V

Non-

enzymatic secretion or microbial lysis?

K+ secretion; filament propagation and mixing with ingested soil

Alkaline hydrolysis at pH up to >12.0

Fermentation in core, and mineralization at periphery of lumen

Neutralization of alkaline pH and aerobic processing of soil organic matter?

FIGURE 4.59. Hypothesis of gut organization and sequential processing in soil-feeding Cubitermes--clade termites. The model emphasizes the role of filamentous prokaryotes, the extremely high pH reached in the P1, and the existence of both aerobic and anaerobic zones within the hindgut. Major uncertainties have question marks. Not to scale (from Brauman et al., 2000).

types were found for the first time. Others were monophyletic clusters with sequences recovered from the gut of other termite species. It should be noted that cellulose digestion in termites, which was once considered to be solely due to the activities of fungi, protists, and occasionally bacteria, has now been convincingly demonstrated to be endogenous to termites. Endogenous cellulose-degrading enzymes occur in the midguts of two species of higher termites in the genus Nasutitermes, and in the Macrotermitinae (which cultivate basidiomycete fungi in elaborately constructed gardens) as well (Bignell, 2000).

In contrast to the carbon degradation situation, only prokaryotes are capable of producing nitrogenase to "fix" N2, or dinitrogen. This process occurs in the organic-matter-rich, microaerophilic milieu of termite guts. Some genera have bacteria that fix relatively small amounts of nitrogen, but others, including Mastotermes and Nasutitermes, have from 0.7 to greater than 21 micrograms (|ig) nitrogen fixed •g fresh weight. This equals 20-61 |ig of nitrogen per colony per day, which would double the nitrogen content if N2 fixation was the sole source of nitrogen and the rate per termite remained constant (and the nitrogen content of termites is assumed to be 11% on a dry weight basis) (Breznak, 2000).

Termites are one of the three major earth-moving groups of invertebrates (the other two are earthworms and ants). Mound-building termite species have a major impact on the distribution and composition of soil mineral and organic matter. Where there is rich, well-drained grassland (e.g., in the Ivory Coast), humivorous and fungus-growing termites are common. In areas of poor drainage, these species are absent and grass feeders such as Trinervitermes and some Macrotermes spp. are common. In some cases, farmers grow crops on mounds. This is advantageous in areas that are flooded, such as paddy fields in Southeast Asia (e.g., Thailand) (Pearce, 1997). In desert regions of North America (e.g., the Chihuahuan desert of southern New Mexico), termites are considered "keystone arthropods," removing and processing large amounts of dead and dying net primary production every year (see Table 4.9) (Whitford, 2000). For a masterful exposition of the role of termites in ecosystems worldwide, refer to Bignell and Eggleton (2000).

Other Pterygota

As we noted above, most terrestrial insect orders have members that participate in soil systems, either by burrowing, pupating, or even feeding there. At times they may be present in some numbers, or exert an unusual influence on food webs.

The Orthoptera, grasshoppers and crickets, lay eggs in soils and some are active on the soil surface. Crickets, Gryllidae, may be abundant in pitfall traps set in meadows or agroecosystems (Blumberg and Crossley, 1983).

The Psocoptera, psocids, are a small order of insects that occasionally become abundant in leaf litter. They feed on organic detritus, algae, lichens, and fungus (Aldrete, 1990).

The order Homoptera, cicadas, aphids, and others, has members important as belowground herbivores and as soil movers. Cicadas are noisy, active flyers as adults. The immature stages feed upon the roots of perennial plants until mature, a time period that may last 13-17 years for periodical cicadas (Magnicicada spp.). In tallgrass prairie soils, cicadas are abundant insects; their annual emergence can result in a significant flux of nutrients from belowground to aboveground (Callaham et al., 2000).

Gastropoda

Terrestrial gastropods (snails and slugs) (Fig. 4.60) (Burch and Pearce, 1990) are major players among herbivores and detritivores in many ecosystems, particularly agroecosystems (Byers et al., 1989).

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FIGURE 4.60. Terrestrial gastropods (snails and slugs): At top, a nonoperculated (pulmonate) snail. It does not have a protective operculum to seal the shell aperture when the animal has withdrawn into its shell. (a) Active snail; (b) Inactive snail withdrawn into its shell, with only the surface of its foot showing. At bottom, slug body terminology (from Burch and Pearce, 1990).

FIGURE 4.60. Terrestrial gastropods (snails and slugs): At top, a nonoperculated (pulmonate) snail. It does not have a protective operculum to seal the shell aperture when the animal has withdrawn into its shell. (a) Active snail; (b) Inactive snail withdrawn into its shell, with only the surface of its foot showing. At bottom, slug body terminology (from Burch and Pearce, 1990).

They have been studied much less than the arthropod fauna in forests. They tend to require moist conditions and the presence of significant amounts of calcium for their metabolic needs, but some gastropods exist successfully in low pH and low calcium environments (Burch and Pearce, 1990). The terrestrial gastropod faunas have become rich and diversified as they have invaded many habitats. Nearly a thousand species occur in North America north of Mexico, and similar rich faunas occur throughout Europe (South, 1992). Land gastropods are quite speciose and ubiquitous in the eastern United States, with at least 500 species, including both snails and slugs, being described from the eastern United States (Hubricht, 1985, cited in Hotopp, 2002). Snails seem to key in on structural attributes of their environment. In an extensive study of community patterns of 108 snail species in the Great Lakes region, Nekola (2003) found that soil surface architecture—deep organic horizon soils (deeper than 4 cm, "duff') versus thin horizon soils subtended by live roots (less than 1 cm deep, "turf')—accounted for 43% of the variability of snail distributions.

In Danish beech forests, Petersen and Luxton (1982) measured 822 mg • m-2 gastropod biomass, an amount that was exceeded by only diplopods and earthworms in the fauna of that forest floor. Terrestrial gastropods feed primarily on plants, but may prefer decaying or senescent tissues. Numerous basidiomycetes are consumed as well, including some that are highly toxic to mammals. Only a few gastropods feed on animals, but several may feed on carrion. The feces of gastropods retrieved from the wild include soil particles, which may be due to humic acids as a required substrate in their diet (this is particularly true for helicid snails grown in culture) (Speiser, 2001). Feeding rates on leaf litter range from 9.3 to 28.1 mg • g-1 live weight of slugs in European forest floors (Jennings, 1975). Assimilation rates of Agriolimax reticulatus feeding on fresh Ranunculus repens (lotus) leaves were greater than 78%, and snails' assimilation rates ranged between 40 and 70%, feeding on either fresh leaf or leaf litter material (Mason, 1974). Pallant (1974) estimated rates of assimilation for several slug species to average 161.3 Joules 100 mg-1 dry weight in grasslands and 141.5 J 100 mg-1 dry weight in woodland, in Europe.

For a review of extensive phylogenies of terrestrial gastropods, using 28S rDNAand morphological data, see Barker (2001).

Sampling Techniques for Gastropods

Two different sampling techniques were employed by Hotopp (2002), working in the forests of the central Appalachian Mountains: (1) timed searching, and (2) sieving litter. The former approach constitutes a 10-

minute search of leaf litter surface, rocks, woody debris, and live plant stems across a 200 square meter sample plot. It tends to be more efficient in finding large snails and slugs. The latter method consists of placing litter from Oi and Oe horizons on a 10-mm sieve placed to a depth of about 10 cm, shaking it 50 times, turning it over, and shaking it 50 more times. This method is more efficient in retrieving small specimens including the ecological dominant, Punctum minutissimum, which is about 1mm in width.

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