The U.S. Fish & Wildlife Service currently lists the Point Arena Mountain Beaver as an endangered species. As such, the USFWS has written a recovery plan for the beaver that can be viewed at the following link.

The mountain beaver recovery plan (written in 1998 by Dale Steele and Laurie Litman) describes its geographic distribution on page 2, with a range map on page 3, its habitat on pages 5-10, and its population density on page 14. A map on page 8 indicates that there are 26 known populations. The criteria for considering de- listing the species from the Endangered Species List are outlines on page iv (in the Executive Summary).

These criteria are an example of the metrics that scientists would used to consider the mountain beaver as recovering and threatened, rather than endangered.

The following California Dept. of Fish and Game website is an interesting place where you can find more information.