Detritivory

Although they are rarely mentioned as such in soil science or ecology texts, the majority of cockroach species can be classified as soil fauna (Eisenbeis and Wichard, 1985). Many live in the upper litter horizon, some burrow into the mineral soil layer, and still others inhabit suspended soils. Cockroaches are also associated with decaying logs and stumps, rocks, living trees, and macrofungi, which are physically distinct from, but have biological links to, the soil (Wallwork, 1976). In the majority of these habitats, the core cockroach diet consists of dead plant material.

Because all species examined to date have endogenous cellulases (Scrivener and Slay-tor, 1994b; Lo et al., 2000), cockroaches may act as primary consumers on at least some portion of ingested plant litter. There is no question, however, that the direct impact of any higher-level primary consumer does not rate mention when compared to soil microorganisms, which are universally responsible for breaking down complex carbohydrates and mineralizing nutrients in plant detritus in all ecosystems. As with other arthropod decomposers (Wardle, 2002), then, the most profound impact of cockroaches is indirect, and lies in their complex and multipartite interaction with soil microbes. The physical boundaries between cockroaches and microbial consortia in soil and plant litter, however, are not always obvious (Fig. 5.3), and the relationship is so complex as to make discrete classifications or discussion of individual roles arbitrary. Here we center on how cockroaches alleviate factors that constrain microbial decomposition, namely, the microbial lack of automotion and their dependence on water.

Although microbial communities account for most mineralization occurring in soil, they are dormant the majority of the time because of their inability to move toward fresh substrates once nutrients in their immediate surroundings are exhausted. Macroorganisms such as cockroaches remove this limitation on microbial activity via their feeding and locomotor activities, by fragmenting litter and thereby exposing new substrate to microbial attack, and by transporting microbes to fresh food (Lavelle et al., 1995; Lavelle, 2002). The physical acts of burrowing and channeling cause small-scale spatial and temporal variations in microbial processes (Meadows, 1991). These, in turn, effect major changes in the breakdown of woody debris (Ausmus, 1977) and leaf litter (Anderson, 1983), and may also influence ecological processes in other cockroach habitats such as soil, guano, abandoned termite nests, and the substrate under logs, bark, and stones. In addition to making substrate available for mi-crobial colonization via physical disturbance and fragmentation, cockroaches transport soil microbes by carrying them in and on their bodies. This is particularly important in surface-foraging species that diurnally or seasonally take shelter under bark, in crevices, or in voids of rotting logs, where they inoculate, defecate, wet surface wood, affect nitrogen concentration, and contribute to bark sloughing (Wallwork, 1976; Ausmus, 1977).

A second factor that limits microbial decomposers is dependence on water (Lavelle et al., 1995). Cockroaches and other detritivores are able to mitigate this constraint, as the gut provides a moist environment for resident and ingested microbes. The hindgut also furnishes a stable temperature and pH, and a steady stream of fragmented, available substrate. In short, the detritivore gut provides an extremely favorable habitat if ingested microbes can elude the digestive mechanisms of the host. Fecal pellets, the end products of digestion, are similarly favorable habitats for microorganisms. Cockroaches on the floor of tropical forests consume huge quantities of leaf litter (Bell, 1990), thereby serving as mobile fermentation tanks that frequently and periodically dispense packets of microbial fast food. This alteration in the timing and spatial pattern of microbial decomposition may dramatically influence the efficient return of above-ground primary production to the soil. Fecal pellets also provide food for a legion of tiny microfauna, including Collembola, mites, protozoa, and nematodes. These feed on the bacteria and fungi growing on the pellets, as well as the fluids and metabolites resulting from excretory activity (Kevan, 1962).

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