Xeric Habitats

Cockroaches are known to participate in the breakdown of plant organic matter in deserts and other arid and semiarid landscapes, and have a direct and substantial impact on nutrient flow. Anisogamia tamerlana is the main consumer of plant litter in Turkmenistan deserts (Kaplin, 1995), and cockroaches in the genus Hetero-gamia are the most abundant detritivore in the Mediterranean coastal desert of Egypt. The latter dominate the arthropod fauna living beneath the canopy of desert shrubs, with up to 116,000 cockroaches/ha, comprising 82% of the arthropod biomass (Ghabbour et al., 1977; Ghabbour and Shakir, 1980). The daily food consumption of An. tamerlana is 17-18% of their dry body mass, with 57-69% assimilation. Females and juveniles con sume 840-1008 g/ha dry plant debris and produce 259320 g/ha of excrement (Kaplin, 1995). These cockroaches improve the status of desert soils via their abundant fecal pellets, the nitrogen content of which is 10 times that of their leaf litter food source (El-Ayouty et al., 1978).

Many of the ground-dwelling, wingless cockroaches of Australia are important in leaf litter breakdown. This is particularly true in stands of Eucalyptus, where litter production is high relative to other forest types, leaves decompose slowly, and more typical decomposers such as earthworms, isopods, and millipedes are uncommon (Matthews, 1976). The beautiful Striped Desert Cockroach Desmozosteria cincta, for example, lives among twigs and branches at the base of eucalypts (Rentz, 1996). In hummock grasslands and spinifex, genera such as Anamesia feed on the dead vegetation trapped between the densely packed stems (Park, 1990). The litter-feeding, soil-burrowing Geoscapheini are associated with a variety of Australian vegetation types ranging from dry scle-rophyll to rainforest, and have perhaps the most potential ecological impact. First, they drag quantities of leaves, twigs, grass, and berries down into their burrows, thus moving surface litter to lower soil horizons. Second, they deposit excreta deep within the earth. Fecal pellets are abundant and large; those of Macropanesthia rhinoceros are roughly the size and shape of watermelon seeds. Third, burrowing by large-bodied insects such as these has profound physical and chemical effects on the soil. Burrows influence drainage and aeration, alter texture, structure, and porosity, mix soil horizons, and modify soil chemical profiles (Anderson, 1983; Wolters and Ekschmitt, 1997). The permanent underground lairs of M. rhinoceros have plastered walls and meander just beneath the soil surface before descending in a broad spiral (Fig. 10.2). The deepest burrows can be 6 m long, reach 1 m below the surface, and have a cross section of 4-15 cm. Burrows may be locally concentrated; the maximum density found was two burrows/m2, with an average of 0.33/ m2 (Matsumoto, 1992; Rugg and Rose, 1991).

Cockroaches in arid landscapes nicely illustrate two subtleties of the ecological role of decomposers: first, an often mutualistic relationship with individual plants, and second, the key role of gut microbiota. In sparsely vegetated xeric habitats, the density of cockroaches generally varies as a function of plant distribution. In deserts, Polyphagidae are frequently concentrated under shrubs (Ghabbour et al., 1977), and the burrows of Australian Geoscapheini are often associated with trees. Macro-panesthia heppleorum tunnels amid roots in Callitris-Eucalyptus forest, and Geoscapheus woodwardi burrows are located under overhanging branches of Acacia spp. in mixed open forest (Roach and Rentz, 1998). Not only are

Fig. 10.2 Burrow of Macropanesthia rhinoceros. Although it does not descend deeper than about 1 m, the gently sloping spiral may be up to 6 m long. Near the bottom the tunnel widens to become a nesting chamber to rear young and to cache dried leaves. Drawing by John Gittoes, courtesy of Australian Geographic.

Fig. 10.2 Burrow of Macropanesthia rhinoceros. Although it does not descend deeper than about 1 m, the gently sloping spiral may be up to 6 m long. Near the bottom the tunnel widens to become a nesting chamber to rear young and to cache dried leaves. Drawing by John Gittoes, courtesy of Australian Geographic.

these cockroaches ideally located to collect plant litter, they are also positioned to take advantage of the shade, moisture retention, and root mycorrhizae provided by the plant. Reciprocally, the burrowing, feeding, and excretory activities of the cockroaches influence patterns of aeration, drainage, microbial performance, decomposition, and nutrient availability in the root zone of the plants (Anderson, 1983; Ettema and Wardle, 2002). This mutualistic relationship therefore may allow for peak performance by both parties in a harsh environment. It is a tightly coordinated positive feedback system in which decomposers improve the quantity and quality of their own resource (Scheu and Setala, 2002).

Another alliance of ecological consequence occurs at a much smaller scale. Because the activity of soil microbes is dependent on water, decomposition in deserts occurs in pulses associated with precipitation. Ciliates, for example, occur in the soil in great numbers, but are active only in moisture films. As a consequence, microorganisms remain dormant most of the time and plant litter accumulates in deserts, restricting nutrient flow (Kevan, 1962; Taylor and Crawford, 1982). A significant resolution to this bottleneck lies in the digestive system of de-tritivores such as cockroaches. The gut environment allows for a relatively continuous rate of microbial activity, even during periods inimical to decomposition by free-living microbes in soil and litter. This relationship is present wherever cockroaches feed, but has a profound ecological significance in deserts and other extreme environments because it allows for decomposition during periods when it would not normally occur—in times of drought or excessive heat or cold (Ghabbour et al., 1977; Taylor and Crawford, 1982; Crawford and Taylor, 1984).

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