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The Nature Center

The Llano Estacado

Habitats of the Llano Estacado
Shortgrass Plains and Chihuahuan Scrub

Introduction

Shallow gravel soils fringe the western edge and southern end of the Llano Estacado. There is generally less biological diversity in areas of shallow gravel soils than in the other habitats of the Llano. The main sources of naturally occurring water are the shallow playas that only fill with the occasional rains. Plants adapt to the shallow soils by having fibrous and wide-spreading root systems. Many plants also have small leaves and the ability to become dormant for long periods. Many animals adapt by getting their water needs met by extracting metabolic water from their foods and less on drinking freestanding water.

The western edge of the Llano Estacado is a prairie dominated by buffalo grass and other short grasses. The southern end is transitional to the more arid Chihuahuan Desert. The southern end is dominated by an immigrant shrub species from South America, the creosote bush.

Birds

The black-throated sparrow is a specialist suited for hot, arid environments. The “desert sparrow” seems impervious to the rigors of heat and scant rainfall. The black-throated sparrow can get by for most of the year from moisture extracted from insects, seeds, and plant shoots from its diet, although drinking water becomes more significant in the heat of summer.

The verdin is another desert specialist. It is common, but often overlooked, in mesquite and other desert scrub. For such a small bird it builds an extraordinarily large dome-shaped nest usually located in mesquite. Like the black-throated sparrow, the verdin evidently gets its water needs met through its diet of insects and seeds. It flits about in shrubs searching for food, often seen clinging to and hanging head downward at the ends of twigs.

Harris’ hawks are social raptors, gathering in small flocks in winter, hunting in pairs, and sharing in nesting duties. After the young have fledged, families of Harris’ hawks stay together for several months. Team hunting is advantageous when preferred prey is agile or too large for one bird to handle. Harris’ hawks have the curious habit of simultaneous polyandry where two males share the same mate at the same time, remaining together through the nesting season.

Chihuahuan ravens make their home in the arid grasslands, mesquite thickets, and desert scrub of the Southwest. Their bulky nests consist of a platform of thorny twigs lined with wool, fur, or other soft materials. The neat may be located in a tree, on an old windmill, or on a utility pole. After breeding, Chihuahuan ravens assemble in large flocks for the fall and winter. They are highly intelligent and seem to be able to learn new behaviors even from other kinds of animals.

Mammals

Javelinas, or peccaries, are pig-like animals that occupy brushy semi-desert where prickly pear cactus abounds. They are active mainly in the early morning and late afternoon, and often bed down in dense brush or prickly pear thickets during the heat of the day. They travel in bands ranging from a few animals to several dozen. Javelinas are chiefly herbivorous and are especially fond of prickly pear, mesquite beans, and other succulent vegetation. Javelinas seldom frequent water holes where prickly pear abounds because this cactus provides them with both food and water.

The pronghorn is not a true antelope, but rather is the sole remnant member of an American family of mammals from the Miocene and Pliocene times. The pronghorn is America’s swiftest mammal; sometimes reaching speeds over 40 mph. Pronghorns have a highly developed sense of curiosity, examining at close range, any unrecognized object, particularly one that moves. A peculiarity of the pronghorn is its hollow horns that grow over a bony core. Every fall the horns are shed and new ones grow over the core. The male’s horns are much larger than those of the female are, and some females have none. The pronghorn is physiologically adapted to arid conditions. It can go for long periods without drinking water by conserving body water and extracting water from its food.

Plains pocket mice feed almost exclusively on seeds of grasses and weeds stuffed into their cheek pouches. Their metabolism is such that they convert the carbohydrates of seeds into metabolic water. They commonly excavate burrows beneath clumps of yucca or prickly pear. Usually the main entrance to their burrow is plugged with soil from within during the day.

Plants

The key to the creosote bush’s success in harsh habitats is that its stems and evergreen leaves are covered with a sticky resin that smells like (but does not contain) the wood preservative, creosote. This resin screens leaves against ultraviolet radiation, reduces water loss, and poisons or repels microbes and most herbivorous animals. The creosote bush monopolizes soil nutrients, starving competing grasses and other plants. Its taproot can extend 15 feet into the soil, while lateral roots fan out into more than 50 square yards of surface soil. Creosote bush frequently self-pollinates and is an opportunistic bloomer. It is able to grow and reproduce even when water is virtually absent. These advantages have enabled creosote bush to spread extensively across North American deserts after arriving (possibly from seeds attached to migrating birds) sometime during the last glacial episode between 10,000 and 14,000 years ago.

The creosote bush is a long-lived plant and goes through several forms during its life. It begins as a single-stemmed plant and may live in this form for 50-200 years. Gradually as the shrub ages, the center stems die out, leaving dead branches in the center while the edges continue to grow outward. After hundreds (and even thousands) of years, the edges of the growing plant will form a large ring of small creosote bushes. The oldest living creosote bush in North America is estimated to be over 11,000 years old!

Buffalo grass is a native North American short grass. It is drought, heat, and cold resistant. It is a warm-season perennial. Buffalo grass sends out numerous branching stolons and occasionally produces rhizomes. Its roots are numerous and thoroughly permeate the soil forming a dense sod. Buffalo grass is also well adapted to fire and grazing pressure. Its root system holds soil together after grazing animals have eaten the taller grasses.

The Torrey yucca is tree-like, growing from 3-24 feet in height with a simple or few-branched trunk. The crowded, radiating leaves and persistent thatch of dead, reflexed leaves give it an untidy appearance. The Torrey yucca is sometimes called the “Spanish-dagger” because of its stiff, sharply pointed leaves. Deer feed on the flowers, many insects lay their eggs in the seedpods, and ants feed on the sugary pulpy fruit.

Catclaw acacia is a shrub or small tree bearing numerous branches armed with recurved thorns as the claws of a cat. It often forms impenetrable thickets providing cover for small mammals and birds. The seeds require physical scarification in order to germinate. This prevents germination unless there is enough water for seedlings to establish deep enough roots to survive the dry season. Catclaw is fully drought deciduous and usually lacks leaves most of the year. Glandular extra floral nectaries on the stems attract insects. Ants are known to use nectaries as a food source and in turn provide the plant some defense against herbivorous insects. Lac insects inhabiting catclaw produce a residue that is used for varnish and shellac.

Invertebrates

Creosote bush is distinctive in both chemistry and appearance. It hosts several insect species, such as the creosote bush grasshopper, that live nowhere else but on creosote bush. It is presumed that the creosote bush grasshopper (a North American insect) did not exist in its current form before the arrival of the creosote bush (a South American plant). Presumably this species of grasshopper evolved as a response to the introduction and eventual dominance of a new resource.

Female gray velvet ants resemble creosote bush seeds and share range almost exclusively with creosote. The winged males are black with orange hair and look nothing like the females. The females are parasites of the larvae of sand wasps. The females lay their eggs in the burrows of sand wasp larvae. After hatching, the velvet ant larvae feed upon the sand wasp larvae. Pupation occurs in the larval chamber of the host.

Whip scorpions are related to spiders. They are nocturnal and hide during the day beneath leaves, rocks, logs, and other debris. The pedipalps are stout and end in pincers used for seizing insect prey. The first pair of legs is very long and is not used for walking, but instead has sensory-tactile function. A long tactile flagellum is at the end of the abdomen. When irritated, the whip scorpion elevates the end of its abdomen and discharges a stream of formic or acetic acid from a pair of anal glands in self-defense. The repellent odor of this fluid has given the whip scorpion the nickname “vinegarroon.” Vinegarroons are not venomous and are completely harmless to humans.

Colonies of honeypot ants develop specialized workers, called repletes, with tremendously swollen abdomens for nectar storage. The repletes gorge on food that is collected on the outside of the nest in times of abundance by normally proportioned worker ants. The repletes hang quietly in clusters from the ceilings of deep underground chambers. In times of need, the repletes regurgitate food back for the colony members.

Reptiles

The Texas banded gecko inhabits rocky desert areas. It is not a good climber, so it is found near ground level. After dark it emerges from hiding under rocks to forage for insects and spiders. When alarmed it may curl its tail over its back and run with it elevated resembling a scorpion. It emits faint squeaks when molested, involved in territorial disputes, and during breeding.

The roundtail horned lizard has a crown of four short horns that are about equal in length and separated at the base. It can also be distinguished from other species of horned lizards by the absence of a fringe of scales along its sides and a lack of ear openings. The overall coloration varies depending on its immediate habitat: it so closely matches the color and texture of the soil of its environment as to be nearly invisible unless in motion. It inhabits arid or semiarid desert plains with scrub vegetation and sandy or gravelly soil. It primarily feeds on ants.

Trans-Pecos rat snakes are widespread and sometimes locally common, though they only venture out at night and in warm weather. They predominantly occur in rocky terrain in the northern Chihuahuan Desert and oak-juniper woodland islands of West Texas’ mountains. The Trans-Pecos rat snake is adapted to its environment by restricting its movements on the surface to dry, temperate summer nights. During the day it withdraws into mammal burrows or to subterranean niches in broken rock of the desert floor. The Trans-Pecos rat snake is non-venomous and seldom defends itself against humans.

Related Resources

The following resources for learning more about the Shortgrass Plains and Chihuahuan Scrub habitat are available on this website:

Photo Essays:

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Sibley Nature Center
1307 E. Wadley, Midland, Texas 79705
phone 432.684.6827
email bwilliams@sibleynaturecenter.org