Dung Decomposers

In grazed grasslands, dung from herbivorous mammals is a major input to the soil and is initially decomposed by distinctive communities of fungi and invertebrates. Dung fungi play a key role in the catabolism of the lignocellulose and the microbial polymers (from intestinal bacteria, protozoa and fungi), although leaching/dispersal by rainfall and invertebrate activity leads to the rapid incorporation of dung into soil (Dickinson and Craig, 1990). Relative to plant litter or soil organic matter, dung is a high quality resource (C:N ratio ranging from 20 to 40 depending on host and diet; Richardson, 2001; Reijs et al., 2003). Enhanced resource quality, partial digestion of plant polymers (with the exception of lignin) by gut microbes and increased access to microbial exoenzymes (due to comminution) lead to rapid decomposition (Nagy and Harrower, 1980).

Basidiomycetes and other fungi adapted to growth on dung tend to have pigmented spores, permitting them to withstand ingestion and digestion by herbivores (enterophilic), so they are already present in the faeces on excretion (Harper and Webster, 1964; Webster, 1970). It was originally thought that the fruiting of dung fungi exhibited a succession, but with regard to biomass and activity, it is more likely that the various groups of dung fungi all develop in parallel but achieving critical biomass for fruiting at different times (Webster, 1970), and culminating, for example, in the formation of basidiocarps (mostly Coprinus spp.) after 10-50 days (Richardson, 2001). Significant decomposition of lignin occurs in dung (Waksman et al., 1939), and the activity of basidiomycetes is correlated with this process (Wicklow et al., 1980b). Dung also comprises a proportion of debris from intestinal microbes, and some grassland basidiomycetes can decompose bacterial cell wall polymers effectively (Fermor, 1988). Dung from different herbivore species exhibits different patterns of fungal colonization (Ebersohn and Eicker, 1997). However, it is unclear whether this is due to variations in the unit resource size, differences in fungal inoculum present or differences in the resource quality of the dung (Wicklow et al., 1980a).

Dung invertebrates generally inhibit fungal activity (Lussenhop and Wicklow, 1985), through nutrient competition, grazing by larvae on hyphae and physical disruption of the resource (McGranaghan et al., 1999). However, invertebrates are susceptible to freezing in winter, possibly explaining the increased abundance of fruit bodies in winter (Richardson, 2001). Application of anthelminthics, some with selective antifungal activity (Edgington et al., 1971), also inhibits invertebrates (Hutton and Giller, 2003; Warren and Paul, 2006). There can be considerable competition between microbial colonizers (Harper and Webster, 1964; Safar and Cooke, 1988), dung microcosms inoculated with combinations of fungi showing slower decomposition than when singly inoculated (Wicklow and

Yocom, 1981). Several dung fungi, notably Coprinus spp. (Ikediugwu and Webster, 1970), are able to disrupt the hyphae of competing species and there are several examples of production of inhibitory metabolites.

Coprinus spp. fruit abundantly in laboratory microcosms (Webster, 1970), whereas in nature there is a greater diversity of basidiomycetes, for example, species of Conocybe, Panaeolus, Psathyrella, Psilocybe and Stropharia. The highly fluctuating moisture conditions of the grassland environment (possibly providing triggers for primordium formation; Chapter 5) and interaction with underlying soil, absent from microcosms, may explain this difference. Wicklow and Moore (1974) did not find any significant colonization by soil microbes, suggesting that competition from the enterophilic dung fungi prevented subsequent colonization by soil fungi. However, several species, termed subcoprophilous, are more often associated with dunged fields rather than dung itself (Lisiewska, 1992) and these species generally have melanized spores (e.g. Panaeolina foenisecii, Psilocybe semilanceata), potentially able to tolerate gut passage.

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