Point Arena Mountain Beaver

(Last Revised 8/30/2002)

Despite the name, the mountain beaver is not related to true beaver species (Castor) and is classified in a genus considered to be one of the oldest groups of living rodents. The Point Arena mountain beaver (PAMB, Aplodontia rufa nigra) is one of seven mountain beaver subspecies occurring in the United States, and one of four that typically occurs in California. The geographical range of the PAMB is limited to 24 square miles in western Mendocino county.

The PAMB is a medium-sized rodent with dark black coloration. Mountain beavers, including the PAMB, are seldom seen and are generally identified by finding their burrows. Their burrows are small cylindrical openings that occur in groups. Also, clipped vegetation is generally found near the base of the burrow, indicating that the burrow is active.

Most mountain beavers live in extensive underground burrow systems with multiple entrances. Along with nest chambers, four other chambers occur in mountain beaver burrows including: food storage, refuge, fecal deposit, and a chamber for depositing rocks and clumps of hard clay ("earthballs") encountered during digging activities. Interestingly, despite the fact that many mountain beavers may live in the same burrow system, they do not interact socially, and tend to be solitary creatures. Mountain beaver burrows provide habitat for several other species of vertebrates and invertebrates. Other vertebrates that have been found in burrows include: skunks, salamanders, moles, voles, shrews, chipmunks, and several other species. Invertebrates that can be found in mountain beaver burrows include the world's largest flea (Hystricopsylla schefferi).

Mountain beavers face many threats including habitat destruction, predation, poisoning, trapping, residential development, cattle grazing, herbicide use, invasion by exotic plants, and genetic isolation. Due to these threats and the small geographical range of the subspecies, the PAMB was listed as federally endangered in 1991.

MRC staff work with the United States Fish and Wildlife Service (USFWS) to protect mountain beavers and their habitat on MRC land. All PAMB burrow systems are reported to USFWS and are protected from disturbance by timber harvest or other activities.

Currently, MRC staff is continuing to locate PAMB burrow systems on the ownership as well as counting the number of active burrows in several systems. MRC staff will monitor these burrows on a yearly basis using the number of active burrows as an index of PAMB abundance on the property. With this information, biologists will be able to determine if this endangered species is declining, or hopefully increasing, on the ownership. MRC biologists are also working on a study to examine habitat used by PAMB in industrial forestlands--this information will help define potential habitat for PAMB on the property.

Although there is a lack of research on the PAMB, there is some speculation that timber harvesting may provide new habitat for PAMB on industrial forestlands. Timber harvesting removes overstory vegetation and allows for the growth of lower-growing vegetation more typical of PAMB habitat. There is no documented evidence of PAMBs moving into an area after harvest, but further research needs to be completed before we fully understand if timber harvest can provide habitat or expand habitat around existing burrow systems. MRC is also seeking coverage for the PAMB under a potential habitat conservation plan (HCP). It is likely that an HCP for PAMBs would provide a greater understanding of their population numbers on MRC ownership and would benefit the recovery effort for the subspecies.

For further information on mountain beavers, see the Point Arena Mountain Beaver website: www.pointarenamountainbeaver.com