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Edwards Plateau savanna

The Edwards Plateau Savannas form an important part of the Texas Hill country, a moderately-sized ecoregion separated from adjacent units by a distinct soil type (mollisols) and a vegetation type distinguished by juniper-oak savanna and mesquite-Acacia savanna underlain by mid- to short grasslands (Küchler 1964). In other aspects, the Edwards Plateau Savannas is an ecoregion that is intermediate among the dry grassland and savanna ecoregions in terms of rainfall, temperature, and length of growing season. It is estimated that up to 90 percent of this ecoregion has been converted to pasture, urban areas, and agricultural crops. Previously, major disturbance regimes were dominated by fire, drought, and perhaps grazing by bison (Bison bison).

  • Scientific Code
    (NA0806)
  • Ecoregion Category
    Nearctic
  • Size
    23,900 square miles
  • Status
    Critical/Endangered
  • Habitats

Description
Biological Distinctiveness
The limestone bedrock of the Edwards Plateau helps to contribute to the distinctiveness of the biota. An array of species are specialists on limestone habitats, including caves. Some of the largest assemblages of cave-dwelling bats anywhere in the world, indeed the largest aggregations of mammals anywhere in the world, roost in several of the large caves of this ecoregion. Millions of Mexican free-tailed bats (Tadarida brasiliensis) use these caves as maternity roosts, creating a globally outstanding phenomena. The high number of endemic invertebrate cave species also qualify this ecoregion as a global hotspot for cave-dwelling species. The aquatic vertebrates endemic to the Plateau include the widemouth blindcat (Satan eurystomus), San Marcos salamander (Eurycea nana), Comal blind salamander (Eurycea tridentifera), Texas blind salamander (Typhlomolge rathbuni), Blanco blind salamander (Typhlomolge robusta), and the Texas salamander (Eurycea neotenes).

This ecoregion also ranks among the top ten ecoregions for reptiles and birds. The Edwards Plateau Savannas contains most of the breeding habitat for an endemic migratory warbler, the golden-cheeked warbler (Dendroica chrysoparia), which nests only in mature oak-juniper savannas, or cedar brakes. Some important breeding habitat for the black-capped vireo (Vireo atricapillus), a species endangered by habitat loss, occurs in this ecoregion. A disjunct population of maples (Acer spp.) occur in mesic pockets, an unusual example of a relict population in a presently dry region.

Conservation Status

Habitat Loss and Degradation
Only about two percent of the remaining habitat in this important ecoregion is considered intact. Important ecological processes such as fire have been suppressed, leading to changes in vegetation structure and composition. Extensive soil loss has contributed to changes in the disturbance regimes of riparian communities. Overgrazing has fragmented and eliminated native grasslands and contributed to the expansion of woody species.

The southern and western portions of the Edwards Plateau are heavily altered by major encroachment of shrubs as a result of fire suppression. The eastern section of the plateau is in better condition and contains much second growth.

Remaining Blocks of Intact Habitat
The Edwards plateau contains only a few relatively small areas of intact habitat. These occur around Austin, where about 30 percent of a 182 km2 tract has been acquired and put under conservation management. About 50 percent of a 122 km2 tract in the Balcones Canyonlands has been acquired. Other intact blocks remain in Real County along the Frio River and in small patches outside San Antonio in northwest Bexar County.

Degree of Fragmentation
All remaining blocks of habitat are highly fragmented.

Degree of Protection
When acquired, the new wildlife refuge near Austin and the Balcones Canyonlands will improve protection of biodiversity. Until then, many of the area's threatened species and habitats face considerable risk.

Types and Severity of Threats
Urban and suburban sprawl around Austin and San Antonio pose a high threat to remaining habitats. Invasion of exotic species and overgrazing will also increase habitat degradation unless mitigated by conservation measures. Cedar choppers in the hill country have removed much of the old growth juniper-oak woodland. Change in riparian disturbance regimes affects the dynamics of this system and threatens species dependent upon this habitat.

Suite of Priority Activities to Enhance Biodiversity Conservation
Priority sites for conservation include:

•areas along the Balcones Escarpment
•areas in western Travis County and northwest Bexar County that preserve the best remaining contiguous habitat of the golden-cheeked warbler
•karst areas that contain the highest concentration of Mexican free-tailed bat maternity colonies in the world
•Fort Hood, which contains the largest population of golden-cheeked warblers and black-capped vireos under a single management unit, as well as significant karst features
•areas of high salamander diversity and endemism. Some individual springs, streams, and artesian wells are the only known localities for some species.
•karst areas rich in endemic invertebrates
Conservation Partners

•Bat Conservation International
•Bexar Land Trust
•Department of Texas Parks and Wildlife
•Hill Country Foundation
•Land Trust Alliance
•National, State, and local Chapter of the Audubon Society
•Native Plant Society of Texas
•The Nature Conservancy
•The Nature Conservancy - Southeast Regional Office
•The Nature Conservancy of Texas
•Save our Springs Association
•Sierra Club, Lone Star Chapter
Relationship to other classification schemes
The boundary of the Edwards Plateau Savannas is taken from Omernik (1995). It generally corresponds to Küchler’s (1985) juniper-oak savanna (87) and Bailey (1994) section 315D (Edwards Plateau).

Prepared by: T. Cook and D. Olson