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:
- pea- or bean-size granules of soil in the form of worm casts;
- thumb-size blocky soil peds shaped by cicada nymphs while tunneling through B horizons (Hugie and Passey, 1963);
- pits and channels that are excavated by antlions, spiders, beetles, ants, termites, scorpions, worms and, in wet soils, crayfish; and
- filled channels, or tubules, and chambers, or glaebules, packed with excreta, brood structures or edible plant and animal materials.
Although the precise ways soil invertebrates interact in soil communities are mostly unknown, important groups have been identified and their roles established.
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
to turnover of available nutrients and to enhancement of biochemical activity in soils (Stout and Heal, 1967).
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
the soil surface from silt-loam to loam in some areas (Thorp, 1949).
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.
SDI = M(WRA)/S
M = Moisture vulnerability. This ranges from 1.0, dry or frozen soil, to 4.0, saturated soils.
W = The mean force exerted on the soil by an average hoofprint expressed as psi.
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 has the added benefit of naturally eliminating odors.
3. Foraging Walls
Whether you have a small tank or a huge vivarium, a foraging wall will multiply the amount of usable space. A foraging wall is simply a piece of cardboard that perfectly fits your tank wall, with steps, passageways, and hides for your mouse to explore. Every morning and evening, I scatter treats in new locations on the foraging walls. The mice do their rounds, following their noses, and never know what they will find and where they will find it. This is a great way to add variety and excitement to their lives. I love the walls in Mumblebee’s shop, and incorporate them into my mouse house.
Foraging walls and other structures should be made of clean cardboard (with no labels, stickers, or paint), plain popsicle sticks, unbleached coffee filters, plain printer paper, phone book paper, brown paper, Elmer’s glue, and hot glue. I also incorporate small glass baby food and yogurt jars, being careful to glue them on very securely. All foraging items get thrown out after 1-4 weeks, depending on how dirty they get and how many mice you have.