5. Animals and Soil in Arizona
Arizona's animal life includes about 751 vertebrate species and more than 20,000 invertebrate species. Among the vertebrates are 64 species of fish, 22 species of amphibians, 94 species of reptiles, 434 species of birds and 137 species of mammals (Lowe, 1972). This diversity results from the extremes in climate (Plates 5 through 10), topography (Plates 2 and 4) and vegetation (Plates 11 and 12) that create many different environments. These environments range from the hot, dry deserts at low elevations in southern Yuma County through rich upland deserts, grasslands and woodlands at the mid-elevations, to cold, moist montane and alpine habitats such as those in the San Francisco Peaks and the White Mountains. In some places the division between habitats is quite distinct, particularly where relief is abrupt such as it is in the San Francisco Peaks. It was there in 1889 that C. Hart Merriam made observations that eventually led to the Life-Zone System for classifying plant and animal associations. Vegetative communities, representative animals and soil types within each life zone are illustrated in Figure 36.
Biotic provinces also have been used to describe the distribution of animals. Biotic provinces differ from life zones in that life zones are fully biogeographic systems whereas biotic provinces are founded primarily upon the distribution of animals (Lowe, 1972).
Factors involved in delimiting the distribution of animals are complex and include topographic, hydrologic, pedogenic, climatic and biotic elements. The general relationships between some of these factors can be seen by comparing Plates 2 through 10 and 13, and Plates 11, 12, and 17. Specific information about the associations between animal populations and plant communities in Arizona has been collected and organized by Patton ( 1978) and is available as a computer printout or on microfiche (see Patton, 1979).
Many animals that inhabit these zones or provinces directly affect the characteristics of soil. Soil properties in turn can influence the distribution and abundance of animals living in (Kevan, 1962; Kuhnelt, 1976) and on (Allen, 1962;
The precise ways that soil organisms interact with plant material, each other and the soil are difficult to describe. This is due to the lack of information about soil biochemistry, physiology and ecology. However, many soil animals depend either directly or indirectly on dead plant tissues as sources of energy. Thus, the decomposition of organic litter on the soil surface is of critical importance in soil ecology. The processes of decomposition are controlled largely by soil organisms. Consequently, the soil surface and immediate subsurface are regions of greatest biological activity.
In addition to their varying roles as decomposers, invertebrates such as protozoa, worms, insects and mites also contribute to the soil community by mixing, loosening and aerating soil (Evans, 1948; Englemann, 1961). Vertebrates influence soils primarily through their burrowing activities, and all organisms contribute to the soil by adding organic matter and chemicals via their bodies, excrement and food residues. See Figure 37 for representative soil animals.
FIGURE 36. Cross Section of the Grand Canyon and Associated Life Forms
Soil fauna have been classified by numerous authors including Kevan ( 1962), Schaller ( 1968) and Wallwork ( 1970). Five major groupings are widely accepted: classification based on body size; time spent in the soil; location or habitat in the soil profile; feeding strategies; and method of locomotion in the soil (Wallwork, 1970).
Soil fauna generally are small and have simplified appendages (Kuhnelt, 1976). Body size ranges from 0.0002 cm (0.00008 in) to more than 20 cm (8 in) and can be divided into microfauna, mesofauna and macrofauna (Figure 38). Microfauna range in body size from 0.0002 to 0.002 cm (0.00008 to to 0.0008 in) and consist of protozoa. Mesofauna range from slightly more than 0.002 to 1 cm (0.0008 to 0.4 in) and include mites, springtails, spiders, pseudoscorpions, pot-worms, insect larvae and the smaller millipedes and isopods. Macrofauna are at least 1 cm (0.4 in) or greater and include earthworms, the largest insects and arachnids, and the soil-dwelling vertebrates.
The amount of time organisms spend in the soil ecosystem also is used as a criterion for classification. Some organisms such as protozoans, nematodes, isopods and mites spend their entire lives in the soil, whereas other organisms like ground-nesting birds may be only tangentially associated with the soil community.
A third way of classifying soil fauna is by their location in the soil profile. Soil animals generally fit into one of three profiles: the epigeon, or vegetation layer above the soil surface; the hemiedaphon, or organic layers; and the euedaphon, or mineral layers. Soils have numerous microhabitats and soil organisms can be classified according to their use of them.
Feeding strategies also are used in classification of soil fauna. Feeding classifications include carnivores, which feed on other fauna; phytophages, which feed on green plant and woody material; saprophages, which eat dead and decaying material; microphytic-feeders, which feed on fungal hyphae and spores, algae, lichens and bacteria; and miscellaneous feeders, which fit into two or more of the other categories.
An organism's method of locomotion through the soil provides yet another way of classifying soil animals. Distinctions are made between burrowing animals and those that move through the soil by making use of pore spaces, cavities or channels. Other classification schemes are used when applicable such as the burrowing behavior and activity patterns of desert animals briefly described below.
Of the relationships that exist between animals and the soils of Arizona, the interactions between fauna and desert soils are the most remarkable. Deserts are defined in numerous ways including kinds of vegetation and soil, and amount of precipitation. In this discussion, deserts are defined as being areas where annual evapotranspiration exceeds precipitation. By this definition, much of Arizona is desert and includes parts of the Great Basin, Chihuahuan, Mohave and Sonoran deserts.
Desert soils are sandy in texture and mineral in character since there is little cover vegetation to provide organic material to the soil (Wallwork, 1976). A classification of desert soils is provided by Wallwork ( 1982). It is in these desert soils that biological activity and productivity reach their lowest levels in Arizona. High temperatures and low amounts of precipitation limit the growing season of plants and the activities of many animals to short intervals during the year. In the lower Sonoran Desert, where annual precipitation may reach 200 mm (8 in) per year (Crosswhite and Crosswhite, 1982), a variety of perennial plants such as sage, saltbush, creosotebush, acacia and numerous cacti and succulents thrive. These plants provide food and refuge for a diverse group of desert animals.
Survival of desert animals depends on their ability to avoid extremely high temperatures. Soil temperatures beneath the desert floor are lower than those at the surface and provide relief from heat for desert fauna. Three basic faunal groups are common in deserts: small mammals, reptiles and arthropods. Individuals in these three groups frequently survive by burrowing during hot times of the day and being active during cooler periods (see Wallwork, 1982). Desert tortoises, lizards, rodents, foxes, and termites, bees and wasps all use this method. The burrowing behavior and the activity patterns of desert animals provide a simple means of classifying desert soil dwellers. They fall into four classes:
Several groups survive through tolerance; being adapted physiologically to require little water (see Wallwork, 1982). Reptiles and insects are ideally suited in this respect because their uric acid needs little water for elimination from the body. In the case of some small mammals such as the kangaroo rat, the urine is extremely concentrated. These animals rely almost entirely on metabolic water or the preformed water on their food for their moisture requirements (see Chew, 1965).
This discussion has merely highlighted soil fauna classification systems currently used. For detailed coverage of this aspect of soil ecology the reader is referred to Burges ( 1969), Hooper ( 1969), Kevan ( 1962), Schaller ( 1968), Sims ( 1969), Wallwork ( 1969, 1970, 1982) and Williams, Davies and Hall ( 1969).
About 90 percent by weight and 99.9 percent by numbers of the animals of Arizona live in the soil and, for the most part, are so small that they are unnoticed (Hole, 1980; Jenny, 1980). Microscopic protozoa live in water films on soil particles and feed on bacteria and yeasts. Snails, slugs and elongate animals such as earthworms, flatworms and nematodes degrade organic matter. Among the most abundant arthropods are soil mites, springtails and various forms of insects including ants, termites, beetles and flies. Some centipedes, millipedes, spiders, scorpions, harvestmen and, in wet soils, crayfish are abundant in Arizona. The live weight of soil invertebrates in moist soil is about 3,335 kg per ha (3,000 lbs per ac), or about the weight of three horses (Jenny, 1980). When soil becomes dry, faunal biomass decreases.
Special features of soil that are fashioned by soil invertebrates include the following:
Protozoa. Protozoa are represented in the soil mainly by rhizopods, ciliates and flagellates. Literally millions of protozoa inhabit a square meter of soil (slightly more than a square yard). These organisms generally are regarded as bacteria-feeders. Some ingest organic litter and fungi and even may be able to digest cellulose. Protozoa are decomposer organisms. Their actions may contribute significantly
Nematodes. The feeding habits of soil nematodes vary considerably (Freckman and Mankau, 1977). Some inhabit decaying organic matter and ingest liquified components of decomposing animals and plants. Others feed on bacteria or fungi, while still others parasitize plants, beetles, worms and slugs. The feeding activity of nematodes generally does not contribute significantly to the decomposition of organic material or to the formation of soil humus, but they do provide an important food source for other members of the soil community (Wallwork, 1970).
FIGURE 37. Representative Soil Animals 37a Termites 37b Harvester Ants 37c Spadefoot Toad 37d Kangaroo Rat 37e Shovel-Nosed Snake
Worms. The best known of all soil animals are earthworms. They have a definite impact on the structure and properties of soils. Charles Darwin ( 1890) first examined the influence of earthworms on the decomposition of organic material. Subsequent investigations have examined the role of earthworms in the formation of organic-mineral complexes (Evans, 1948; Gerard, 1967; Satchell, 1967; and Thorp, 1949). Earthworms contribute to the soil community by ingesting and mixing decaying organic material and mineral soils. This action converts the bound nitrogen in organic complexes to ammonia, nitrites and nitrates that are more readily available to vegetation. Earthworms also influence soil drainage, fertility and stability (Wallwork, 1970) and promote the redistribution of organic debris.
Molluscs. Molluscs are represented in soil communities by slugs and snails. Land molluscs exhibit several types of feeding habits including herbivory, fungivory, predation and detritus feeding. This group probably influences soil most by feeding on surface vegetation, then moving into soil subsurface layers, thus incorporating organic material into the mineral structure of the soils.
Arthropods. Arthropods are another important and conspicuous part of the soil community. These organisms frequently dominate all other groups of the soil meso- and macrofauna, both in numbers of individuals and species. Shaller ( 1968) divides arthropods into crustaceans (wood lice), arachnids (scorpions, pseudoscorpions, harvestmen, soil spiders and mites), myriapods (millipedes and centipedes) and insects.
Crustaceans generally are not terrestrial and many have retained characteristics associated with aquatic life; most are not important soil organisms. However, the wood lice have established themselves as terrestrial forms and are abundant in a variety of soils ranging from humid litter in forests to the hot, dry soils of Arizona's deserts (Wallwork, 1982). They are omnivorous, feeding on dead plant material, feces and invertebrate carrion, and they play an important role in the decomposition of organic material.
Arachnids are predatory arthropods and frequently inhabit vegetation on the soil surface and loose leaf litter. The role of arachnids in the soil community has not been studied thoroughly, but they are important predators of insect populations and like all animals contribute organic matter to the soil when they die.
FIGURE 38. Soil Microfauna, Mesofauna and Macrofauna Classification (after J. A. Wallwork, 1970)
Millipedes and centipedes are common in many soils. Millipedes generally feed on plant detritus (Wallwork, 1982) to assist in the decomposition of organic matter, while centipedes are primarily predators. More information is required about both to determine the importance of their specific roles in soil ecology.
Numerous orders of insects are represented in soil fauna, but perhaps the most groups with respect to soil ecology are the termites and ants (Wallwork, 1982). Members of both groups construct numerous galleries in the soil, and many species transport large amounts of organic material from the surface to underground chambers; termites are particularly important in this respect (Schaefer and Whitford, 1981). These activities can contribute significantly to nutrient cycling.
Other species of insects may use the soil during part of their life cycles, larvae that overwinter in the soil, for instance. Most species, however, play minor roles in soil dynamics and should be considered passive members of soil communities.
Larger vertebrates help shape the microtopography of soil landscapes. A moderate-size colony of prairie dogs may build clusters of mounds over an area of 1.2 ha (3 ac). Pocket gophers make conical mounds and thick, rope-like soil fillings in tunnels in basal layers of snow-banks. Wood or pack rats pile litter up to 1 m (3.3 ft) high and 2 m (6.6 ft) wide in a retreat or nest site.
Other vertebrates mix soils. Skunks, javelinas, coatis, whiptail lizards, roadrunners, Gambel's Quail and other animals dig and scratch through the upper soil layers in search of seeds, roots, tubers, insects, lizards and other small animals. Some snakes, such as the western shovel-nosed snake, and desert tortoises move or ‘‘swim’’ in sand. Spadefoot toads bury themselves in soil during dry periods and dig themselves out again when rains come.
Excavation of underground passageways and chambers affects the soil climate and alters soil horizons, in some instances to the point of obliterating argillic horizons. Pocket mice, ground squirrels, prairie dogs, skunks, pocket gophers, cottontail rabbits, Kit foxes, kangaroo rats, pack rats and wood rats make extensive systems of tunnels, shafts and chambers, and make dens under mounds in Fluvents. Badgers in pursuit of rodents enlarge burrows. Trampling by hooved animals collapses burrows and exposes soil material to wind erosion.
Animals also redistribute materials. Ground squirrels, rats, mice and gophers store plant materials, including seeds, in subsurface chambers. Bodily wastes of animals constitute local concentrations of nitrogen, phosphorus and potassium.
A great variety of rodents, birds, bats and other vertebrates, including coyotes, make their dens and nests in openings between masses or rocks. Mice and bats use crevices in the faces of high cliffs. Raptors build nests on ledges. In so doing, these animals introduce organic matter, some of which undoubtedly promotes weathering of bedrock and its conversion to new soil that supports vegetation. Bat and bird excreta are natural organic fertilizers, and large concentrations of these animals may have a significant influence on the chemical nature of soil.
The impact of horses, wild burros and, above all, cattle on soil landscapes has been enormous since the arrival of Europeans in Arizona. Loosening the sandy soils by overgrazing has accelerated both wind and water erosion. At some sites, sandy soil has blown short distances and collected in linear and oval deposits around mesquite trees. These deposits do not suppress growth of mesquite, but do provide an environment suitable for growth of new vegetation. These mounds are called ‘‘coppice dunes’’ and are Torripsamments (Gile and Grossman, 1979). The pattern of alternating bare and grassy patches and strips, then, may be ascribed to accelerated water erosion, resulting from overgrazing. Exposure of argillic soil horizons on the bare areas perpetuates movement of runoff and sediment into adjacent grassy areas where vegetative growth is fostered and soil is protected from erosion.
Although some vertebrates spend part of their time in the soil, they usually feed on the surface and their importance in the food web of soil ecosystems is often overlooked or deemed minimal. Because methods of study differ for vertebrates and invertebrates and because scientists tend to specialize, vertebrates are seldom included with invertebrates in investigations of soil fauna. Some vertebrates, however, do have an impact on soil ecosystems.
Vertebrates with Minimal Effect on Soils. ‘‘Periodic’’ vertebrates are those that associate with soils but have little impact on soil communities. These vertebrates include birds that nest in lagomorph or rodent dens; lizards that sleep in the ground; toads or frogs that lie dormant in soil when temperatures are high or that occasionally burrow in the soil in search of food; and foxes, badgers, coyotes, lagomorphs and desert tortoises all of which create dens in the soils.
The dens or chambers created by mammals and reptiles often become miniecosystems. When unoccupied by their creators, these underground chambers frequently are used by nonburrowing animals, such as beetles and frogs. The buildup of organic debris in the dens promotes growth of fungi, which, in turn, is eaten by insects and mites that become food for vertebrates. However, the overall effects of these chambers on soil communities probably are small.
Vertebrates with Substantial Impact on Soils. Many mammals have considerable influence on soil communities. The most important are the burrowing rodents including pocket gophers, kangaroo rats, ground squirrels and prairie dogs. Burrowing mammals raise soils from lower profiles to the surface where they are broken down, incorporated with organic matter and carried off by water and wind. Mixing deep and surface materials also may have significant effects on the texture and composition of soils at various levels (Koford, 1958).
Rodents also are responsible for moving large amounts of soil. Grinnell ( 1923) reported that pocket gophers moved more than 2.7 metric tons (3 tons) of soil per 2.6 km2 (1 mi2) during one winter. Prairie dogs also move soil; soil in the mounds excavated from 25 burrows may weigh as much as 27 to 36 metric tons (30 to 40 tons) (Thorp, 1949; Koford, 1958).
The net influence of vertebrates on soil composition is not easy to measure, but the following examples demonstrate important relationships. Badgers are strong diggers and can move large rocks. They have been known to change completely
Rodents and lagomorphs influence the soil by adding organic material. Feces alone is a significant contribution to soil communities. On the Santa Rita Experimental Range in Arizona, Vorhies and Taylor ( 1933) found an average of 16 kg per ha (14 lb per ac) of jackrabbit feces. If this value were extrapolated to include the entire 20,240 ha (50,000 ac) Santa Rita Experimental Range, fecal weight would be about 315 metric tons (350 tons), nearly 30 times the combined weight of the jackrabbits that lived on the range.
Soil chemical composition is altered by mammalian activities. Feces, urine and animal remains are rich in the salts of important soil chemicals. Greene and Reynard ( 1932) evaluated kangaroo rat dens on the Santa Rita Experimental Range and found increased quantities of soluble salts and nitrates in them. There was an average of 0.6 kg (3 lb) of nitrogen per hectare (1 ac) in kangaroo rat dens and 1.5 kg (8 lb) of stored food per hectare (1 ac).
Mammals also alter soil structure. Soil structure is determined by the size and arrangement of soil particles. Structure is important because it affects the ability of soils to absorb water and subsequently yield it to plants. Burrowing animals usually improve soil structure by loosening soil particles (Koford, 1958). Kangaroo rats in Arizona produce a measurable increase in the water-holding capacity of surface soils near their burrows (Greene and Murphy, 1932).
Not all mammal-soils associations are beneficial. Soil disturbance caused by burrowing animals can increase erosion and prevent natural revegetation. These changes can cause, in turn, the mortality of beneficial soil organisms such as earthworms. The extent to which mammals cause erosion is unclear. Wallwork ( 1970) maintained that the activities of burrowing animals can lead to soil erosion. But Koford ( 1958) maintained that although mammals may increase the speed of erosion after it is started, overgrazing by livestock, not burrowing activities of mammals, is most often the initial cause of excessive soil erosion.
Livestock and native ungulates affect soils by compacting them. Hungerford ( 1980) attempted to measure the effects of ungulate movement on soil by establishing soil stability classes. He based these classes on the amount of vegetational cover, litter or rock on the soil surface. Other indicators of soil stability included the number of seedling perennial plants, observed soil movement, amount of litter against rocks or plants, presence of rills and gullies without perennial vegetation. Animal trailing, grazing, playing, fighting and walking to and from water also were measured. Impact of animals on soils and vegetation was assessed using a site disturbance index (SDI). Hungerford ( 1980) calculated the SDI as shown below.
R = The daily range of an animal. Cattle are used as the norm with their range being represented as 1.0. Ranges lesser or greater than cattle would be ±1.0. (Less than cattle, then, would be 0.0 and more than cattle would be 2.0.)
A = Relative activity. This value is estimated for each month of the year. Breeding, fighting and playing are some behavioral elements that cause more impact by one species than another. Cattle have a value of 1.0 so other ungulates have relative activity values of ±1.0, if they are more or less active than cattle.
S = Range shift. A migration through and off the site would have impact during the movement, yet it would preclude any action adding to the movement for the subsequent months and would therefore be for only a portion of the time interval; the value of S would be 0.0. If no shift occurred during the time interval, S = 1.0.
Hungerford's SDI for mule deer, elk, cattle and horses in Carson National Forest, New Mexico, are in Table 5. Use of the SDI can quantify the physical impact of large herbivores on soil communities to ascertain when management practices should begin. The SDI is a comparative measure of the impact of one class of animal upon a site when compared with another animal. The effect herbivores have on soil communities may be nearly as important as the amount of vegetation they consume.
Soil properties that most affect the distribution and abundance of animals include soil structure, texture, moisture, aeration and chemical composition (Kevan, 1962; Kuhnelt, 1976). These properties do not influence animals independently, but act in concert with each other and with biotic factors such as the presence of food, symbiots and predators. The following discussion briefly outlines the role of soil as it influences animals that live in and on soils.
Soil Structure and Texture. The structure and texture of soil affect the distribution of burrowing animals. Compact soils or very rocky soils may reduce the rate of burrowing by earthworms, for instance (Guild, 1955), or preclude burrowing altogether. Soils that are too fine may not be suitable for burrowing, except for those animals with special adaptations (Kuhnelt, 1976). These generalities hold not only for animals that spend all their lives in the soil, but also for vertebrates such as burrowing rodents that spend varying amounts of time underground. For example, Koford ( 1958) found that most prairie dog towns were on deep alluvial soils of medium to fine texture, whereas few were on shallow, sandy or rocky soils.
Another aspect of soil structure, the size and number of spaces between soil particles, influences the species composition and verticle distribution of nonburrowing animals. A clear, positive correlation exists between the average size of pore space in soils and the animals that inhabit them (Kuhnelt, 1958). Furthermore, the size and number of spaces in soils affect soil moisture and carbon dioxide content. Thus, soil structure may indirectly influence animal distribution in other ways described below.
Moisture. The species composition of animals that live in the microcaverns of the soil is influenced not only by the size of the microcaverns, but by the moisture content of these small cavities. Generally, the smallest of the soil organisms are the most susceptible to desiccation. They often are found deep in soils in the most narrow crevices. These small spaces hold water for the longest time due to surface tension. The thin film of water that forms around soil particles provides habitat for a number of small organisms (Kevan, 1962; Kuhnelt, 1976). Lack of moisture may limit the number of soil animals, as illustrated by the limited biological activity in the soils of Arizona deserts (Wallwork, 1976).
Aeration. Soil aeration is difficult to separate from soil moisture because they generally correlate inversely (Kevan, 1962). The resistance of soil organisms to high carbon dioxide levels is extremely variable, and little is known about aeration requirements of many soil animals. But some species such as nematodes (Wallace, 1956) apparently depend on specific levels of oxygen for successful emergence.
Chemical Composition. The pH of soils depends to a large extent upon the soil parent material, but the kind of vegetation on the surface and the level of aerobic and anaerobic decomposition processes also influence soil pH (Kevan, 1962; Wallwork, 1976). Soil organisms vary considerably in their preferences for soil pH, but most avoid very acid soils (Kevan, 1962).
Perhaps the most significant influence that soil has on the distribution and number of animals that live aboveground is through the relationships between soil and vegetation. In the broadest sense, the distribution of vegetation on the surface of the Earth is controlled primarily by climatic conditions. Temperature and precipitation are the most important climatic factors (Ricklefs, 1979). However, superimposed upon these general vegetative patterns are numerous other factors, both biotic and abiotic, that influence plant communities. Soil conditions are among the more important of these factors and play a significant role in determining the characteristics of vegetation on a particular site.
The importance of the structure, composition and general vigor of vegetation in determining the distribution and health of animal populations is difficult to overstate. The general configuration of vegetation is thought to be an important proximate factor in habitat selection by birds (Hilden, 1965; James, 1971). For some mammals, such as elk and deer, the structure of vegetation determines the suitability of a site for thermal or hiding cover (Thomas et al, 1979). The species composition and vigor of vegetation are of obvious importance to animals that feed directly on plants, but also are important to predators. In the latter group, selection of a particular species of plant or vegetation configuration when foraging may occur because the number of prey is greater there, or because the configuration allows the predator to search more easily for and capture prey (e.g. in birds; Holmes and Robinson, 1981). In the Southwest, vegetation even may be the primary source of water for some birds and mammals such as Gambel's Quail and bighorn sheep. Ultimately then, vegetation can provide most of the elements necessary for survival: food, shelter and water. And changes in soil conditions that produce changes in vegetation potentially have tremendous impacts on animal populations.
Allen ( 1962) recognized that soils influence animals indirectly through vegetation, and emphasized the correlations that exist between soil fertility and the density and health of animal populations. His examples from the eastern United States clearly showed that white-tailed deer, rabbits, raccoons, muskrats, wild turkeys, pheasants and Bobwhite Quail were in better condition on fertile soil than on poor soil. Similar examples can be found in the Southwest. For instance, the most dense populations of Gambel's Quail inhabit areas of residual soils of decomposed granite, or of floodplain soils of river bottoms (Johnsgard, 1976). Both types of soils support the relatively luxuriant and diverse vegetation preferred by Gambel's Quail.