Cockroache; Ecology, behavior & history - W.J. Bell

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 microbial
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).
Forests
In temperate climates, cockroaches are usually relegated
to a minor role in soil biology because population densities
can be low (e.g., Ectobius spp. in central Europe—
Eisenbeis and Wichard, 1985). Similarly, in surveys of
tropical forest litter, ants, mites, and springtails typically
dominate in number, with cockroaches rating an incidental
mention (e.g., Fittkau and Klinge, 1973). Cockroaches
comprised just 3.0% of the arthropod biomass of
the ground litter in a humid tropical forest in Mexico
(Lavelle and Kohlmann, 1984), for example. On the other
hand, cockroaches are very common in the leaf litter on
the floor of the Pasoh Forest in West Malaysia, with 6.7
insects/m 2 (Saito, 1976). They are very well represented
in several forest types in Borneo. Leakey (1987) cites a
master’s thesis by Vallack (1981) in which litter invertebrates
were sampled in four forest types at Gunung Mulu
in Sarawak. Cockroaches contributed an impressive 43%
of the invertebrate biomass in alluvial forest, 33% in
dipterocarp forest, 40% in heath forest, and 2% in a
forest situated on limestone. A specific decomposer role
has been quantitatively established for Epilampra irmleri
in Central Amazonian inundation forests (Irmler and
Furch, 1979). This species was estimated to be responsible
for the consumption of nearly 6% of the annual leaf
litter input. Given that seven additional cockroach species
were noted in this habitat, the combined impact on decompositional
processes may be considerable.
The ecological services of cockroaches are not limited
to plant litter on the soil surface. Those species found in
logs, treeholes, standing dead wood and branches, birds’
nests, and plant debris trapped in epiphytes, lichens,
mosses, and limb crotches in the forest canopy (i.e., suspended
soils) are also members of the vertically stratified
decomposer niche (Swift and Anderson, 1989). Cockroach
species that feed on submerged leaf litter on stream
bottoms and in tank bromeliads may have an impact in
aquatic systems.
Wood Feeders
Wood-feeding cockroach species remove large quantities
of wood from the surface but their contribution to soil
fertility has yet to be explored. Both Panesthiinae and
Cryptocercidae progressively degrade the logs they inhabit.
They not only ingest wood, but also shred it without
consumption when excavating tunnels. The abundant
feces line galleries, pack side chambers, and are
166 COCKROACHES