Box 71 cont

bodies. Many of these species are showing a sharp decline in Europe (52% of basi-diomycetes are on at least one country's red data book list; Arnolds and de Vries, 1993) and heading this way elsewhere. A variety of reasons have been put forward, including air pollution and decline of old forests but intensive collection of the delectable edible fruiting bodies for culinary use may possibly be playing a significant part.

Although less conspicuous, microfungi (especially vegetative or asexually reproducing forms of ascomycetes which produce no fruiting bodies above-ground) are nonetheless important in decomposing plant debris. For example, just 5 grams of litter from tropical forests in Costa Rica have yielded up to 134 different species of microfungi (Bills and Polishook, 1994). A number of the microfungi, such as species of Aspergillus and Penicillium, assist in the breakdown of clay complexes, helping to make the soil structure more crumbly and less clayey in nature.

The degree of host-specificity varies tremendously between different fungal groups. Agarics decomposing fallen leaves rarely show strong host-specificity. Even when they do it is normally very broadly so, with distinctions being between monocotyledonous and dicotyledonous hosts in the tropics, and gymnosperms and angiosperms in temperate forests. In contrast, larger ascomycete fungi of the family Xylariaceae are often restricted to the leaves and fruits of particular hostplant genera or families in the tropics. Of the 13 species of Xylaria ('dead men's fingers') encountered in Puerto Rico, over half were thus restricted. Xylaria aristata, X. axifera, X, meliacearum, X. phyllocharis and X. stromatica were host-specific on leaves, and X. warburgii and X. palmicola on fruits. Others showed no strong preferences: X. apiculata, X. appendiculata, X. clusiae, X. ianthinovelutina on leaves, and X. mellisii and X. multiplex on fruits.

The amount of fungal biomass varies between different types of soil but can be up to around 70% of all the living biomass (including animals) in the soil, and between 60-90% of the microbial mass in forests, especially on acidic soils where bacteria do less well (see Box 7.1 for a discussion of soil fungi diversity). Most fungi grow as thin white threads (hyphae), 2-10 mm wide, which group together to form mycelia. In addition to decomposition, fungi are also important in helping to glue together soil particles into larger 'crumbs' giving the soil a more open structure with more air spaces. Most woody material in forests is decomposed by a sequence of different fungi, discussed in Section 7.7, and the size and type of substrate involved has considerable influence on the decomposer organisms involved in particular degradative successions. At El Verde in the Luquillo Mountains of Puerto Rico, Lodge (1966) looked at the fungi found on six classes of substrate: logs (> 10 cm diameter), branches (1-10 cm), twigs < 1 cm diameter, leaves (including petioles), roots and soil. Of the 705

tropical decomposer fungi involved, 493 species (70%) were confined to one substratum, 246 only on leaves, 43 in soil, 13 on roots, 27 on twigs, 84 on branches and 80 on logs. A further 25% (173 species) were found only on two similar substrata, with a mere 5% on three or more substrata. These results confirm a degree of substratum specificity also demonstrated by others.

Decomposition tends to be rapid when it is primarily bacterial and much slower when fungal. However this does depend to some degree on the abundance of the meso- and macrobiota (worms, beetles, millipedes, etc.). Bacteria are fairly immobile and need fresh litter brought to them and so greatly benefit from mixing by the soil fauna. By contrast, fungal hyphae are very capable of growing into fresh litter and by the same token are readily broken up by the mixing of the litter. So litters with a low soil fauna, where there are summer droughts, very acidic conditions, or low nutritional status, tend to be fungal dominated. Fungi also directly suppress the abundance of bacteria by producing antibiotics (Alexander Fleming named his initial discovery of penicillin from the first fungus he saw with antibiotic properties, Penicillium notatum). This may be partly explained by it being an adaptation of mycorrhizal fungi (see Section 5.4.1) for protecting their partner plants from bacterial disease (Marx, 1969; Rasanayagam and Jeffries, 1992). Whatever its origin it is effective. Experimental elimination of ectomycorrhizal fungus from a Monterey pine stand in New Zealand resulted in a greatly accelerated activity of microorganisms and saprotrophic fungi resulting in more rapid litter decomposition, known as the Gadgil effect after the researchers, Gadgil and Gadgil (1975). This oppressive effect of ectomycorrhizal fungi has been attributed to their inhibition of other organisms, high competitiveness for nitrogen and possibly low soil moisture levels caused by high water uptake by the mycorrhiza (Bending, 2003).

7.2.2 The mechanics of decomposition

Fungi and bacteria work in similar ways: soluble substances such as sugars and low molecular weight compounds are directly absorbed from their surroundings, and larger compounds are broken down by extracellular enzymes secreted into the organic substrata. Some fungi are efficient in breaking down cellulose-containing material, such as plant cell walls (e.g. Chaetomium, Trichoderma) while others have enzymes capable of breaking down keratin in skin and hair (e.g. Myxotrichum) or lignin in wood (e.g. Phanerochaete). The nutrients released by decomposition and absorbed by the microbiota are immobilized in the fungal and bacterial bodies and remain so until the cells die or are eaten by other organisms (see Fig. 7.1 and Section 7.3). These nutrient-rich microbiota are fair game for being eaten just as plants are fair game for herbivores, and a number of animals feed directly on bacteria and fungi. To do this they need the appropriate enzymes to be able to digest their food. Thus a number of mites produce chitinase to attack fungal walls and trehalase to digest the fungal storage compound, the sugar trehalose (Luxton, 1972).

Soil animals that directly eat dead plant material are faced with a complex mass of cellulose and lignin since the more readily assimilated compounds such as sugars and proteins will soon have been leached or exploited by the microbiota (Fig. 7.1). Animals such as snails, wood-boring beetles and at least one species of termite, can produce their own cellulases although these may be supplemented by microbial enzymes. In other organisms there is a much clearer case of symbiosis where microbiota (particularly bacteria but also protista) are given a home in a gut and digest cellulose for the host. To be effective this normally needs a long gut to give time for digestion and absorption of the nutrients; those with a shorter gut can get round the problem by coprophagy, eating of their own faeces so that partly digested food has a second passage through the gut. This habit is well known in rabbits but is also found in certain millipedes. Feeding on the faeces of other species is also a well-used strategy in soil organisms where each successive organism will work over the microbially conditioned remnants of the resource until all usable energy and nutrients have been extracted. This explains the apparent paradox in soil that there often does not seem to be enough litter input for all the soil biota; much of it is reworked several times. For example, detritivores may work their way through one particular food source for long periods of time (months in the case of woody substrates such as beech cupules) until no more than a dung heap remains, but which is ripe for further exploitation by other organisms.

It is fairly easy in the laboratory to investigate what food each particular type of soil organism prefers. Under field conditions, however, it is much more difficult to observe what an organism is feeding upon. One way around this is to examine gut contents, while remembering that more detailed analysis is needed to establish what has actually been digested. The result is surprising. Although soil animals usually show a clear preference for particular microbes or leaves of particular species in the laboratory, allowing the construction of theoretical food webs, the contents of guts from organisms taken in the field are often remarkably similar, even across different types of organism. A similar story is seen when constructing soil food webs by following the journey of stable radioactive isotopes through different organisms. This apparent free-for-all (a lack of 'food niche differentiation') is in distinct contrast to the organization of above-ground food webs. Competition between species would normally force them to be specialized otherwise competition would drive one species to extinction. Possible reasons for this sharing of food sources below-ground may be attributable to litter arriving in bursts and swamping the numbers of detritivores so that food is not limiting. Or there may be a separation in space (by vertical zonation in the soil for example) or in time that helps prevent competition for the same food source. There may also be other factors which control populations, such as disease, predators, climate or limited egg-laying sites, that reduce competition for food. Wardle (2002) gives evidence that fungal-dominated decomposer systems are limited by food resources and bacteria-dominated systems are limited by predation.

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