range map is the Central Valley of California. The riparian woodrat is the only subspecies found on the floor of the Central Valley and it is restricted today to small remnant patches of riparian forest along the Stanislaus river.
The type locality for the riparian woodrat is Kincaid's Ranch, about 3 km (2 mi) northeast of Vernalis in Stanislaus County, California. Hooper's (1938, p. 223) taxonomic analysis used only seven specimens, all from the vicinity of the type locality, but he believed that "it probably ranges south, along the river bottom lands, as far as southern Merced County or northern Fresno County, since the same environmental conditions evidently prevail throughout this area." Hooper further pointed out that the range of the riparian woodrat was disjunct by 1938 because no suitable habitat remained between the type locality and the San Francisco East Bay region, where two other subspecies (N. f. perplexa and N. f. annectens) could be found. Hall and Kelson (1959) assigned a specimen from El Nido, Merced County to this subspecies on the basis of geography.
The range of the riparian woodrat is far more restricted today than it was in 1938 (Williams 1986). The only population that has been verified is the single, known extant population restricted to about 100 ha (250 acres) of riparian forest on the Stanislaus River in Caswell Memorial State Park. Williams (in litt. 1993) estimated the size of this population at 437 individuals. Analysis of California Department of Water Resources land use maps indicate that there were approximately 20 ha (50 acres) of "natural vegetation" present along the San Joaquin River near the type locality in 1988, though no woodrats have been seen in that area. Today there is no habitat for woodrats around El Nido, which is located about 8.9 km (5.5. mi) east of the San Joaquin River, the closest possible riparian habitat.
Although we still do not have a good estimate of the amount of riparian habitat remaining in the San Joaquin Valley, it is only a vestige of what it was 50-100 years ago. Thus, loss and fragmentation of habitat are the principal reasons for the decline of the riparian woodrat. Much of this loss was the result of the construction of large dams and canals which diverted water for the irrigation of crops and permanently altered the hydrology of Valley streams. More was lost through cultivation of the river bottoms. Historically, cattle also probably impacted riparian woodrat populations since the thick undergrowth, which is particularly important to woodrats, is sensitive to trampling and browsing and grazing by livestock.
Order RODENTIA, Family MURIDAE, Subfamily SIGMODONTINAE, Genus Neotoma, Species fuscipes, Subspecies riparia
Hooper (1938) and Hall (1981) recognized 11 subspecies of Neotoma fuscipes: Portola woodrat (N. f. annectens); San Miguel woodrat (N. f. bullatior); Sacramento Valley woodrat (N. f. fuscipes); Santa Lucia woodrat (N. f. luciana); San Diego woodrat (N. f. macrotis); San Pedro Martir woodrat (N. f. martirensis); Rhoads woodrat (N. f. monochroura); Diablo Range woodrat (N. f. perplexa); riparian woodrat (N. f. riparia); Fort Tejon woodrat (N. f. simplex); Streator woodrat (N. f. streatori).
San Joaquin Valley woodrat.
Hall and Kelson 1959; Hall, E. R. 1981. The Mammals of North America. New York: John Wiley & Sons; Hooper, E. T. 1938. Geographical variation in wood rats of the species Neotoma fuscipes. Univ. Calif. Publ. Zool., 42, 213-246; Kelly 1990, Population Ecology and Social Organization of Dusky-footed Woodrats, Neotoma fuscipes. Ph.D. Dissertation, University of California, Berkeley 191 pp; Williams, D.F. 1986. Mammalian Species of Special Concern in California. California Dep. Fish and Game, Wildl. Manage. Div., Admin. Rep., 86-1:1-112; Williams, D.F., J. Verner, H.F. Sakai, and J.R. Waters. 1992. General biology of major prey species of the California spotted owl. USDA Forest Service Gen. Tech. Rep., PSW-GTR-133:207-221; Williams, D. F. 1993. Population censuses of riparian brush rabbits and riparian woodrats at Caswell Memorial State Park during January 1993. California Dept. Parks and Recreation, Lodi, California, Final Report, 15 pp.; Linsdale, J. M. & Tevis, L. 1951. The Dusky-footed Wood Rat. Berkeley and Los Angeles: University of California Press.
Total length (from Hooper 1938):
(434-448) mm (males)
(435-452) mm (females)
Length of Tail (from Hooper 1938):
(207-224) mm (males)
(215-217) mm (females)
Avg. 266 g, St. Dev. 32 g (males)
Avg. 243 g, St. Dev. 30 g(females)
* Data from 10 adult males (225-330 g) and 13 adult females (190-300 g) captured at Caswell Memorial State Park, Jan. 9-16, 1993 (Williams et al., unpub. data).
N. f. riparia differs from other, adjacent subspecies of woodrats by being larger, lighter, and more grayish in color, with hind feet white instead of dusky on their upper surfaces, and a tail more distinctly bicolored, lighter below contrasting more with the darker dorsal color. The riparian woodrat is a medium-sized rodent with a stockier build and a tail that is well furred and not scaled, compared to the coexisting, non-native roof or "black" rats (Rattus rattus).
P.A. Kelly and D.F. Williams