Outlaw Desert Plants

Desert Food Chain 10

by Jay W. Sharp

In the best of times, drought, intense heat, variable weather and organically impoverished soils conspire to make desert food chains a game of chance, a stressful venue in which only the most biologically nimble and resourceful of the plants and animals can adapt and survive.  In the worst of times, drought, intense heat, variable weather and organically impoverished soils, in collusion with “humanized” desert landscapes, changed wildfire patterns and aggressive native and non-native plant and animal species, can completely reshuffle desert food chains, sometimes inflicting severe damage on interconnecting links.  For invasive plants – botanical outlaws that have spread across the desert landscape like the Mongol hordes of Ghengis Khan – the worst of times has been the best of times. 

  

A desert basin, once a patchy grassland, now invaded by mesquite and creosote.

Before the Invasion

Before settlers with European roots came with their vast livestock herds and their iron and steel plows and axes to colonize the Southwest, a “patchy vegetation of grass, shrubby trees, succulents [e.g., the cacti], rosette plants [e.g., the yuccas], and subshrubs—a distinctive species assemblage unlike that of any other North American landscape,” grew in the desert basins, according to Tony L. Burgess’s paper “Desert Grassland, Mixed Shrub Savanna, Shrub Steppe, or Semidesert Scrub?” The Desert Grassland.  Shrubs such as fourwing saltbush, catclaws, various sumacs, and desert willow grew as thickets along arroyos.  Gallery forests of various cottonwood, willow and mesquite species grew in the floodplains of the deserts’ meandering permanent and intermittent streams, especially those connected to the Rio Grande and Colorado River systems.  The plant communities, particularly in the basins, drew much of their character and relative plant abundances from periodic fires ignited by the dry lightning storms of the deserts’ monsoon seasons.  The fast-burning but also fast-growing grasses recovered quickly, maintaining a dominant position.  The shrubby trees, succulents, rosette plants and subshrubs grew back more slowly, restrained to more marginal positions. 

The desert grass and shrublands and the drainages supported a community of vertebrate animals that consisted of mammals such as bison (up to late prehistoric times), elk, mule deer, pronghorn, grizzly bear, black bear, coyote, foxes, big cats and many rodents; numerous bird species ranging from hummingbirds to eagles; reptiles including various lizards and turtles, some one dozen types of rattlers, a coral snake, and numerous non-poisonous snakes; and amphibians including frogs, toads and salamanders.  The deserts’ vegetation also sustained an abundant and diverse collection of invertebrate animals, for instance, protozoans (single-celled animals), arachnids (soil mites, scorpions, spiders, vinegaroons), millipedes and scorpions, beetles, butterflies and moths, crickets, grasshoppers, flies, ants, bees and wasps. 

Desert landscape, a former grassland, now covered by mesquite and creosote.

The Landscape Altered

With the coming of the European colonists, especially those who migrated into the deserts from the eastern half of the United States after the Mexican American War (1846 to 1848), the desert grasses begin to give way before great herds of livestock.  “By the late 1800s and continuing throughout the 1900s,” said Matthew L. Brooks and David A. Pyke in their paper “Invasive Plants and Fire in the Deserts of North America,” “all desert ecosystems experienced unsustainable levels of inappropriate seasons of livestock grazing.”  The grasses could withstand wildfire but not total denudation. 

Trees along drainage areas disappeared from the food chain and reappeared as building timbers and firewood.  Desert grass and shrublands and riverine thickets and forests became fractured, isolated like islands, or vanished altogether in making way for fields and development.  Some species, especially the voracious and almost innumerable prairie dogs, that competed with commercially valuable animals for plant foods became government statistics expressed in terms of body counts.  The deserts’ plant community, with depletion of natural fuel and the misguided prevention efforts of the human community, experienced a substantial change in the frequency and patterns of wildfires. 

Denuded desert landscape with foundation of early house.

In the newly barren, cleared and fragmented desert landscape – now forced open for botanical colonization, virtually free of prairie dogs, encouraged by altered wildfire patterns – the aggressive home-grown and newly introduced plants saw unprecedented opportunities.  They seized the advantage.  Well-adapted native shrubs, long held at bay by the resilient grasses and the wildfires, quickly extended their reach across the desert.  Introduced grasses, weedy plants and trees swiftly conquered the desert basins and the drainages vacated by the natives. 

The Invasion and Its Consequences

Among the most prominent of the native plants that have slipped their traditional botanical bonds and mounted a territorial conquest are the mesquite and the creosote bush, both superbly adapted to the desert environment, together, the Butch Cassidy and Sundance Kid of the Southwest. 

As we have said earlier (see Part 7, Desert Shrubs), the mesquite – which include the western honey mesquite, centered the Chihuahuan Desert; the velvet mesquite, centered in the northern Sonoran Desert; and the screwbean mesquite, centered in the northern Sonoran and the Mojave Deserts – have small compound leaves that minimize transpiration (water evaporation).  They come armed with thorny stems that help protect the plant from large browsing animals.  They have far-reaching root systems that spread both laterally and deeply in a resolute reach for water.  (In fact, velvet mesquite may send its roots 150 feet or more downward to reach a water table.)  They have high resistance to insects and disease.  They produce nutritious and palatable seeds that are consumed by the larger grazing and browsing wildlife and domestic livestock that then serve as agents for distributing the plant across the landscape. 

Almost pure creosote bush stand on a rocky desert slope.

The creosote bush may be even more ideally adapted to the desert, indeed, the only plant that may survive and reproduce in the harshest desert conditions.  It has small leaves with a resinous coating that reflects hot desert sunlight and restrains transpiration.  Further, the plant turns the edges rather than the flat surfaces of its leaves toward the sun, further minimizing exposure to solar heating.  Its root system, according to Jack C. Schultz and Ted Floyd in their article “Desert Survivor – North America’s Creosote Bush,” in the American Museum of Natural History’s Internet site, may extend laterally across 400 to 500 square feet and downward for some 15 feet.  The creosote bush seizes tyrannical control of available soil nutrients.  Its roots may even contain compounds toxic to neighboring plants.  The creosote bush produces small velvety seeds that ride the wind, and animals’ coats, into new territory.  Its lateral roots produce clones that sometimes circle protectively about the mother plant, yielding living tissue with a continuity that may extend back over thousands of years, a history possibly longer than any other plant on earth. 

Together, the mesquite and the creosote bush, no longer constrained by wildfire, have taken the denuded desert landscape as a welcome mat. 

Previously confined primarily to drainage channels, the mesquite have invaded basin grasslands, mesas and mountain foothills throughout the Southwestern deserts, becoming perhaps the most widely distributed of the shrubs.  Beyond the desert, honey mesquites, within my lifetime, have completed the conquest of heavily overgrazed pasturelands (including those that belonged to my father) near my boyhood home in the rolling plains of Texas, virtually eliminating the native grasses.  The mesquite is a designated “Weed of National Significance” in Australia, where the plant was introduced in the late 19th century. 

In an example of its aggressiveness, the creosote bush expanded its coverage at the site of Arizona’s Santa Rita Experimental Range by seventy times in fifty years, from about 950 acres in 1904 to 11,900 acres in 1934 to nearly 70,000 acres in 1954, according to the U. S. Department of Agriculture Forest Service Fire Effects Information System (FEIS) Internet site.  Today, creosote bush dominates, or co-dominates, roughly 40 million acres in the Southwest, blanketing some 40 percent of the Chihuahuan Desert, a large part of the Sonoran Desert and 70 percent of the Mojave Desert. 

As mesquite and creosote bush gain a stranglehold, they crowd out grass cover, depriving an old enemy, wildfire, of fuel.  They can then close their grip still tighter, shutting off virtually any possibility for revival or restoration of a desert grassland. 

Mesquite

If the native-born mesquite and creosote bush have led the charge to convert the overgrazed landscape of the Southwest from a patchy grassland into a desert shrubland, multiple imported outlaw plants such as cheatgrass, Russian knapweed and salt cedar are leading an even more insidious invasion. 

Cheatgrass, an winter annual native to Europe and Asia, found its way into the Southwest as a stowaway contaminant in shipments of seeds during the mid- to late 19th century.  It is even more suited than the native grasses to our arid environment.  According to the Land Use History of North America Internet site, “Cheatgrass usually germinates in fall and grows during the winter, opposite the cycle followed by common native perennial grasses.  By the time the rain stops in spring, cheatgrass already is maturing its seeds.  Unlike native bunchgrasses, cheatgrass then dies by the end of July, avoiding the hottest and driest part of summer.”  Dead cheatgrass then serves as a source of abundant fuel for wildfires that kill out competing native grasses.  It produces seeds that germinate early, capitalizing on nutrients released by the fires.  It develops extensive root systems that commandeer water.  Its spikelets, according to eNature.com, break into sharp-pointed sections that can stick like spines into the faces and eyes of foraging wildlife and domestic animals.  The Land Use History of North America Internet site says that, “...native grasses inevitably decline, and so over time, cheatgrass becomes more and more common until eventually it dominates.  Cheatgrass often opens the way for secondary invaders such as knapweed and thistle.”

Russian knapweed, a creeping perennial plant native to southeastern Russia, southern Ukraine, Iran, Kazakhstan and Mongolia, found its way into the Southwest after arriving in the United States in the early 20th century as a contaminant in imported alfalfa seed.  While it produces seeds, the Russian knapweed relies primarily on vegetative root propagation to stake its claim indiscriminately in rocky, sandy or clay soils or in saline bottoms, according to Colorado State University’s K. G. Beck (see the University’s Cooperative Extension Internet site).  It expands its black to brown scaly root system rapidly, covering as much as 36 square feet within just a couple of growing seasons.  As it grows, it produces biochemicals that inhibit the growth of native plants.  Driving out all neighboring plants, Russian knapweed finally establishes dense solid stands that can survive unchallenged for many decades.  While it's at it, it poisons horses. 

The infamous salt cedar, or tamarisk, a multiple-branched shrub or small tree that originated in southern Europe and Asia Minor, according to the FEIS Internet site, came to the United States as an invited ornamental guest in the 19th century.  Perfectly happy in the desert’s high heat, drought and poor soils, the salt cedar greedily appropriates any available water, including highly mineralized water, through both deep and shallow root systems.  In fact, a single large salt cedar plant can absorb 200 gallons of water a day, according to the Washington State Department of Ecology Internet site.  It exudes salt in the form of crystals through its leaf openings, or salt glands, effectively poisoning the surrounding soil for other plants.  According to Jason Hart in his paper “Invasive Species in the Southwest: Tamarix sp. (Salt Cedar),” the plant “has been shown to tolerate up to 36,000 ppm [parts per million] salt salinity [in the soil], whereas native floodplain species such as willow and cottonwood can only tolerate up to 1,500 ppm...”  Salt cedar spreads not only vegetatively, through its roots, it produces prodigious numbers of tiny seeds that scatter with wind and water along stream bottoms and tributaries.  The plant may grow 10 feet or more in a single season, and it has established dense solid stands along drainages, including the Grand Canyon, with each plant using its 200 gallons of water a day. 

Cheatgrass, Russian knapweed and salt cedar have all asserted their unwelcome presence in the Southwest.  Cheatgrass, in fact, has been introduced throughout the United States, according to the U. S. Department of Agriculture’s Natural Resources Conservation Internet Site.  Well established in the Great Basin, cheatgrass, with its progress accelerated by  overgrazing and wildfires, has begun “invading grasslands in the southwestern United States,” the International Arid Lands Consortium said in a news release, “International Arid Lands Consortium Investigates a Major Contributor to Wildfires in the Southwestern United States.”  Russian knapweed, after finding toeholds along roadsides, riverbanks, ditches and croplands, had established its presence, according to the FEIS Internet site, in some 5,000 acres in Arizona, 15,000 acres in New Mexico, 60,000 acres in Utah and 168,000 acres in Colorado by the year 2000.  Salt cedar, says the FEIS Internet site, now “occurs in every major watershed” in the southwestern United States.  “Salt cedar has almost completely replaced the native forest that historically dominated the riparian corridor from the Grand Canyon to the delta on the Gulf of California.  It is by far the most abundant plant in the Colorado River delta, accounting for 40% of total ground cover.”  It has replaced cottonwood and willow gallery forests that once hosted a great diversity of breeding bird species.

The Effects on the Food Chains

The effects of home-grown and non-native plant invasions and altered wildfire patterns have “repercussions on the higher-order organisms in foodwebs,” said Brooks and Pyke.  One researcher, they said, found that at one Sonoran Desert site, invasive plants had “reduced species numbers of birds (7 of 9 species), rodents (3 of 7 species), and grasshoppers (7 of 9 species).”  Further, “Old-world invasive grasses now dominate many shrublands in the Great Basin, Mojave, and Sonoran deserts.  Such grasses include the annuals cheatgrass, red brome, Mediterranean grass, and medusahead, and the perennials bufflegrass..., fountain grass..., natal grass..., and Lehman lovegrass...”  The outlaw plants undercut biological complexity and diversity—important measures of the health of ecological systems.

In another example offered by Brooks and Pyke, they say, “The invasion of riparian areas by the invasive salt-cedar has caused water tables to drop due to its very high rate of evapotranspiration…  As water tables drop, the availability of surface water declines and native riparian plants begin to senesce [decline].  With the loss of native riparian plants and surface water, native wildlife generally decline[s] as well…such as the desert big-horn sheep that depend so critically on sources of perennial water.”  In tamarisk-dominated stands, said Cameron W. Barrows  in his paper “Tamarisk Control and Common Sense,” “biologists have universally found depauperate [impoverished] wildlife populations...”

The U. S. Geological Survey Western Ecological Research Center, in its Internet site, said, “With the invasion of fine-fuel species like exotic annual grasses the fire cycle has been significantly shortened and the potential for fires to propagate has increased. The result has been conversion of desert scrub landscapes to ‘weedscapes’ dominated by exotic pest plants. It is hard to imagine that this has not had a negative impact on tortoises and other species that coevolved with plants in the desert ecosystem.”

Invasive plants, in the wake of overgrazing, land clearing, dam construction, river channeling, agriculture, development, recreational vehicle and military exercises, are reshaping the face of our deserts, and they are in the process of re-defining the food chains of our deserts.  It is a story that is still unfolding, with an outcome filled with uncertainty. 

In this article, the tenth in the DesertUSA series on the food chains of the desert, we focus on some invasive species of the desert plants and how they are altering the food chain.  For background, you may wish to review “The Desert Food Chain,”  See links at end of this page.

More on the Desert Food Chain


Part 1 Desert Food chain - Introduction
Part 2 Desert Food chain - The Producers
Part 3 Desert Food chain - The Cacti: A Thorny Feast 
Part 4 Desert Food chain - The Yuccas
Part 5 Desert Food chain - The Agave
Part 6 Desert Food chain - Desert Grasslands
Part 7 Desert Food chain - Desert Shrubs
Part 8 Desert Food chain - The Annual Forbs
Part 9 Desert Food chain - Mavericks of the Desert Plant
Part 10 Desert Food chain - Outlaw desert plants (This Page)
Part 11 Desert Food chain - Animals: The Consumers
Part 12 Desest Food chain - The Insects
Part 13 Desest Food chain - The Ugly, the Uglier and the Ugliest

Also see: The Desert Food Chain for the young student

 

      
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