Improvement Of Nutritional Environment For Invertebrates

The carbon:mineral nutrient ratios vary considerably between different plant litter components, the C:N and C:P ratios of undecayed leaf litter ranging between 25:1 to 100:1 and 450:1 to 1,850:1, respectively, and of undecayed wood between 350:1 to 500:1 and 1,250:1 to >3,500:1 (Dighton and Boddy, 1989). As decomposition proceeds, however, the ratios decrease, providing animals with a resource of higher nutritional quality. Further, fungal mycelium itself has seven times better C:N and C:P ratios than undecayed wood (Swift and Boddy, 1984). Similarly, with external symbiotic relationships, the N and P contents in fungus gardens of Termitomyces in the nests of Macrotermes bellicosus are greater than in the food initially collected: food 0.28% N, 0.15% P; food store 0.58% N, 0.12% P; fresh fungus comb 0.85% N, 0.19% P; mycotetes 6.68% N, 0.46% P (see Swift and Boddy, 1984). The attraction of mycophagy is clear: invertebrates need to eat far less fungal mycelium than plant litter to meet their mineral nutrient requirements. Even when invertebrates are feeding on wood and leaf litter a considerable fraction of the nutrients consumed will be within hyphae of Bas-idiomycota.

The benefits of feeding on fungal-decayed resources is seen with the death watch beetle, Xestobium rufovillosum (Anobiidae), that typically attacks wood colonized by the heart-rot (Chapter 11) Basidiomycota Laetiporus sulphurous, Fistulina hepatica and Phellinus cryptarum in oak, and Bjerkandera fumosa, Trametes versicolor and Coniophora puteana in willow and other taxa (Fisher, 1940,1941). The length of the beetle life cycle is inversely related to the state of decay of the wood: in undecayed wood the larvae developed very slowly or not at all, but in decayed wood the life cycle was completed within 10-17 months, in the laboratory. This is not only a nutritional effect, but also relates to physical factors, the larvae being able to process a larger volume of decayed than undecayed wood, and consequently obtain nitrogen more rapidly (Fisher, 1941; Rayner and Boddy, 1988). Similar nutritional and physical considerations are likely to apply to siricid wood wasps that have a mutualistic symbiotic relationship with Amylostereum aerolatum and A. chailletii in conifers, and Cerrena (= Daedalea) unicolor in angiosperms (Martin, 1992; Thomson and Koch, 1999; Slippers et al., 2003). The larvae are wood borers that burrow through wood colonized by the fungi; to ensure the presence of the wood-rotting fungus the female carries oidia and inoculates them into the wood during oviposition.

Ingested enzymes, from the fungus, also play an important role in the digestion of plant litter in siricid wood wasp, fungus-growing termite and fungus-growing ant mutualisms (Martin, 1992). Cellulose and hemicellulose are digested during passage through the alimentary tract, predominantly the midgut of Sirex spp. larvae, and by the termite and attine ant workers. These acquired enzymes survive gut passage and, in the case of the ants and termites, are concentrated in the faecal droplet which is deposited on fresh plant material (Martin, 1992; Ronhede et al., 2004). This prepares plant material for fungal colonization and increases the initial growth.

Basidiomycota also render palatable wood and leaf litter that is initially repellent or unpalatable to invertebrates due to the presence of allelopathic compounds. Again there are well-documented examples for termites (see references in Swift and Boddy, 1984). There are several examples of trees whose central heartwood is resistant to attack from termites when undecayed, but not once decay has begun. Of course, other aspects of enzyme conditioning (e.g. density reduction) may also play a part. Phenolics are also degraded by the mutualistic fungus partner on the fungus comb within termite nests (Taprab et al., 2005).

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