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Southwest Adventure, Living & Travel


Desert Food Chain - Desert Food Web

Overview

A food chain constitutes a complex network of organisms, from plants to animals, through which energy, derived from the sun, flows in the form of organic matter and dissipates in the form of waste heat.  The food chain’s biological productivity and species diversification depend on factors such as the daily duration and angle of seasonal sunlight, the timely availability of water, the daily swings of seasonal temperatures, the chemical content of the soils, and the availability of nutrients. 

The food chain complies with two of the most basic notions in biology.  First, it has an energy source, in this case, the sun, and an energy “sink,” in this instance, space.  The sun fuels the work required for biologic processes.  Space receives the waste heat produced by the work.  Otherwise, temperatures would rise to the point that the community of organisms would perish.  Second, by definition, a food chain comprises a system of interdependent species.   A single isolated species would sooner or later consume the supply of chemicals it needs to live, grow and reproduce.  It would perish. 

Producers and Consumers

In a food chain in our Southwestern desert region – as in a food chain in any other biologically distinctive region, or “biome,” on earth – it is the plants, or the “producers,” that capture the energy from the sun and initiate the flow, becoming the first link in the chain.  In an almost magical-seeming process called “photosynthesis,” which means “gathering of light,” all plants – from one-celled diatoms to mesquite and creosote shrubs to the towering saguaro cactus to riverside cottonwoods and willows – use the sun’s energy, with water and carbon dioxide, to produce a carbohydrate, or sugar, called “glucose,” a basic component in the food chain.  The plants then use the glucose to produce the carbohydrates, proteins and fats required for reproduction and growth, drawing nourishment from various soil nutrients, for instance, nitrogen, phosphorus and potassium.  As producers, the plants, in effect, create storehouses of solar energy, setting the dinner table, often impoverished in the desert, for the animals, the “consumers.” 

Bumblee, primary consumer that feeds on plant nectar.

Plant-eating animals – the herbivores, or “primary” consumers – become the second link in the food chain.  Flesh-eating animals – the carnivores, or “secondary” and even “tertiary” consumers – become the next links.  Plant and flesh eaters – the omnivores, like human beings, for example – span two or three links.    Scavengers, or the detritivores, become the next link in the food chain, and microorganisms, or decomposers, the final consumer link.  Decomposers free up nutrients for recycling within the food chain. 

Ladybird Beetle, secondary consumer that feeds on plant-eating insects such as aphids.

 

In eating plant and/or animal matter, consumers are, in effect, “fueling up” on stored solar energy, although they surrender the great majority of it as waste heat.  At each of the food chain links – called “trophic levels” – the consumers give up roughly 90 percent of the energy they ingest.  This means that 100 units of plant energy are required to sustain the 10 units of herbivore energy that are required to sustain one unit of carnivore energy.  For example, 100 units of grass and shrub energy are required to sustain the 10 units of desert cottontail energy that are required to sustain one unit of red-tailed hawk energy.  It also means that the producers – the plants – constitute 90 percent of all living matter, or “biomass,” in a biological system such as a food chain, and that the consumers – the animals – account for only the remaining 10 percent.  Plant productivity, always tenuous in our hard and unforgiving deserts, can impose severe limits on the consumer population.   

The Southwest Deserts vs Tropical Rainforests

Our Southwest deserts, which rank among the least biologically productive biomes on earth, resemble a biological wasteland in comparison, for instance, to tropical rainforests, which rank among the most biologically productive biomes.  The contrast reflects differences in those factors that impose limits on biological productivity and diversity.

In our deserts, which lie about 2500 to 3000 miles north of the equator, our longest summer days last for about 14 hours and the shortest winter days, about 10 hours.  Energy received from the sun waxes and wanes with the seasons.  Our precipitation, totaling no more than a few inches in an average year, falls erratically, primarily in the late summer, in late summer and winter, or in the winter, depending on location.  Moreover, our evaporation rates, accelerated by a relentless sun and restless winds, can exceed the precipitation rates by tenfold or more.  Our daily air temperatures swing from moderate to very hot in the summer and cold to moderate in the winter.  We have clearly defined growing seasons and dormant seasons.  Our soils, especially in the dry lower basins where brimming lakes stood during the late Pleistocene, or Ice Age, times, often bear heavy concentrations of minerals, especially alkali salts – a poison to the food chain – and they offer relatively little organic matter, or nutrients, for instance, nitrogen, to foster plant growth.

In tropical rainforests – equatorial lands of perpetual summer and a never-ending growing season – daylight lasts for roughly half of the 24 hours all year long.  Energy received from the sun remains fairly constant throughout the year.  Rain comes, not by the inch, but by the foot—from six to 30 feet per year.  Water lost by evaporation is largely trapped in the humid microclimate surrounding a rainforest then simply returns in the form of more rainfall.  Air temperatures range from the high 60s (in degrees Fahrenheit) to the low 90s throughout the year.  Rainforest soils are comparatively free of harmful mineral residue.  Nutrients, freed from rapidly decomposing organic matter by the decomposers, re-enter the food chain almost immediately, always encouraging more growth. 

As a result of the differences between the two biomes, the total organic matter, or “biomass,” produced by the food chains of our Southwestern deserts amounts to no more than a small fraction of the biomass produced by the food chains of comparably sized tropical rainforests.  The variety of species of wild plants and animals supported by our Southwestern desert biome probably numbers in the few tens of thousands.  The number of species supported by a comparably sized rain forest might number in hundreds of thousands or even millions.

Stream that issues from mountain range and disappears into loosely consolidated soils of the desert floor.

The Setting, Origin and Development of Our Deserts

Our biologically demanding Chihuahuan, Sonoran and Mojave Deserts – each a collection of basins – bear the designation of “hot” deserts, contrasting starkly, for example, with the much colder Great Basin Desert.  The hot deserts’ basins lie among a succession of roughly linear, north-south trending mountain ranges, with some peaks reaching 13,000 feet in height, well above the timber line.  The desert basins and their mountain neighbors form the geographic heart of what geologists call the “Basin and Range Province,” which extends across the Southwestern United States from the Pecos River in the east to the Pacific coast in the west. 

Generally, the three deserts, which blanket more than 350,000 square miles – an area larger than France, Great Britain and Portugal combined – have been filled with sandy-to-fine stream-deposited, or “alluvium,” soils, which form the classic broad desert “flats.”  The soils, products not only of flowing water, wind and changing temperatures but also of chemical processes and biological agents, often have upper layers that are impoverished in organic content and lower layers, or hardpans, that are virtually solidified beds of calcium carbonate and silica.  At the mouths of mountain canyons, the basins are marked by semicircular alluvial “fans” or by coalescing alluvial fans (called “bajadas”), which have been formed by loosely consolidated sand, silt, rocks and boulders carried down the drainages by rushing waters in partnership with gravity.  Basin runoff from the mountain slopes and the irregular but often intense desert rain storms empties into either the Rio Grande or the Colorado River drainage systems, or soaks down and disappears into loosely compacted desert soils, or collects temporarily in the highly mineralized normally dry lake beds called “playas.” 

The Basin and Range Province’s stratified sedimentary mountains such as the Sacramentos of south-central New Mexico or the Franklins of western Texas have uplifted and tilted like listing barges along fault lines, leaving a steep slope (like the side of the barge) on one side and a more gentle slope (like the deck of the barge) on the other.  The volcanic ranges such as the Santa Catalinas near Tucson formed when molten rock from deep within the earth erupted through the surface, raising a tortured, mountainous mass of basalt and other igneous materials. 

The basins, already arid, evolved into full deserts beginning about eight to 10 thousand years ago, as the Pleistocene Epoch and the last great Ice Age, drew to a close.  While their annual average temperatures rose gradually through time on one hand, the basins experienced dwindling rainfall on the other hand.  Eastern mountain ranges hijacked much of the moisture from summertime systems moving west and northwest from the Gulf of Mexico.  Western mountain ranges stole most of the moisture from winter storm systems moving onshore from the Pacific Ocean.  The basins’ wide-spread Ice Age pinyon-juniper-oak woodlands – or, “pygmy” forests – retreated, over time, from the basin floors up to the lower slopes of the mountains, giving way to desert vegetation and animal life.  The Chihuahuan, Sonoran and Mojave Deserts, with differing elevations and climates, gave rise to varying plant and animal communities.

Mountain and River Plants and Animals

While the distinctive plant and animal communities of our three hot deserts typify the resilience and adaptability of life under harsh conditions, the plant and animal life of the mountains and river systems enrich the biological stew of the Basin and Range Province. 

Coyote looking for a little lunch.

In the ranges, which rise like islands from the desert floor, precipitation increases (up to an annual average of 30 inches or more) and temperatures decline with rising mountain elevations.  Pygmy forests of juniper, pinyon pine and oaks mixed with shrubs – refugees from the Ice Age basins – cover the lower slopes.  As the slopes ascend, the pygmy forests melt into ponderosa pine forests that give way to mixed conifer forests that, in turn, give way to subalpine forests that finally, at about 11,500 feet elevation, fade into treeless alpine tundra.  Mammals, birds, reptiles, amphibians, fish and invertebrates, occupying environmental niches quite different from those of the desert, form distinctive mountain communities.  Some mammals and many birds migrate between the mountains and the deserts in seasonal quests for food sources and accommodating habitat. 

Along the Rio Grande and its tributaries, which drain most of the northern Chihuahuan Desert, and along the Colorado River and its tributaries, which drain most of the northern Sonoran Desert, gallery forests of cottonwoods, willows and, sometimes, mesquites once covered the flood plains, attracting and nurturing the densest concentration of animal life in the desert basins.  They formed meandering threads of green across the hard desert landscape.  Most of the riverine forests have now been replaced by farm land. 

Combined with the mountains and rivers, the Chihuahuan, Sonoran and Mojave Desert basins form what is perhaps the most diverse landscape in the United States. More on these deserts...

Next - The role of the "Producers"

By Jay W. Sharp

Video available on this subject. Click Here for a short video on how the Food Chain works Video available on this subject.

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


In preparing this article, I have drawn, in no particular order, from various articles in DesertUSA’s Internet site; James A. MacMahon’s Deserts, part of the Audubon Society Nature Guide series; Ann and Myron Sutton’s The Life of the Desert, part of the McGraw-Hill Book Company Our Living World of Nature series; Richard Lachowsky’s The Chihuahuan Desert Internet site; The Tropical Rain Forest Internet site; Peter V. Sengbusch’s “The Flow of Energy in Ecosystems – Productivity, Food Chain & Trophic Level,” Botany Online, The Internet Hypertextbook, site; “Introduction to Food Chains,” Commonwealth Bank Foundation Internet site; “Landscape Changes in the southwestern United States: Techniques, Long-Term Data Sets, and Trends,” USGS Land Use History of North America Internet site; “Deserts,” National Wildlife Federation Internet site; R. C. Brusca’s “Deserts of the Southwest: Lecture Notes,” R. C. Brusca’s Internet site; the Mohave [sic] Desertscrub Internet site; “Physiography,” USGS Our Dynamic Desert Internet site; “What is a Desert?” Desert Biome Internet site; “Ecosystem Productivity,” Geography 210: Introduction to Environmental Issues Internet site; the Bioenergy Information Network Internet site; and “The Ecological Impacts of Human Development in the Southwest,” Earlham College Biology Major Internet site.

  

 


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