Animals: The Consumers

The Desert Food Chain - Part 11

As the name “consumers” suggests, animals, unlike typical plants, eat other organisms to survive.  Additionally, most animals, unlike plants, can move themselves from place to place.  They can seek refuge from extreme environmental conditions such as the high heat and prolonged droughts of the desert.  They have specialized tissues, including, as a few examples, muscles used for movement, a nervous system used for processing and sending signals, and internal chambers used for digesting food.  Animal organisms (excepting those of animals such as sponges, jellyfish and barnacles) have a basically bilateral symmetry, or mirror-image left and right halves.

By comparison, typical plants, the “producers,” manufacture their own food, or carbohydrates, using the process of photosynthesis; that is, plants fabricate glucose, a major component in the food chain, using water and carbon dioxide as raw materials and sunlight as fuel.  They remain anchored in place by root systems.  Since they cannot take refuge from extreme environmental conditions, they rely on various adaptations to withstand desert heat and drought.  They have no muscles, nervous systems or digestive chambers.  Typically, a plant organism lack bilateral symmetry, although some parts (for instance, the compound leaves of a mesquite tree) may have bilateral symmetry.  Stems and flowers have other geometrical arrangements. 

The animals comprise a relatively small fraction – less than one-tenth – of the biomass (total living matter) of the earth; the plants, about nine-tenths.  On the other hand, the animals account for a relatively large fraction – roughly three-quarters – of all of the 1.6 million named species on the earth; the plants, less than one-fifth, according to Michigan University’s Global Change Internet site.  (Bacteria, fungi, protozoa, algae and other life forms make up comparatively small percentages of the biomass and the species population.)

Compared with the animal and plant communities of, say, a highly productive tropical rainforest, those of our deserts, faced with limited and highly variable seasonal rainfall, punishing summer temperatures and organically impoverished soils, produce a disproportionately small part of earth’s total biomass and biodiversity.  (The total biomass, scientists estimate, equals more than a trillion tons of dry, or water-free, organic matter.  The total number of species of animals, plants and the other life forms may range anywhere from 10,000,000 to 30,000,000, including both the ones known and those unknown to science.)

The Relationships

As our deserts have evolved following the end of the last Ice Age, some 8000 to 10,000 years ago, the animals, plants and environment have woven a tapestry of complex, uneasy and often contrary relationships.  Of course, the animals – herbivores, carnivores or omnivores – depend totally on the plants, the foundation of the food chain, for survival.  Simultaneously, the plants depend totally on the unpredictable desert environment: the availability and timeliness of moisture, the intensity of seasonal temperatures and the organic richness of the soil. 

Mother Nature, on the other hand, follows her own agenda, with utter disregard for the desert animals or plants.  Capricious and whimsical, she produces an ever-changing mosaic of “mirco-climates,” or ephemeral localized climatic conditions spawned by irregular “pulses” of rainfall sometimes followed by high heat and winds.  Typically, she delivers most of her annual rains during the monsoonal seasons of late summer in the Chihuahuan Desert, late summer and winter in the eastern Sonoran Desert, and winter in the western Sonoran Desert and the Mojave Desert. 

In a late-summer thunderstorm in the Sonoran Desert, she may pound a talus slope along a mountain range with a torrential rain, producing a surge of water than rushes away before it can soak into the soil.  At the same moment, she may leave a neighboring slope totally dry, teasing with towering cumulus clouds and a brilliant rainbow.  She may bring a more gentle and soaking rain to a drainage basin, favoring established plants with radiating root systems but essentially ignoring seeds of species that are not prepared for germination.  Other times, she brings no rain at all.  In the hottest of our deserts, she routinely raises the midday temperature of the air in summer to more than 120 degrees Fahrenheit and soil temperature to 150 to 180 degrees.  She may turn up the winds of spring to gale force levels, raising dust clouds that envelop lower mountain ranges and accelerating already high water evaporation rates.  By inhibiting the prosperity of the animals and plants, she limits the organic richness of the soils.  Mother Nature makes the survival of the animals and plants of the desert a game of chance. 

Adaptation and Survival

During periods of prolonged drought and heat, the animals, especially those with no access to free-standing water, can become severely tested.  Herbivores and omnivores may have to depend heavily on plants for moisture, taken from the tissues, fruits and flowers.  Carnivores and omnivores may depend on prey for moisture.  Scavengers such as the Turkey Vulture may depend on carrion for moisture.  Some, for instance, beetles, have a hard shell encasing their bodies, helping them preserve their store of moisture.  As summer sets in, smaller animals look to the shade of plants or to the shelter of burrows to escape the desert heat.  Some larger animals, for instance, mountain sheep, may turn to the coolness of natural caves.  Other animals, for instance, the Black-tailed Jackrabbit with its strikingly large, heat-dispensing ears, rely on physiological adaptations to help cope.  Highly mobile animals, including numerous birds and larger mammals, simply migrate to areas that promise more water and cooler temperatures.

By comparison, the desert plants, immobile and fully exposed, have developed several basic strategies for survival.  Some, for example, the cacti, yuccas and agaves, endure drought and heat by conserving and rationing water within spongy tissues encased in waxy coatings.  Other plants, for instance, some of the shrubs, avoid drought and heat by shedding leaves and twigs so they can reduce their need for water, or they put down deep tap roots in a reach for ground water.  Still other plants such as grasses and forbs (non-woody plants other than grasses) escape the drought and heat by racing – when Mother Nature does deliver timely and sufficient rainfall – to produce prolific crops of seeds, prudently banking them in the surrounding soil to await the next timely and sufficient rainfall, perhaps years later. 

From year to year in the desert, the animals depend on a variable and uncertain plant menu to survive, creating a dynamic and constantly changing food chain. 

Our Deserts’ Animal Population

Broadly, our deserts’ animal population, like all of earth’s animal population, falls into one of two main groups, the invertebrates – those without backbones – and the vertebrates – those with backbones.  Our desert invertebrates, stunningly complex in their diversity, include, for a few examples, the arthropods (insects, spiders, scorpions, centipedes, millipedes, desert shrimp and many others), the mollusks (snails) and annelids (segmented earthworms).  Our desert vertebrates consist of representatives from all five of the best known categories: reptiles, amphibians, fish, birds and mammals. 

Our native invertebrates include perhaps 10,000 to 20,000 known species of arthropods, several dozen species of mollusks, and the communities of earthworms.  The native vertebrate population comprises more than 100 species of reptiles, perhaps two dozen species of amphibians, several dozen species of freshwater fishes, over 500 species of birds and well over 100 species of large and small mammals. 

Some Invertebrates of the Desert

The number of species of insects far exceeds the number of species of all other animal life in the desert combined.  In a single example, the “University of Arizona insect collection has more than 13,000 identified species of only Arizona insects,” Floyd Werner and Carl Olson said in their 1994 book Insects of the Southwest.  “There are many more that we have been unable to name or are waiting description.”  The insects have forged a labyrinthine web in the food chain.  Most herbivorous species feed on a few related plants throughout their lives.  Others feed on a wide selection of plants.  Carnivorous insects, including the predators, blood-suckers and parasites, feed on animal tissue. 

The spiders of the desert, eight-legged carnivorous arthropods that total roughly 1000 species, “can create fear and hysteria in movies and homes,” but they “really are gentle predators,” Werner and Olson said.  The spiders do, however, have a strange way of expressing their gentleness.  Most trap, ambush or attack insects or other spiders, injecting them with a venom that liquefies the insides, which become a nutritious cocktail for the predator.  Tarantulas, the largest spiders in the desert, prey not only on insects, but also small reptiles (sometimes including even young poisonous snakes), amphibians and even mammals.  In some species, female spiders, in an act of feminine cannibalism, prey gently on their males. 

The scorpions, with an ancestry dating back hundreds of millions of years, include “many species” in the Southwest, according to Werner and Olson.  Most, according to the University of California at Berkeley Museum of Paleontology Internet site, “are nocturnal, hiding under rocks, in crevices, or within burrows during the day, and coming out after sunset” to hunt.  Primarily, the scorpions eat insects, using powerful pincers to catch and crush their prey.  Amazingly, the scorpions, supremely adapted to the desert environment, can survive by eating as little as one insect a year, according to Brian Handwerk, writing in National Geographic News, June 24, 2003.  They have the uncanny ability, he said, to reduce their need for food by slowing “their metabolism to a third of the rate of another typical arthropod”

The various species of centipedes and millipedes, with their segmented and elongated bodies and multiple legs, would seem to hold much in common, but they have fundamental differences, as Werner and Olson point out, and they play quite different roles in the desert food chain.  The centipedes, swift carnivorous creatures typically three to six inches long, have fairly flat bodies with a single pair of legs on each segment.  The larger species may have a pair of fanglike claws – actually modified legs near their mouths – that they use for injecting venom into their prey or, for that matter, into unwitting human beings.  Nocturnal, the centipedes remain secreted under rocks or within burrows during the day, emerging to hunt at night, seeking out, for instance, beetles and other insects.  By comparison, the millipedes, slow-moving herbivorous or scavenging animals typically three to six inches long, have fairly cylindrical bodies with two pairs of legs on each segment.  They have no poisonous claws or fangs or stingers, but they do have orifices along the sides of their bodies that emit evil-smelling chemicals they use to repel predators.  Normally secretive, millipedes feed on plants and organic material, but they come out after a rain to celebrate the event.

Desert Shrimp, which live in ephemeral playas and water holes, rank as true crustaceans, like the shrimp, crabs and lobsters of the oceans.  The Desert Shrimps’ eggs, provided they dry completely, hatch in vast numbers when rain brings water to their playas and water holes.  Adults, depending on the species, range from a half inch to two inches in length.  Omnivores, Desert Shrimp eat fungi, algae and microscopic organisms.  Remarkably adapted to the desert, they produce eggs that may lie desiccated for years awaiting the hatching cues prompted by rainfall.  Some species breathe through their feet, where gills are located.  Their great numbers following a hatch attract large populations of waterbirds during migratory seasons.  The shrimp die as their water evaporates. 

Snails, members of the mollusks, occupy widely diversified environments.   They live in mountain ranges, rock slides, ephemeral water holes and the deserts’ few permanent springs.  Ranging from a mere speck to thumbnail in size, they likely descended from species that covered wide areas of the Southwest during the Ice Ages.  Constrained by limited mobility and sensory systems, they have, in many instances, evolved into species unique to their restricted individual habitats.  “The average snail moves at a speed of 0.0000362005 miles per hour [roughly five feet a day],” according to the AmusingFacts.com Internet site.  Desert snails survive the heat and drought by taking refuge in stony crevices or burrowing into mud, relying on their shells to preserve their moisture until the next rains bring more water.  “They will withdraw into their shells, and hibernate or sleep, for as much as 2-3 years, until conditions improve,” says AmusingFacts.  Snails feed on plants, fungi and plant detritus, and they serve as prey for several animals.

“Worms,” said Charles Darwin in The Formation of Vegetable Mould, the last of his books, “have played a more important part in the history of the world than most persons would at first suppose.”  The earthworms’ ancestors have been stirring the soil of the earth for perhaps 120 million years, according to the spring 2004 edition of the Utah Agriculture in the Classroom Bulletin.  In the desert, the earthworms live, not in the organically poor desert sands, but primarily in the richer riverine floodplains where, daily, each worm can ingest its weight in decaying organic materials and minerals, converting them into nutrients, enriching the soil.  Numbering as many as hundreds of thousands per acre, earthworms not only contribute in a major way to increasing the fertility of the soil, they also serve as an important food source for a diverse array of other animals, including the vertebrates. 

Some Vertebrates of the Desert

Like all reptiles, those of our deserts, including snakes, lizards, turtles and tortoises, have thick scaly skins, an especially valuable feature for terrestrial species because it inhibits the loss of water.  They eat less than comparable sized mammals because they have slower metabolic rates.  The several dozen species of snakes, including at least 10 rattlesnakes and the Arizona Coral Snake, all feed on other animals.  Their prey, depending on their species, ranges from small mammals to birds, reptiles, amphibians, insects and even centipedes.  The various lizards, many of them active during the day even during the desert summer, eat a wide range of foods. 

Most prey on other animals, especially insects, although some eat other vertebrates.  A few, for instance, the Chuckwalla, primarily eat plants.  The fearsome-looking Gila Monster scavenges, feeding on the new-borns of small mammals, birds and reptiles.  The some half-dozen turtles and a tortoise live in diverse environments.  Some live in the few waterholes of the desert, feeding on animals such as snails, tadpoles, worms and aquatic insects.  The Desert Box Turtle, an omnivore and scavenger, lives in the open grasslands, feeding on plants, insects, worms, reptile eggs and carrion.  The endangered Desert Tortoise, 10 to 15 inches in length, leads an entirely terrestrial life, feeding on various cacti, herbs and grasses.  

The amphibians, which include a relatively few frog species and salamanders, inhabit the deserts’ occasional streams and ephemeral ponds, where they find the moisture they require for breeding.   The frogs, primarily toads and spadefoots, have developed several distinctive adaptations for survival in the desert.  For instance, during drought, the Couch’s Spadefoot may excavate a two-foot-deep burrow, where it can spend two or more years in a dormant state, according to James A. MacMahon in his book Desert.  When rain finally comes, the spadefoot replenishes its need for moisture, takes a position at an ephemeral pond, issues a reverberating call for a mate, consummates one or two nights of romance, and quickly produces a new generation of tadpoles.  The adults eat enough insects to meet their nutritional needs for another period of dormancy.  The tadpoles eat plant and animal matter and even each other should resources be limited.  The three- to six-inch-long Tiger Salamanders, the most common in our deserts, live on the desert floor, occupying their own burrows or appropriating other animals’ burrows.  Cued by monsoonal rains, they head for the nearest water to breed.  Voracious, night-feeding carnivores, they prey on insects, spiders, earthworms, other amphibians and small mammals. 

The several dozen native fishes of the desert of the Southwest live in the Colorado River drainage system, the Rio Grande drainage system or the rare permanent springs.  “The fishes in these communities range from long-lived, large-bodied fishes found in large, highly variable rivers to small specialized fishes that have been isolated for thousands of years in relatively stable environments,” according to the U. S. Geodetic Survey Internet site, Science for a Changing World.  Like their terrestrial vertebrate bretheren, they have had to develop adaptations for surviving in the desert environment. 

Desert fishes can, for instance, tolerate wide fluctuations of temperature, mineralization and oxygen content.  In fact, says MacMahon the desert pupfishes “have survived at the lowest oxygen concentration known for any fish…”  The larger species may prey on smaller fish and aquatic insects, and the smaller, for instance, the pupfish, feed on algae, detritus and aquatic invertebrates.  Unfortunately, the native fishes of our deserts rank among the most imperiled in the United States.  Their range and water quality have been altered by dams in the Colorado and Rio Grande drainage basins.  They suffer from predation and competition from introduced species.  For a specific example, according to the Phoenix Zoo’s Mike Demlong, Conservation Spotlight: Desert Fish, “the Bonytail Chub is the most endangered fish in the Colorado River Basin, perhaps in the entire United States.”  Across the Southwest, says the USGS, 85 percent of the fish fauna are threatened in Arizona; 72 percent, in California; 30 percent, in New Mexico, and 42 percent, in Utah. 

Our desert bird population, with maybe 500 species, mirrors the diverse, intersecting environments of the Southwestern landscape.  They range in size from the Black-chinned Hummingbird, with a wingspan of perhaps three inches, to the Sandhill Crane, with a wingspan of perhaps four feet.  They vary in color from the American Goldfinch, with a bright yellow body, to the Curve-bill Thrasher, with a dull grayish brown body.  Some, for instance, the quails, stay close to home all their lives.  Others, for instance, the Black-chinned Hummers and the Snow Geese, migrate hundreds to thousands of miles every year to spend a season in the desert.  According to MacMahon, the birds of the desert cope with the heat and drought by capitalizing on physiological adaptations, feeding in the early mornings and late afternoons or (for the large soaring birds) flying at higher and cooler altitudes.  They find water in plants or in drainages or in ponding areas.  They feed on a range of foods as varied as their sizes, colors and behaviors.  The hummers sip the nectar from the flowers of the desert blooming season. 

The herbivorous White-winged Doves, abundant across much of the desert brushlands, eat the seeds of the ephemeral plans and the fruits of prickly pear cacti.  The carnivorous American Dippers, which may appear at streams issuing from the mountains into the desert during the winter months, feed on aquatic animal life at the bottom of the rushing waters.  The carnivorous Roadrunner feeds on arthropods, reptiles, rodents and other bird species’ nestlings.  The carnivorous Golden Eagles feed on Black-tailed Jackrabbits and other large rodents.  The opportunistic omnivore Common Raven, or Crow, feeds on seeds, insects, small rodents, garbage and carrion.  The scavenging Turkey Vulture, so elegant in its soaring flight, eats the rotting flesh of dead animals.

While some stay active through the day, the mammals – the fur-bearing vertebrates that nurse their young – really take center stage in the desert during the cooler hours from late afternoon through the night into the early morning.  Most turn to burrows and natural shade as shelter from the fierce midday summer heat.  The smaller desert mammals, like the Black-tailed Jackrabbit with its large ears, rely heavily on physiological adaptations to cope with the desert.  The Merriam’s Kangaroo Rat, for another example, has kidneys designed to reabsorb water before urination, according to MacMahon.  Many small mammals have slow metabolic rates, slowing the use of water.  Under periods of high stress, the smallest rodents can go into an energy- and water-saving torpor. 

The larger mammals can follow a different strategy for desert survival.  With much greater range than their smaller relatives, they can travel miles to reach streams and ponds to meet their water needs.  Their greater mass tempers the rise and fall of body temperatures.  Like the birds, mammals feed on a wide range of foods.  Bats, for instance, depending on the species, feed on nectar and insects.  The rodents, depending on the species, eat seeds, nuts, plant matter and arthropods (including scorpions).  The nocturnal, carnivorous Ringtail, said MacMahon, “ambushes prey, then pounces, forcing the prey down with its paws and delivering a fatal bite to the neck.  Its diet includes grasshoppers, crickets; small mammals, small birds; fruit, spiders, and frogs.”  The skunks, omnivores, eat vegetable matter, insects, bird eggs, amphibians, and small mammals.  Badgers eat small mammals.  The Raccoon “will eat almost anything.”  The Collared Peccary can lay waste to a stand of prickly pear cacti, thorns and all.  Coyotes, like Raccoons, will eat almost anything.  Pronghorns graze on grasses, forbs, cacti and, in winter, sagebrush.  Mule Deer browse primarily on a wide range of woody plants. 

Diversity

The diversity of the animal life of in the punishing venue of our Southwestern deserts validates the resourcefulness of nature.  As the eminent naturalist Roy Chapman Andrews said in his book Nature’s Ways: How Nature Takes Care of Its Own, “one of the most fascinating aspects of nature is the way it equipped every creature, be it of high or low degree, to withstand enemies and to obtain the necessities of life.  Some animals had to change their entire physiology or anatomy to enable them to meet competition and to survive; more often less drastic adaptations in skin, color, or habits made the difference between life and death of a species in the struggle for existence.”

Next The Insects

 

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

 

 

 

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