Mancos Milk-vetch

Astragalus humillimus

Description

Mancos milk-vetch (Astragalus humillimus ) is a perennial with short spiny stems that bear compound leaves measuring about 0.6 in (15.2 mm) in length. Each light green, oval leaflet is only about 0.1 in (2.5 mm) long and is covered with minute hairs. Plants grow in a low tufted mat up to 18 in (46 cm) in diameter. Small, white-veined lavender flowers, blooming between late April and early May, give off a sweet, pungent aroma. The fruit— a tiny oblong pod—ripens by early June.

Clumps of blooming Mancos milk-vetch are often covered with butterflies, particularly the painted lady butterfly (Vanessa cardui ), which has been identified as a pollinator of the plant.

Habitat

Mancos milk-vetch is restricted to Cretaceous sandstone outcroppings (Point Lookout and Cliff House formations) at elevations of 5,070-5,400 ft (1,545-1,646 m). It grows in bowl-shaped depressions in the bedrock that have filled with sandy soils, and it appears to prefer a southern exposure. Dominant associated plants are Oryzopsis hymenoides, Gutierrezia sarothrae, Yucca angustissima, and Artemisia tridentata.

Distribution

This plant is endemic to extreme southwestern Colorado and extreme northwestern New Mexico. The pattern of distribution follows a line north and south along a ridge formed of the preferred sandstone formation (known as hogback).

First collected in 1875 from the vicinity of Mancos, Montezuma County, Colorado, Mancos milk-vetch slipped from sight and was considered extinct for almost 50 years. In 1980 it was rediscovered near Waterflow, San Juan County, New Mexico.

Subsequent surveys located three additional populations along the ridge in San Juan County within 10 mi (16 km) of Waterflow. The total population in 1985 was about 7,000 plants. The largest of these populations is atop a sandstone mesa that is flanked on three sides by active oil wells. The Navajo Indian Tribe owns the land and the surface rights; the leasable mineral rights are privately owned. One site is managed by the Bureau of Land Management (BLM).

In the late 1980s a small population was discovered near its original collection site on the Ute Mountain Indian Reservation near Mancos, Colorado.

Threats

This species is restricted to bedrock basins and is extremely vulnerable to disturbance. Twenty years after a power line was constructed through the middle of one population, it still had not repopulated the disturbed zone. Remaining populations grow in the vicinity of utility corridors, drilling pads, oil wells, pipelines, and roads. Activities associated with these installations pose a grave threat to surviving populations of Mancos milk-vetch.

Conservation and Recovery

One of the issues facing conservationists trying to save this plant are the conflicts over the need to protect the species versus programs which had or have been authorized by the BLM and tribal authorities, specifically a right-of-way for transmission lines and assigned mineral leases for oil, gas, and minerals development.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
P. O. Box 1306
Albuquerque, New Mexico 87103-1306
Telephone: (505) 248-6911
Fax: (505) 248-6915
http://southwest.fws.gov/

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
P.O. Box 25486
Denver Federal Center
Denver, Colorado 80225
http://www.r6.fws.gov/

References

Barneby, R. C. 1964. "Atlas of North American Astragalus." In Memoirs of the New York Botanical Garden. Vol. 13. Part II.

Rydberg, P. A. 1905. "Astragalus and Its Segregates in Colorado." Bulletin of the Torrey Botanical Club 32: 657-665.

U. S. Fish and Wildlife Service. 1986. "Mancos Milk-vetch Recovery Plan: Technical/Agency Draft Review." Endangered Species Office, Albuquerque.

U. S. Fish and Wildlife Service. 1987. "Endangered and Threatened Species of Arizona and New Mexico (with 1988 Addendum)." U. S. Fish and Wildlife Service, Albuquerque.