Cockroaches potentially influence biogeochemical cycles via two known pathways: nitrogen fixation and methane production. Cryptocercus is the only cockroach currently known to harbor gut microbes capable of fixing atmospheric nitrogen (Breznak et al., 1974), but spirochetes found in the hindgut of other species also may have the ability (Lilburn et al., 2001). Acetylene reduction assays indicate that adults and juveniles of Cryptocercus fix nitrogen at rates comparable to those of termites on a body weight basis (0.01-0.12 mg N day-1 g-1 wet weight) (Breznak et al., 1973; Breznak et al., 1974, 1975). The process provides a mechanism for nitrogen return to the ecosystem and may have a significant ecological impact (Nardi et al., 2002), particularly in the food chains of the montane mesic forests where Cryptocercus is the dominant macroarthropod feeding in rotting logs.
A more universal characteristic of cockroaches is an association with methanogenic bacteria in the hindgut and the consequent emission of methane. Almost all tropical cockroaches tested emit methane, regardless of the origin of specimens and their duration of laboratory captivity. Methane, carbon dioxide, and water are released synchronously in a resting cockroach, in slow periodic cycles that suggest the gases are respired (Bijnen et al., 1995, 1996). Among temperate species, North American C. punctulatus emits the gas (Breznak et al., 1974), but the European genus Ectobius does not (Hackstein and Strumm, 1994). Cockroaches (n = 34 species) produce an average of 39 nmol/g methane/h, with a maximum of
Fig. 10.4 Scorpion feeding on the ground-dwelling cockroach Homalopteryx laminata, Trinidad. Photo courtesy of Betty Faber.
450 nmol/g/h (Hackstein, 1996). On a global scale, estimates of methane production by cockroaches vary widely and are debatable, given first, the paucity of data on which to base biomass estimates of field populations, and second, the finding that methane production varies with cockroach age and diet fiber content (Gijzen et al., 1991; Kane and Breznak, 1991). It has been suggested that cockroaches make a significant contribution to global methane, particularly in the tropics (Gijzen and Barugahare, 1992; Hackstein and Strumm, 1994). However, methane oxidation by bacteria in the soil may buffer the atmosphere from methane production by gut Archaea, and although cockroaches may be a gross source of methane, little to none of it may be escaping into the atmosphere. The sink capacity of the soil may exceed methane production by cockroaches, just as it does for termites (Eggle-ton et al., 1999; Sugimoto et al. 2000). Nonetheless, their typically large body size (relative to termites), and the tendency of many species to live in aggregations in enclosed spaces (e.g., treeholes, caves, logs) may engender atmospheric changes at a local level. Mamaev (1973), for example, collected more than 400 C. relictus from a single cedar log. On a per weight basis methane production by C. punctulatus is comparable to the termite Reticulitermes flavipes and may surpass levels emitted by ruminants (Breznak et al., 1974; Breznak, 1975).
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