The Desert Grasslands

The Desert Food Chain - Part 6


“Man’s existence,” said Edward Knobel in his Field Guide to the Grasses, Sedges and Rushes of the United States, "depends directly or indirectly, almost entirely on the grasses, a fact which should make this part of the vegetable kingdom the most interesting to us.  Aside from their usefulness, their beauty and graceful forms are unsurpassed by any other plants…”  In our desert Southwest, the Puebloan peoples, for one example, depended on a grass, specifically corn, so heavily that it stood not only at the core of their food supply but of their spiritual life as well.

Origin and Range

According to the Audubon Society’s Grasslands, written by Lauren Brown, the grasses began to evolve 70 to 80 million years ago, late in the Cretaceous Period, when the Southwest’s Rocky Mountain chain arose, the polar ice melted, the seas advanced and the dinosaurs still thrived.  Emergent grasses, as producers, gave rise to new invertebrate and vertebrate animal communities, or consumers, giving new dimension to the food chain.  Benefiting from frequent wildfires, which released nutrients from dead plant matter and inhibited the growth of botanical competitors, the grasses, with some 10,000 species, would lay claim to a quarter of the earth’s surface, especially the flat or rolling plains.  They became the “third largest family of flowering plants in the world…” according to Brown.  “Grasses are found from pole to pole,” she said, “and contain more species that are distributed worldwide than any other  family of plants.”

Our Desert Grasslands

At the time of the Puebloan zenith in the Southwest, the desert grasslands began at the Pecos River.  They spanned what is now western Texas, southern New Mexico and northwestern to southeastern Arizona plus much of northern Mexico.  Unlike the Great Plains of the American midwest, where solid stands of “tall,” “mixed” and “short” grasses blanketed the landscape like an ocean from horizon to horizon, the desert grasslands emerged in a crazy-quilt pattern.  They developed “in the basins and valleys that skirt the hills and mountain ranges of southwestern North America,” Mitchel P. McClaran said in “Desert Grasslands and Grasses,” The Desert Grassland.  They took on the character, not of classic fertile prairie grasslands, but more of an arid shrub savannah, with stands of grass punctuated by a diversity of plants such as Creosote Bush, the mesquites, cacti, yuccas and forbs (plants – other than grasses, sedges and rushes – that have non-woody stems).  The desert grasslands have long produced most of their growth during the late summer, when the deserts receive most of their annual rainfall. 

Anatomy of Grass

 Unlike prairieland grasses, which usually grow in more or less uniformly across the landscape, the desert grasses, often called “bunch” grasses, typically grow as closely spaced mounds, some as ephemerals (blooming and dieing within a single season), others as perennials (living for more than a single season).

Like its kin throughout the world, a desert grass plant produces several stems, or “tillers,” from its base, or “root crown.”  Each tiller resembles a series of segments joined, like a pipeline, end to end.  (The tiller of a bamboo, the largest of the grasses, serves as a vivid illustration of the jointed segment appearance.)  Each segment of the tiller has its own two-part leaf.  One part, the sheath,  fits like a sleeve along all or most of the length of its host segment.  The second part, the blade, flares away from the tiller at the segment’s uppermost “joint,” or “node.”  The leaf blades therefore alternate, one occurring at each node, along the length of the tiller.   The tiller produces its flowers near its top, within structures called “spikelets.”  In our deserts, the flowers’ pollen scatters with the wind from plant to plant, according to McClaran.

The desert grass plant’s primary adaptation to an arid environment lies in its shallow, dense and labyrinthine root system, which botanists describe as “fibrous.”  Given such a root structure, a desert grass is a “very effective” competitor for “the limited shallow soil moisture,” according to Tony L. Burgess, “Desert Grassland, Mixed Shrub Savanna, Shrub Steppe, or Semidesert Scrub?” The Desert Grassland.  The plant’s first, or “primary,” roots form during the seedling stage.  Its later, or “secondary” or “crown,” roots form during maturation.  Some grass plants’ root system biomass (organic tissue) exceeds that of the stems, leaves and spikelets.  

Typical Desert Species

“The dry climate of the desert grassland dictates a dominance of short, warm-season bunchgrasses,” said Lauren Brown.  During the Puebloan times, the “southern,” or warm-season gramas (especially Black Grama), the Tobosa, the Sacaton and the Curly Mesquite probably ranked high on the list of the most prevelant grasses across our deserts. 

Black Grama, which stands one to two feet tall, grows in sandy basins below 7000 feet in elevation.  Highly nutritious, it has crooked, tangled, wooly grayish-green stems, according to New Mexico State University’s Circular 374, “New Mexico Range Plants.”  Its spikelets resemble a human eyebrow. 

Tobosa, also one to two feet tall, grows in basins and on talus slopes at 3000 to 6500 feet in elevation.  Especially nutritious during its growing season, Tobosa has smooth bluish-green stems with leaf blades about six inches in length.  Its mature spikelets, white in color, weigh on the stems, causing them to droop, assuming a zigzag shape. 

Sacaton, three to six feet tall, grows in flood-prone basins and flats between 3100 to 7000 feet in elevation.  Palatable to grazing animals during its growing season, Sacaton has grayish-green stems with foot-long leaf blades.  Its open, multiple-branched spikelets may cover one to two feet of the stem. 

Curly Mesquite, less than a foot tall, grows in gravelly soils between 3800 and 8500 feet in elevation.  Palatable, persistent and aggressive, the grass has woolly, bluish-green stems with a profusion of leaves that curl tightly as they cure in late summer and fall.  The spikelets cause the slender stems to assume a characteristic zigzag shape. 

Other widespread desert grassland species include several other gramas, Green Sprangletop, Bush Muhly and Sand Dropseed.  

The desert grasses and their associated plants in the shrub savannas support complex communities of invertebrate and vertebrate animals. 

The Invertebrates

“The invertebrate fauna of desert grasslands is incredibly diverse…” Walter G. Whitford, Gregory S. Forbes and Graham I. Kerley said in their paper, “Diversity, Spatial Variability, and Functional Roles of Invertebrates in Desert Grassland Ecosystems,” The Desert Grassland.  “While mammal, bird, reptile, and vascular plant species occur in the tens to hundreds, invertebrate species in desert grasslands number in the thousands or tens of thousands” 

Examples of desert grassland invertebrates that feed on the plants range from the microscopic protozoans and nematodes to soil mites to molluscs to the multi-legged millipedes to, of course, the insects.  The protozoans – single cell animals that feed on single-celled plants as well as on bacteria – occur in mind-boggling abundance, more than three quarters of a million in an ounce of dry desert soil, according to Whitford, Forbes and Kerley.  The nematodes – elongated cylindrical worms that feed on roots and other plant structures as well as bacteria and fungi – also occur in abundance, more than 100,000 in a square meter of desert soil.  The mollusks, or snails and slugs, feed on plant materials above the surface of the ground.  Some species of mites – relatives of spiders and scorpions – feed on grasses and plant debris.  The seven- or eight-inch-long millipedes, non-venomous animals that look something like segmented worms with two pairs of legs for each segment, feed on dead plant matter. 

Many insects, for instance, various grasshoppers, cicadas and seed-eating harvester ants, take their customary places at the desert grasslands dining table.  Female grasshoppers “lay their eggs in very specific kinds of places, usually on a gentle hillside with isolated clumps of grass,” according to Floyd Werner and Carl Olson in their Insects of the Southwest.  “The hoppers that hatch in the spring depend on a good local supply [of the plants for] food.”  Cicadas, for instance, the common Apache Cicadas, live underground for years as nymphs, “making smooth tunnels that they use to get to the roots of plants, into which they insert their sucking mouth parts.”  When they emerge from the earth, the males call for mates, issuing the signature sound of a summer evening in the desert.  Harvester ants seek out grassy areas that promise a high level of seed production.  In fact, said Werner and Olson, “A good grass stand generates a large number of [harvester ant] colonies.” 

Although the invertebrates have usually attracted comparatively little attention because they are small, inconspicuous and often nocturnal creatures, they comprise the vast majority – well over 90 percent –  of the animal species of the desert grasslands.   Moreover, as suggested by the University of Arizona’s Books of the Southwest, “5. Animals and Soil in Arizona,” the combined weight of the invertebrates may exceed the combined weight of the vertebrates by several times within a given area.

The Vertebrates

“Desert grasslands form an important habitat for a large number of southwestern vertebrates—not only the specialists that live exclusively in the grasslands, but also the many species that occupy adjacent riparian zones, desertscrub, and pinyon-juniper woodland,” said Robert R. Parmenter and Thomas R. Van Devender in their “Diversity, Spatial Variability, and Functional Roles of Vertebrates in the Desert Grassland,” The Desert Grassland.  Many of the vertebrates of the neighboring environments “make frequent forays into nearby desert grassland in search of food.”  The principal plant-eating vertebrates in the food chain of the desert grassland include birds and mammals. 

“Bird species,” say Parmenter and Van Devender, “are far more numerous in desert grassland than any other group of vertebrates.”  Bird species become especially abundant, reaching the hundreds, in those areas where the desert grasslands converge with other ecological communities such as riverine systems and mountain foothills.  The ebb and flow of the bird population and diversity in the desert grasslands largely reflect the availability of seeds, say Parmenter and Van Devender.  “Peaks in resident bird densities generally coincide with maximum seed production in late summer,” following the rapid growth triggered by the seasonal rains.  “Many species of northern birds overwinter in the desert grasslands, where temperatures are relatively mild and grass seeds left over the from the summer crop are widely available.”

Mammals include numerous rodents, the omnivorous Javelina and the browsers—the  Mule and Whitetail Deer and the Antelope, or  Pronghorns.  Surprisingly, well into the Puebloan era, the mammal population also included that symbol of the grassland grazing animals, the Buffalo, or Bison.  For poorly understood reasons, this animal had largely disappeared from the desert grasslands by the time the Spanish began to explore and colonize the Southwest, abandoning the grasses to various rodent species, for instance rabbits and the prairie dogs, which became the primary consumers of the grasses.

In years of high mammal populations, Desert Cottontails and Blacktail Jackrabbits probably accounted for the consumption of much of the available forage.  “On the Santa Rita Experimental Range in southern Arizona, desert cottontails (Sylvilagus auduboni) and jackrabbits can consume as much as 40 percent of the forage production,” said Parmenter and Van Devender. 

Prairie dogs took on the proportions of a plague in the desert grasslands.  Early in the 20th century, “prairie dog towns were estimated to have covered 2.5 million ha [one ha, or hectare, equals about 2.47 acres] of choice grazing land in [Arizona],” according to Parmenter and Van Devender.  One writer, quoted by Parmenter and Van Devender, said that the “Animas Valley [roughly 300 square miles in southwestern New Mexico] was an almost continuous prairie dog town for its whole length and breadth.  In many places where rain had missed a part of the valley the prairie dogs had taken all the season’s vegetation and made barren deserts miles in extent.”  The writer estimated that the prairie dog population of southwestern New Mexico’s Grant County would equal 6,400,000.  Since 256 prairie dogs ate as much grass as one cow, the population would consume forage that would feed 25,000 cows.  Ranchers did not like prairie dogs.  They exterminated them.

The Human Factor

As it turned out, the only mammals that would affect the food chain of the desert grasslands more than prairie dogs were cattle, which have changed the botanical face of the landscape.  According to the U. S. Geological Survey’s web site, Science for a Changing World, the desert grasslands “have been declining for more than a century,” as a result of livestock grazing.  “Grazing pressure was severe across the range in the last half of the nineteenth century; in Arizona alone livestock numbers rose from 5000 in 1870 to 1,500,000 in 1891  As a result, these grasslands were greatly altered by 1900 and this trend is continuing   Only a few relict areas in southern Arizona continue to support semidesert grasslands in the Sonoran Desert.

“Historically, in the Chihuahuan Desert, most of the range was grazed extensively; it continues to be grazed even though plant and livestock production is low.  As a result, desertification resulting in conversion of desert grassland to desert shrubland since the 1850’s has been well documented ”

As a consequence of the overgrazing – human intervention in the natural food chain – “Extant Chihuahuan semidesert grasslands in good condition in New Mexico and western Texas are rare today.”

“Man’s existence,” said Edward Knobel in his Field Guide to the Grasses, Sedges and Rushes of the United States, “depends directly or indirectly, almost entirely on the grasses…”

Next Desert Shrubs

by Jay W. Sharp

Index

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
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


We also have a Desert Food Chain series aimed at explaining the desert food chain to young people, we invite you to explore with us the pathway that energy, beginning with sunlight, follows as it flows through communities of plants and animals that make up food chains. Click Here

 

 

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