The American Beaver in Arizona

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The American beaver (Castor canadensis) is the largest rodent in North America with adults reaching a weight of up to 70 pounds with an average of 40 pounds. (The porcupine is the second largest rodent, see my  article: Porcupines.) The American beaver ranges throughout the U.S. and Canada. It has also been introduced to parts of South America. A different beaver species (Castor fiber) occurs in Eurasia.

Beavers are semi-aquatic mammals that are most active at night. They are excellent swimmers and can stay submerged up to 15 minutes. They have webbed hind feet and a broad, flat tail which propel them through water.

Beavers build “lodges” and dams in lakes and rivers using sticks, stones, grass, and mud. Their structures often have a great (and beneficial) effect on the local ecology.

Beavers eat the leaves, twigs, and inner bark of many tree species. Fermentation by special intestinal microorganisms allows beavers to digest 30 percent of the cellulose they ingest. (source)

Beavers are prized for their fur which consists of long, coarse outer hairs and short, fine inner hairs. The fur has a range of colors but usually is dark brown. Scent glands near the genitals secrete an oily substance known as castoreum, which the beaver uses to waterproof its fur. Beaver meat tastes similar to beef, but care must be taken in preparation to avoid getting the castoreum on the meat. The characteristics of the fur led to extensive hunting and in some place, extirpation of the beaver. For more extensive general information see this Wikipedia article.

Beavers in Arizona:

According to the Arizona Game & Fish, beavers were at one time found nearly everywhere in Arizona that had permanent water. With settlement, and the desiccation of the state’s streams, beaver populations declined. This habitat loss, and in some cases, heavy trapping pressure, caused beavers to disappear from such former strongholds as the San Pedro and Santa Cruz rivers. Reintroduction programs and natural colonizations have since enabled the beaver to recover much of its former distribution, if not numbers, and these animals can now be found along several permanent streams, some of the larger river stretches, certain shallow lakes, and even a few dirt-lined canals.

The Winter, 2014, issue of Sonorensis pubished by the Arizona-Sonora Desert Museum has a good article on Arizona beavers (pages 18-23).

The article entitled “Aquatic Architects at Work: the Return of Beaver to the San Pedro River National Conservation Area” begins: “In 1826, trapper James Ohio Pattie dubbed the lower San Pedro River “Beaver River.” He and his party had been trapping beaver along the Gila River and its tributaries, but found that northern stretch of the San Pedro “very remarkable for the number of its beaver…At this place we collected 200 skins.”

The article claims: Beavers influence community diversity and ecosystem structure through tree felling and dam building. Beaver herbivory tends to result in large woody material in the floodplain, which enhances the amount of water that may be captured and stored. Water impounded behind beaver dams increases the area of riparian habitat, and, through groundwater recharge, leads to elevated water tables.

The United States Geological Survey, in cooperation with the University of Arizona recently published a paper which evaluated ecological benefits of restoring beavers to the San Pedro (read full study).

The paper’s abstract says:

We measured bird abundance and richness along the upper San Pedro River in 2005 and 2006 to investigate how beavers (Castor canadensis) may act as ecosystem engineers after reintroduction to a desert riparian area in the Southwestern United States. In areas where beavers colonized, we found higher bird abundance and richness of bird groups, such as all breeding birds, insectivorous birds, and riparian specialists, and higher relative abundance of many individual species—including several avian species of conservation concern.

After accounting for environmental factors, such as presence or persistence of surface water, and extent of Frémont cottonwood (Populus fremontii) and Goodding’s willow (Salix gooddingii), the relative influence of beaver activity was not as strong as these other environmental factors. However, there was still evidence of an association between beaver activity and bird abundance and richness, as models that included beaver-related variables better explained variation in bird abundance and richness for 71 percent of species groups and 46 percent of individual species for which we built models. Although the effect sizes associated with the beaver influence on the bird community were smaller than similar studies conducted in other regions of North America, the biological significance of beaver activity in the upper San Pedro River riparian area will likely become even stronger with increasing time.

AZ Game & Fish studied the suitability of re-establishing beavers in Cienega Creek. The 2008 study concluded that certain reaches were suitable.

You can see beavers at the Arizona-Sonora Desert Museum. ASDM has a story about a “beaver breakout” from the Museum. “One night, the beavers left their pond, waddled through the bighorn enclosure and escaped into the desert. The next morning Museum people followed their tracks, which headed west and before long stopped near a set of coyote tracks. Swirling beavertail prints in the sand showed how the beavers had made an about-face and headed back to the safety of their pond. They never attempted another escape.” source

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