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Habitats of the Llano Estacado
Alkaline Soils

Introduction

Alkaline soils contain high concentrations of salt. In arid lands rainwater dissolves salts and minerals out of the soil. In areas where water collects into playas the soil becomes salty. Silt and dust blow out of large salt playas forming alkaline deposits. Some streams on or near the Llano Estacado, such as the Pecos River, have alkaline (gyp) water and soil deposits with high pH values. Alkaline soils are corrosive and often contain high concentrations of heavy metals. Gyp water has a laxative effect.

The bodies of plants and animals are composed of cells surrounded by semi-permeable membranes. Osmosis is the passage of water through a semi-permeable membrane from a region of high water concentration (low solutes) to a region of low water concentration (high solutes). Because alkaline soil environments have low water but high solute concentrations, water is lost from the tissues of plants and animals.

Plants and animals must be adapted to living in a “physiological desert” in an alkaline environment. The process that regulates the amount of water and salts in living tissues is called osmoregulation.

Plants

Alkaline soils make up less than two percent of the total habitat area of the region. Some alkaline-adapted plants are endangered and many are considered rare. Plants can either be succulent or excrete excess salts as an adaptation for living in an alkaline environment. Pickleweed has succulent stems and scale-like leaves. Water is stored in the tissues in the form of a gel. Very few grazing animals or insects can eat pickleweed. Saltbush excretes excess salts. Often there are tiny salt crystals on the leaves. Deer and cattle can eat young saltbush leaves.

Invertebrates

An insect’s small size makes it readily susceptible to water loss by evaporation from the body surface. The insect’s body wall provides protection from evaporation, though. Insects that live in saline environments are able to get rid of excess salts by means of structures called Malpighian tubules that arise from the anterior end of the hindgut. There are also glands in the rectum that remove excess water and salts from digested materials. Not all waste products and salts are removed by the Malpighian tubules. Some excess salts and other substances are deposited in the cuticle to be disposed of at ecdysis---the shedding of the exoskeleton.

Tiger beetles are swift predators of other insects, hunting along the waterline of a playa. Their larvae live in vertical burrows in the soil. They prop themselves at the entrance of their burrows, and with huge jaws open, wait to capture a passing insect.

Odonates (dragonflies and damselflies) are among the most beautiful and beneficial of insects. They are voracious predators, both as aquatic larvae and adults, feeding on other insects, especially mosquitoes. Many species are adapted to alkaline environments.

Adult brine flies (shore flies) are found in moist places. The larvae are aquatic and many species occur in strongly alkaline water. The flies often occur in enormous numbers and arise from the ground in clouds.

Mammals

Life depends on the maintenance of an internal balance of water and salt within fairly narrow limits. Water loss can occur rapidly in an alkaline environment. The kidneys are the main organs of osmoregulation in mammals. Mammalian kidneys filter the blood, reabsorb water and salts, regulate pH of body fluids, excrete nitrogen-containing compounds from protein metabolism, and excrete metabolic waste products in the form of urine.

Some rodents, such as the western harvest mouse, obtain water from succulent plants that contain high salt concentrations. They have kidneys that are able to produce highly concentrated urine with little water relative to the contained solutes.

The banner-tailed kangaroo rat maintains its water balance by a combination of physiological and behavioral adaptations. Since they do not drink water, kangaroo rats obtain their water from the metabolism of high-carbohydrate foods such as seeds. The absence of evaporation from the skin, the reduction of respiratory water, and the production of highly concentrated urine minimize water loss.

Birds

The maintenance of the correct proportions of water and salts within cells, the spaces between cells, and the blood is critical to the survival of an animal. All birds have paired salt glands located in a depression on the skull near the eye sockets. Salt glands vary in size, shape, and degree of function, but are best-developed in seabirds. Excess salts in the diet are dissolved in a clear fluid that flows from the salt glands through ducts in the nasal cavities and, by way of the nostrils or mouth, out to the tip of the bill in droplets which the bird shakes off. The size of the salt glands appears to be determined by individual adaptation to the amount of salt in the diet.

Sandhill cranes feed in agricultural fields on grains, insects, rodents and small reptiles and birds. In the evenings they flock to stand in salt playas to avoid coyotes. In the summer, sandhill cranes migrate to their nesting territory in Alaska, northern Canada, and Siberia.

Snowy plovers favor the shores of salt playas. They forage at the water’s edge eating insects such as tiger beetles and brine flies. They lay their eggs in scrapes on bare alkaline soil. In the winter, snowy plovers migrate to South America.

Fishes

There are great differences among freshwater fishes with regard to salt tolerance. Freshwater fishes that are generally adaptable to changes in water salinity are called euryhaline. Euryhaline fishes have the ability to osmoregulate efficiently in waters of highly variable salinity and can adapt rapidly if sudden changes occur.

Several species of Gambusia and pupfish are adapted to alkaline water and are unique to specific springs or rivers. Some species, such as the Leon Springs pupfish, are threatened by the drying of the spring and hybridization with other species of pupfish. Introductions of non-native fish such as the mosquitofish, gulf killifish, and the sheepshead minnow have contributed to the elimination of many populations of native fishes with similar ecological requirements.

Related Resources

The following resources for learning more about the Alkali habitat are available on this website:

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1307 E. Wadley, Midland, Texas 79705
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