Fauna the Engine for Microbial Activity and Transport

David C. Coleman Diana H. Wall

Introduction

The Microfauna

Rotifera

Nematoda

Microarthropods

Enchytraeids

Macrofauna

Summary

References introduction

Animals, another group of major heterotrophs in soil systems, can be viewed as facilitators of bacterial and fungal activity and diversity in soils. They exist in food webs containing several trophic levels. Some are herbivores, since they feed directly on roots of living plants, but most subsist upon dead plant matter (saprophytes), or living microbes associated with it, or a combination of the two. Still others are carnivores, parasites, or top predators. Analyses of food webs in the soil have emphasized numbers of the various organisms and their trophic resources. The structure of these food webs is complex, with many "missing links" poorly described or as yet unknown (Walter et al., 1991; Scheu and Setala, 2002).

Animal members of the soil biota are numerous and diverse and include representatives of all terrestrial phyla. Many groups of species are not described taxonomically, and details of their natural history and biology are unknown. For the microarthropods only about 10% of populations have been explored and perhaps 10% of species described (André et al., 2002). We feel protection of biodiversity in ecosystems clearly must include the rich pool of soil species. This is because data for some of these species individually and collectively indicate tight connections to biodiversity aboveground, major roles in ecosystem processes, and provision of ecosystem benefits for human well-being (Wardle et al., 2004; Wall et al., 2005; Wall, 2004).

When research focuses at the level of the soil ecosystem two things are required: the cooperation of multiple disciplines (soil scientists, zoologists, and microbiologists) and the lumping of animals into functional groups. These groups are often taxonomic, but species with similar biologies and morphologies are grouped together for purposes of integration (Coleman et al., 1993; Hendrix et al., 1986; Hunt et al., 1987).

The soil fauna also may be characterized by the degree of presence in the soil 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. Gnats (Diptera) are temporary residents of the soil, since 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 are temporary soil residents, whose larvae feed on seedlings by night. Nematodes that parasitize insects and beetles spend part of their life cycle in soil. Periodic residents spend their life histories belowground, with adults, such as the velvet mites, emerging to reproduce. The soil food webs are linked to aboveground systems, making trophic analyses much more complicated than in either subsystem alone (Wardle et al., 2004). 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. 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 apparatus (furcula). Collembolans living within mineral soil tend to be smaller, with unpigmented, elongate bodies, and possess a much reduced furcula.

A generalized classification by length 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 ca. 1-2 pm for the microflagellates to >2 m for giant Australian earthworms. Body width of the fauna is related to their microhabitats (Fig. 7.1). The microfauna (protozoa, rotifers, tardigrades, small nematodes) inhabit water films. The mesofauna inhabit existing air-filled pore spaces and are largely restricted to existing spaces. The macrofauna have the ability to create their own spaces, through their burrowing activities, and like the megafauna, can have large influences

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