The Macrofauna

Macroarthropods

Larger insects, spiders, myriapods, and others are considered together under the appellation "macroarthropods." Typical body lengths range from about 10mm to as much as 15 cm (Scolopendromorph centipedes) (Shelley, 2002). The group includes an artificial mix of various arthropod classes, orders, and families. Like the microarthropods, the macroarthropods are defined more by the methods used to sample them rather than by measurements of body size.

Large soil cores (10 cm in diameter or greater) may be appropriate for euedaphic species; arthropods can be recovered from them using flotation techniques (Edwards, 1991). Mechanical or hand sorting of soils and litter is more time-consuming but yields better estimates of population size. In rare instances, capture-mark-recapture methods have been used to estimate population sizes of selected macroarthropod species, but the assumptions for this procedure are violated more often than not (Southwood, 1978).

Pitfall traps have been widely used to sample litter- and surface-dwelling macroarthropods (Banerjee, 1970; Greenslade, 1964; Michail, 1993) (see Chapter 9). This method catches arthropods that blunder into cups filled with preservative. Absolute population estimates are difficult to obtain with pitfall traps (Gist and Crossley, 1973) but the method yields comparative estimates when used with caution.

Many of the macroarthropods are members of the group termed "cryptozoa," a group consisting of animals that dwell beneath stones, logs, under bark, or in cracks and crevices (Cole, 1946). Cryptozoans typically emerge at night to forage; some are attracted to artificial lights. The cryptozoa fauna is poorly defined and is not an ecological community in the usual sense of the term. The concept remains useful, however, for identifying a group of invertebrate species with similar patterns of habitat utilization.

Importance of the Macroarthropods

However they are sampled, the macroarthropods are a significant component of soil ecosystems and their food webs. Macroarthropods differ from their smaller relatives in that they may have direct effects on soil structure. Termites and ants in particular are important movers of soil, depositing parts of lower strata on top of the litter layer (Fig. 4.39). Emerging nymphal stages of cicadas may be numerous enough to disturb soil structure. Larval stages of soil-dwelling scarabaeid beetles sometimes churn the soil in grasslands. These and other macroarthro-

FIGURE 4.39. Fire ants (Solenopsis invicta) as soil movers (D. A. Crossley, Jr. photo).

pods are part of the complex that has been termed "ecological engineers."

Some macroarthropods participate in both above- and belowground parts of terrestrial ecosystems. Many macroarthropods are transient or temporary soil residents (see Fig. 4.1), and thus form a connection between food chains in the "green world" of foliage and the "brown world" of the soil. Caterpillars descending to the soil to pupate or migrating armyworm caterpillars are prey to ground-dwelling spiders and beetles. A ground beetle species, Calosoma sycophanta ("the searcher"), was imported from Europe for biological control of the gypsy moth (Kulman, 1974).

Macroarthropods may have a major influence on the microarthropod portion of belowground food webs. Collembola, among other micro-arthropods, are important food items for spiders, especially immature stadia, thus providing a macro-to-micro connection. Other macroarthro-pods such as cicadas emerging from soil may serve as prey for some vertebrate animals (Lloyd and Dybas, 1966), thus providing a link to the larger megafauna.

Among the macroarthropods there are many litter feeding species, such as the millipedes, that are important consumers of leaf, grass, and wood litter. These arthropods have major influences on the decomposition process, thereby impacting rates of nutrient cycling in soil systems.

And, the reduction of vertebrate carrion is largely accomplished through the actions of soil-dwelling insects (Payne, 1964).

The vast array of macroarthropod species in soil systems constitutes a major reservoir of biodiversity. As with the mites and collembolans, the functional significance of this diversity is not evident. Intuitively, it would seem that the large number of species participating in belowground food webs should increase their stability and enhance the recovery following disturbance, but the concept has remained elusive.

Isopoda

Terrestrial isopods (Fig. 4.40) are crustaceans, but are typical crypto-zoa, occurring under rocks and in similar habitats. Although they are distributed in a variety of habitats, including deserts, they are susceptible to desiccation. Adaptations to resist desiccation include nocturnal habits, the ability to roll up into a ball, low basal respiration rates, and restriction of respiratory surfaces to specialized areas. Considered to be general saprovores, isopods can feed upon roots or foliage of seedlings. Isopods possess heavy, sclerotized mandibles and are capable of considerable fragmentation of decaying vegetable matter. They display some selectivity in preferences for different leaf species. Digestive processes in the terrestrial isopods encompass a wide extent of biochemical complexity, with detoxification of ingested phenolics in the foregut, digestion by endogenous and bacterial enzymes in the anterior hindgut, absorption of nutrients, and microbial proliferation in the posterior hindgut (Fig. 4.41) (Zimmer, 2002). In the laboratory, terrestrial isopods feed upon fecal pellets dropped by themselves or by any other isopod (Zimmer, 2002). There is some doubt about how common this trait is

FIGURE 4.40. Aterrestrial isopod, Armadillidium vulgare. Left, extended; right, rolled into a ball (from Metcalf and Flint, 1939).
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