Animals, the other group of major heterotrophs in soil systems, exist in elaborate food webs containing several trophic levels. Some soil animals are true herbivores, because they feed directly on roots of living plants, but most subsist upon dead plant matter, microbes associated with dead plant matter, or a combination of the two. Still others are carnivores, parasites, or top predators. Actual heterotrophic production by the soil fauna is poorly known, because turnover of the faunal biomass, feeding rates, and assimilation efficiencies are difficult to assess. Estimates of biomass of soil animals are not common, and knowledge of the rates of energy or material transfer in food webs is fragmentary (Moore and de Ruiter, 1991; 2000). Analyses of food webs in the soil have emphasized numbers of the various organisms and their trophic resources. Analysis of the structure of these food webs reveals complex structures with many "missing links" poorly described (Walter et al., 1991; Scheu and Setâlâ, 2002). Communities of soil fauna offer opportunities for studies of phenomena such as species interactions, resource utilization, or temporal and spatial distributions.

Animal members of the soil biota are numerous and diverse. The array of species is very large, including representatives of all terrestrial phyla. Many groups of species are poorly understood taxonomically, and details of their natural history and biology are unknown. For the microarthropods (discussed later in this chapter), only about 10% of populations have been explored, and perhaps only 10% of species described (André et al., 2002). Protection of biodiversity in ecosystems clearly must include the rich pool of soil species.

Soil ecologists cannot hope to become experts in all animal groups. When research focuses at the level of the soil ecosystem, two things are required: the cooperation of zoologists and the lumping of animals into functional groups. These groups are often taxonomic, but species with similar biologies are grouped together for purposes of integration (Coleman et al., 1983; 1993; Hendrix et al., 1986).

The soil fauna also may be characterized by the degree of presence in the soil (Fig. 4.1) or microhabitat utilization by different life forms. There are transient species exemplified by the ladybird beetle, which hibernates in the soil but otherwise lives in the plant stratum of the garden. Gnats (Diptera) represent temporary residents of the soil, because the adult stages live aboveground. Their eggs are laid in the soil and their larvae feed on decomposing organic debris. In some soil situations, dipteran larvae are important scavengers. Cutworms also are temporary soil residents, whose larvae feed on seedlings by night and hide by day. Periodic residents spend their lives belowground, with adults such as the velvet mites emerging perhaps to reproduce. From this perspective, the soil food webs are linked to aboveground systems, making trophic analyses much more complicated. Even permanent residents of the soil may be adapted to life at various depths in the soil.

Among the microarthropods, collembolans are examples of permanent soil residents (see Fig. 4.1). The morphology of collembolans reveals their adaptations for life in different soil strata. Species that dwell on the soil surface or in the litter layer may be large, pigmented, and equipped with long antennae and a well-developed jumping appa-


e.g., Hippodamia sp. e.g., Tipula spp. e.g., Forficula sp. e.g., Batrisodes spp. (Coleoptera: Coccinellidae) (Diptera) (Dermaptera) (Coleoptera: Pselaphidae)


e.g., Hippodamia sp. e.g., Tipula spp. e.g., Forficula sp. e.g., Batrisodes spp. (Coleoptera: Coccinellidae) (Diptera) (Dermaptera) (Coleoptera: Pselaphidae)

FIGURE 4.1. Categories of soil animals defined according to degree of presence in soil, as illustrated by some insect groups (from Wallwork, 1970).

ratus (furcula). Within the mineral soil, collembolans tend to be smaller with unpigmented, elongate bodies and much reduced furculae—there is no place to jump to.

Numerous researchers have marveled at the many and varied body-plans and size differences of the soil fauna. A generalized classification by length (Fig. 4.2) illustrates a commonly used device for separating the soil fauna into size classes: microfauna, mesofauna, macrofauna, and megafauna. This classification encompasses the range from smallest to largest (i.e., from about 1 to 2 micrometers [|im] of the microfla-gellates to several meters for giant Australian earthworms).

Body width of the fauna is related to their microhabitats (Fig. 4.3). The microfauna (protozoa, small nematodes) inhabit water films. The mesofauna inhabit air-filled pore spaces and are largely restricted to

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