Microarthropods

Large numbers of the microarthropod group (mainly mites and collembolans) are found in most types of soils. A square meter of forest floor may contain hundreds of thousands of individuals representing thousands of different species (Fig. 4.11). Microarthropods have a significant impact on the decomposition processes in the forest floor (see Chapter 5) and are important reservoirs of biodiversity in forest ecosystems.

Microarthropods also form an important set of linkages in food webs. Many microarthropods feed on fungi and nematodes, thereby linking the microfauna and microbes with the mesofauna. Microarthropods in turn are prey for macroarthropods such as spiders, beetles, ants, and centipedes, thus bridging a connection to the macrofauna. Even some of the smaller megafauna (toads, salamanders) feed upon microarthro-pods. We emphasize, again, the need to study soil as an ecosystem. Analysis of one part of the food web, the microarthropods for example, falls short if other components are ignored.

In the size spectrum of soil fauna (see Figs. 4.2 and 4.3), the mites and collembolans are found among the mesofauna. Members of the microarthropod group are unique, not so much by their body size as by the methods used for sampling them. Microarthropods are too small and numerous to be sampled as individuals. Instead, small pieces of habitat (soil, leaf litter, or similar materials) are collected and the microarthropods extracted from them in the laboratory. In this manner they resemble certain of the microfauna such as nematodes, rotifers, or tardigrades. Most of the methods used for microarthropod extraction are either variations of the Tullgren funnel ("Berlese funnel"), which uses heat to desiccate the sample and force the arthropods into a collec-

Extraction Organisms Soils Images
FIGURE 4.11. An example of the thousands of species of soil microarthropods (D. A. Crossley, Jr. photo).

tion fluid, or flotation in solvents or saturated sugar solutions followed by filtration (see Chapter 9). Edwards (1991) gives an extensive review of these procedures. Both approaches to sampling microarthropods have their proponents. Generally, flotation methods work well in low organic, sandy soils whereas Tullgren funnels perform best in soils with high organic matter content. Flotation procedures are much more laborious than is Tullgren extraction.

Choice of method also depends upon the objectives of the sampling program. If numbers of individuals are to be measured, a large set of small samples may be needed. Estimations of species number may be better served by fewer, larger samples. In any case, extraction methods are never completely efficient and, indeed, efficiency of sampling is seldom estimated (André et al., 2002). Consequently, Walter and Proctor (1999) concluded that enumeration of microarthropods was an "intellectually vacuous" exercise. However, valid comparisons of microarthropod abundance in different habitats may be obtained even if extraction efficiencies, though unknown, are similar.

Microarthropod densities vary during seasons within and between different ecosystems (Table 4.2). Generally, temperate forest floors with large accumulations of organic matter support high numbers, whereas tropical forests where the organic layer is thin contain lesser numbers of microarthropods (Seastedt, 1984b). Disturbance or perturbation of soils usually depresses microarthropod numbers. Tillage, fire, and pesticide applications typically reduce populations but recovery may be rapid and microarthropod groups respond differently.

Soil mites usually outnumber collembolans but these become more abundant in some situations. In the springtime, forest leaf litter may develop large populations of "snow fleas" (Hypogastrura nivicola and related species). Among the mites themselves the oribatids usually dominate but the delicate Prostigmata may develop large populations in cultivated soils with a surface crust of algae. Immediately following cultivation, numbers of astigmatic mites have been seen to increase dramatically (Perdue, 1987).

TABLE 4.2. Abundance of Microarthropods in Soils from Various Ecosystems

Ecosystem

Microarthropods (103 per m2)

Reference

Fallow crop fields, Nigeria

0 0

Post a comment