Official Status: ENDANGERED, The Point Arena Mountain Beaver is Federally listed under the Endangere

Official Status: ENDANGERED, the Point Arena mountain beaver is federally listed under the Endangered Species Act as endangered, and is considered a mammalian species of special concern by the State of California.

Mountain beavers are the only extant member of the family Aplodontidae, and are considered the most primitive living rodents. Mountain beavers are stout, compact and cylindrical and have a broad, massive, laterally compressed skull. They average about one foot in length and two to four pounds in weight. Mountain beavers have small eyes, rounded ears, and a distinctive cylindrical stump of a tail. Each forepaw has an opposable thumb and all digits have long curved claws for digging. Mountain beavers are not closely related to true beavers (Castor spp.), which are semi-aquatic. The Point Arena mountain beaver is one of seven subspecies of mountain beaver. Certain cranial and other characteristics separate the Point Arena mountain beaver from other subspecies of mountain beavers. The most obvious of these characteristics is the unique blackish coloration.

Since their discovery by Lewis and Clark, mountain beavers have been known to occur in many areas of the Pacific Northwest. The Point Arena subspecies, however, is only found within a disjunct, 24-square mile area in western Mendocino County, California. The U.S. Fish and Wildlife Service considers the range of the Point Arena mountain beaver to include areas five miles inland from the Pacific Ocean extending from a point two miles north of Bridgeport Landing south to a point five miles south of the town of Point Arena. Point Arena mountain beavers can be found along Nulls Creek, Mallo Pass Creek, Irish Gulch, Alder Creek, Manchester State Park, Lagoon Lake, Lower Hathaway Creek, City of Point Arena, Lower and Middle Brush Creek, and Hathaway Creek.

The ratio of males to females is thought to be 1:1, with an estimated lifespan of 5-6 years. Mountain beavers are thought to have a very low reproductive output, with females starting to breed in their second year and then producing a single litter each year consisting of two to three offspring. The breeding season of the Point Arena mountain beaver is December 15 through June 30, with dispersal occurring from April 15 through September 30. Other subspecies of mountain beaver have been recorded dispersing up to 1,850 feet from natal dens. Among the other subspecies of mountain beaver, wide variation in home range size has been reported, ranging from 0.3 to 10.3 acres per animal. Studies are currently underway on various aspects of the life history of the Point Arena subspecies.

Mountain beavers are semi-fossorial, spending much of their time in underground burrow systems, but surface above ground to forage on vegetation. Sites with burrows are typically found on moist and steep north-facing slopes or gullies with well-drained and friable soil. Studies suggest that the most important factors in habitat use are a cool thermal regime, adequate soil drainage and softness, an abundant food supply, and a high percent cover of lush herbaceous and small diameter woody plants. Point Arena mountain beavers are found in a variety of habitat types including mesic coastal scrub, northern dune scrub, the edges of conifer forest, and riparian plant communities. Some subpopulations at Manchester State Park are found in stabilized sand dunes dominated by bush lupine, European beach grass, and ice plant. Away from the coastal slope, Point Arena mountain beaver are found adjacent to redwood forests in riparian strips dominated by alder and sword fern. Mountain beavers are strictly herbivorous, and are know to eat many plants toxic to other animals, including bracken fern, sword fern, stinging nettle, thistle, and larkspur. Probable or known foods of the Point Arena mountain beaver include ice plant, sword fern, cow parsnip, wild radish, angelica, Douglas iris, miner’s lettuce, and many others.

The total number of individual Point Arena mountain beavers throughout their range is unknown. It is also unclear exactly how many separate Point Arena mountain beaver subpopulations currently exist, but there are probably at least 26 separate subpopulations. The amount of occupied, and unoccupied but suitable, Point Arena mountain beaver habitat throughout the range is unknown. Within Manchester State Park, there are an estimated 481 acres of suitable habitat, 57 acres of which are considered occupied.

The primary reasons for the decline in Point Arena mountain beavers is habitat loss and fragmentation, primarily due to construction of recreational facilities, urban development, conversion to agricultural use (farming and livestock grazing), and construction of transportation and utility corridors. Cattle and sheep can also alter vegetation and crush mountain beaver burrows. The high rate of urbanization in coastal Mendocino County is expected to continue or increase in the future. Rodent control activities, domestic pets, invasive plants, and vegetation fires are among the other threats to the Point Arena mountain beaver. Timber harvest might kill or disturb mountain beavers, but over time the removal of overstory trees is thought to increase and enhance the herbaceous and brushy habitats they favor. This is especially true if adequate down woody material is retained on site after harvest. Mountain beavers have highly developed tactile senses, suggesting susceptibility to disturbance from loud noises or ground vibration during the breeding season.

The most important and immediate conservation needs for the Point Arena mountain beaver are to stop the loss of suitable habitat within its range, the direct take of any individuals, and the disturbance to breeding animals. Accordingly, any site within the range where an activity may alter vegetation, compact the soil, or cause disturbance should be carefully assessed for the presence of mountain beavers during project planning. These assessments typically involve evaluating whether or not suitable habitat exists in or near the project site, and then conducting thorough surveys in all areas deemed suitable. Livestock grazing should be carefully managed in suitable habitat areas. Many invasive plant species, such as conifers, reduce habitat suitability and should be removed from in and around mountain beaver sites. Any rodent control activities within the range of the Point Arena mountain beaver should follow the guidelines in the Interim Measures for Use of Pesticides in Mendocino County that can be found at the Department of Pesticide Regulation web site at http://www.cdpr.ca.gov/. Any timber harvest should be conducted so as to avoid crushing mountain beavers or their burrows. Activities that produce loud noise or ground vibration, such as directional boring, road or building construction, timber harvest, and mineral extraction, that are to occur in or near occupied mountain beaver habitat should be conducted outside of the mountain beaver breeding season.

Over the long-term, a network of protected occupied sites with adequate interconnectivity for dispersal must be established before the subspecies can be considered for delisting. These sites should be distributed throughout the current range of the Point Arena mountain beaver. Protected sites should be large enough and should have enough individuals to be considered a stable subpopulation that could then contribute to the larger metapopulation. This approach may require that some currently unsuitable sites or travel corridors be restored to suitable mountain beaver habitat.