South Texas Ambrosia
South Texas Ambrosia
|Listed||August 24, 1994|
|Description||Herbaceous, erect, silvery to grayishgreen, rhizomatous perennial.|
|Habitat||Low elevations in open clay-loam to sandy-loam prairies and savannas.|
|Threats||Habitat destruction and fragmentation through alteration and conversion of native plant communities to commercial uses; displacement by invasive non-native grasses; and low population numbers.|
South Texas ambrosia is an herbaceous, erect, silvery to grayish-green, rhizomatous perennial plant, 0.3-1.0 ft (0.1-0.3 m) tall. Its simple leaves are usually opposite on the lower portion of the plant and alternate above. The staminate flower heads are arranged in inconspicuous terminal racemes 2-4 in (5-10 cm) long. The pistillate flower heads are in small clusters in the leaf axils just below the staminate racemes. Due to its rhizomatous growth, a single plant may be represented by hundreds of clonal stems.
South Texas ambrosia grows at low elevations in open clay-loam to sandy-loam prairies and savannas. Much of the original native habitat for south Texas ambrosia has been converted to agricultural fields, improved pastures, or urban areas. Many savanna areas have been cleared and planted with non-native grasses, such as buffelgrass (Cenchrus ciliaris ), which outcompete and eventually displace much of the native vegetation. Other potential prairie habitat may now be invaded by thorny shrub and tree species as a result of fire suppression or overgrazing. South Texas ambrosia does not appear to survive intensive plowing, blading, or discing; however, some lesser soil disturbance may enhance its growth. Associated native grasses found at the existing sites include Texas grama (Bouteloua rigidiseta ), buffalo grass (Buchloe dactyloides ), Texas spear-grass (Stipa leucotricha ), and tobosa (Hilaria mutica ). Invading non-native grasses found at the sites include buffelgrass, King Ranch bluestem (Bothriochloa ischaemum var. songarica ), bermuda grass (Cynodon dactylon ), and St. Augustine grass (Stenotaphrum secundatum ). Associated native woody species found scattered throughout the existing sites include mesquite (Prosopis glandulosa ), huisache (Acacia smallii ), huisachillo (Acacia schaffneri ), brasil (Condalia hookeri ), granjeno (Celtis pallida ), and lotebush (Ziziphus obtusifolia ).
Historically, south Texas ambrosia occurred in Cameron, Jim Wells, Kleberg, and Nueces Counties in south Texas, and the state of Tamaulipas in Mexico. The current status of any Mexican populations is unknown. The historic populations in Cameron and Jim Wells Counties have not been relocated.
Only one location noted in the 1983 status report is known to be still extant. Three populations, two in Nueces County, and one in Kleberg County, were discovered by Ruth O'Brien. Three Nueces County populations were discovered in 1992 and 1993 by William Carr. The extant populations occur on private land, highway and railroad rights-of-way, and the Kingsville Naval Air Station. Four historic locations for south Texas ambrosia, one extirpated and three extant, also support the endangered slender rush-pea (Hoffmannseggia tenella ), which was federally listed on November 1, 1985, because of threats similar to those affecting south Texas ambrosia. One known location for south Texas ambrosia also supports the endangered black lace cactus (Echinocereus reichenbachii var. albertii ), which was federally listed on October 26, 1979, because of habitat destruction and collecting threats.
Habitat destruction is the primary threat to south Texas ambrosia. The past and current practices of converting native south Texas brush and woodlands to agricultural fields, improved pastures, and urban areas, or clearing brush and woodlands for urban water development, industrial development, or flood control have destroyed 95% of this native vegetation. Most native Texas Gulf Coast prairies have been converted to agricultural fields or improved pastures. The amount of conversion of these plant communities in Mexico is similar though not quantified. The remaining remnant native prairie, brush, and woodland tracts are often surrounded by agricultural fields, pastures, or urban development. These modified habitats pose potential threats to the native areas through agricultural chemical drift from aerial spraying; chemical runoff following rains; invasion of non-native grasses such as buffelgrass, guineagrass (Panicum maximum ), King Ranch bluestem, and Angleton bluestem (Dichanthium aristatum ); and trampling and possible collection pressures due to easy accessibility from nearby urban areas. The few remaining populations of the species are vulnerable to extinction if any of their remaining habitat is modified.
Even roadside remnants of native vegetation in south Texas are often bladed, or plowed and seeded with exotic grasses such as buffelgrass and King Ranch bluestem. Herbicides are often used to control vegetation around signs, guard rails, and bridge abutments, and to kill shrubby vegetation encroaching on the right-of-way. Due to the rarity of Texas ayenia and south Texas ambrosia, the likelihood they will be directly impacted by roadway maintenance is small, but almost any impact could lead to extinction of either species.
South Texas ambrosia may be vulnerable to extinction due to lowered genetic variability. Populations are clonal, so despite having many stems, the populations may actually represent very few genetically different individuals. It has been noted that species like south Texas ambrosia that were once more widespread, but are now reduced to low numbers, may be more vulnerable to the detrimental effects of lowered genetic diversity than species that were always rare.
Conservation and Recovery
Some federal actions that may affect south Texas ambrosia include brush clearing for flood control by the International Boundary and Water Commission, management recommendations to landowners by the Soil Conservation Service for activities funded by the Agricultural Stabilization and Conservation Service, and agricultural pesticide registration by the Environmental Protection Agency. Additionally, a population of south Texas ambrosia occurs on Kingsville Naval Air Station and may be affected by maintenance or construction activities at this facility. These agencies have been notified of the occurrence of the endangered plants so that appropriate conservation steps may be taken.
U.S. Fish and Wildlife Service. 24 August 1994 "Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for the Plants Ayenia limitaris (Texas Ayenia) and Ambrosia cheiranthifolia (South Texas Ambrosia)." Federal Register. 59(163): 43648-43652.