Walker's Manioc

Manihot walkerae

Description

Walker's manioc is a multi-branched perennial in the spurge family that grows to a height of about 18 in (50 cm). The alternate leaves are palmately five-lobed. Separate male and female flowers are fleshy and white, about 0.4 in (1 cm) long. The fruit is a capsule, about 0.4 in (1 cm) in diameter, containing three seeds.

Habitat

This species grows in loamy and sandy-loamy soil along the Rio Grande in south Texas. It has also been found in Mexico on sandy prairie overlying caliche, a hard desert soil containing calcium carbonate. Walker's manioc occurs in Tamaulipan brushland which is dominated by acacia, mesquite (Prosopis glandulosa), Texas ebony (Pithecellobium fiexicaule), and cenizo (Leucophyllum frutescens).

Distribution

Walker's manioc was first collected in 1853 at Ringgold Barracks, Starr County, Texas, but was misidentified. It was not described as a new species until 1942. Plants have been found at seven sites in Starr and Hidalgo counties, Texas, and the State of Tamaulipas, Mexico.

The species is now found at a single site in the United States. Despite extensive searches in the late 1980s, no Walker's manioc plants were found at historic sites. However, in 1990 the species was discovered at a new site in Hidalgo County, Texas. Plants may still exist in Mexico, but their status is unknown. Botanists familiar with the area around the Rio Grande are hopeful that new populations will eventually be discovered.

Threats

The main threat to this extremely rare species is the clearing of its unique habitat for agriculture, urban development, and recreation. It is endemic to Tamaulipan brushland, which is found only in south Texas and northeastern Mexico. During the twentieth century, 95% of this habitat has been cleared for development. Along the Rio Grande 99% of native brushland has been destroyed. Almost all remaining native vegetation is on land that is being heavily grazed by livestock.

Conservation and Recovery

Because Walker's manioc is related to cassava (Manihot esculenta ), an important food plant in the tropics, it is a possible source of genetic material valuable in improving cultivated strains of that crop. Wild relatives of crop species are sometimes able to confer increased resistance to disease or harsh climatic conditions. Walker's manioc plants are currently under cultivation at the University of Texas, Austin, and at the San Antonio Botanical Gardens.

The Fish and Wildlife Service prepared a Recovery Plan for the Walker's manioc in 1993. Although the species is not found on federally owned land, two federal agencies are active in areas where there is hope of finding additional plants. The Army Corps of Engineers has jurisdiction over pipeline construction in the area, and the International Boundary Commission is in charge of brush clearing to maintain canals on the U.S. side of the Rio Grande River. Any Walker's manioc plants discovered on federal land administered by these agencies will require them to consult with the Fish and Wildlife Service about ways to conserve the species. More research is needed into the effects of prescribed vegetation management and wildfire on the Walker's manioc, and its populations must be monitored.

Contacts

U.S. Fish and Wildlife Service
Division of Endangered Species and Habitat Consevartion
2105 Osuna Road N.E.
Albuquerque, New Mexico 87113-1001
Telephone: (505) 346-252
E-mail: [email protected]
http://ifw2es.fws.gov/

U.S. Fish and Wildlife Service
Corpus Christi Ecological Services Field Office
c/o Texas A&M University at Corpus Christi
6300 Ocean Drive, Campus Box 338
Corpus Christi, Texas 78412-5599
Telephone: (361) 994-9005
Fax: (361) 888-3198
E-mail: [email protected]
http://ifw2es.fws.gov/CorpusChristiTexas/

References

Collins, K. 1984. "Status and Management of Native South Texas Brushlands." U.S. Fish and Wildlife Service, Corpus Christi, Texas.

Jahrsdoerfer, S. E., and D. M. Leslie, Jr. 1988. "Tamaulipan Brushland of the Lower Rio Grande Valley of South Texas: Description, Human Impacts, and Management Options." U.S. Fish and Wildlife Service, Biological Report 88(36).

Rogers, D. J., and S. G. Appan. 1973. "Mani-hot, Manihotoides (Euphorbiaceae)." Flora Neotropica, Monograph No. 13, Hafner Press, New York.

U.S. Fish and Wildlife Service. 1993. Walker's manioc (Manihot walkerae ) Recovery Plan. U.S. Fish and Wildlife Service, Albuquerque, New Mexico. (Unpublished)