Biodiversity is generally considered to be fundamental to ecosystem functioning and stability. Thus, if this tenet is extrapolated to fungi in forest ecosystems, fungal biodiversity is critical to global geochemical cycles. A well-cited global estimate for fungal biodiversity is in the range of 1.5-M species (Hawksworth 1991; 2001). However, at a local level, appropriate monitoring of fungal diversity can identify ecological and climatic trends, provide a database for spatio-temporally predictive biodiversity models, and inform appropriate conservation strategies at both the regional and global scale (Arnolds 1997).
Most local diversity studies are based on survey of conspicuous fruit bodies of higher fungi, either represented as presence, number, or biomass within a plot. Diversity studies of ephemeral and sometimes sensitive fruit bodies will only represent a partial account of the resident fungi. Nevertheless, such studies are often valid because of their ease of (usually nondestructive) analysis over extensive sampling domains. Fruit body surveys are further justified because of the ecological significance of spore dissemination for the establishment of new genets, and due to the involvement of sporocarps in soil mineralization processes owing to developmental regulation of extracellular enzyme activities (Ohga et al. 1999). Moreover, the fruit bodies of many species are a valuable edible resource. However, sporocarp inventories of species with varying resource allocation to reproduction, especially for comparison of taxonomically and ecologically different groups, are fallible as a bioindicator of diversity. Thus, ruderal strategists are generally likely to fruit more frequently and transiently than combative strategists. Moreover, sporocarp productivity of different species are known to vary spatially (clustered or dispersed and associated with distributed substrata), temporally (yearly, seasonally or weekly), can mature at differing rates and persist for varying time periods (Egli et al. 1997; Vogt et al. 1992). Fruiting may also be influenced by environmental factors such as temperature and precipitation, natural and artificial amendments to soil and by local or nonlocal ecological disturbance (Straatsma et al. 2001; Watling 1995). In the case of certain mycorrhizal fungi fruit body development is affected by host genotype, age, provenance, edaphic factors, climatic factors, and supporting photosynthetic activity (Dix and Webster 1995). The relationship between individual visible fruiting structures and mycelium activity within substrata is therefore varied and complex, and considerable disparity has been identified between the two in numerous studies (Cotter and Bills 1985; Gardes and Brans 1996; Yamada and Katsuya 2001). Notably, only about half of the ectomycorrhizal community has been indicated by production of conspicuous epigeous fruit bodies (Dahlberg et al. 1997; Peter et al. 2001).
Sampling plot size, frequency, and distribution are also likely to influence the data set, as different ecological groups will operate over a range of spatio-temporal scales (Newton and Haigh 1998; Schmit et al. 1999). Indeed, it has been suggested that different ecological guilds would require differently sized and spatio-temporally distributed sampling protocols (Villeneuve et al. 1991). To compound such problems, Tofts and Orton (1998) declared that 21 years of recording species accumulation was insufficient to reliably estimate (extrapolate) the fungal biodiversity of the site, as the species accumulation-time curve had not yet started to plateau. At a practical level, the quality of a data set is even dependent on the expertise of the collector (Straatsma et al. 2001). However, sporophore surveys may be justified as a suitable method for warning of species disappearance due to detrimental external factors within a site, as evidence suggests that fruiting may be more sensitive to environmental stress than survival of the supporting mycelium itself (Termorshuizen and Schaffers 1991).
Despite these limitations evidence accrues, particularly from Europe and more recently the United States, suggesting that fungal biodiversity is in decline. The likely causes of decline are due to habitat loss and/or pollution. Harvesting of wild edible mushrooms is believed to have little detrimental effect on fungi, except where collection has involved damaging or exhausting the mycelium, or trampling or raking the soil (Arnolds 1995). Nevertheless, the environmental impact of large-scale commercial harvesting remains a contentious issue. Fungal habitat may be lost, with implicit reduction of fungal species diversity, either by deforestation, or because of commercial forestry management practices, such as the conversion to less-mixed or monoculture plantations, stand felling of a particular age, and the removal of course woody debris (Fridman and Walheim 2000; H0iland and Bendiksen 1996; Lindblad 1998; Norden and Paltto 2001; Ohlson et al. 1997; Straatsma et al. 2001). Red-List (endangered, vulnerable, or rare) species may be particularly sensitive, often showing a preference for large diameter logs in late decay stages, of which managed forests are largely deficient (e.g., Humphrey et al. 2000; Kruys et al. 1999).
Pollutants such as lead, sulfur dioxide, ozone, and oxides of nitrogen (direct or as acid precipitation) are known to affect biodiversity (Wainwright and Gadd 1997). Ectomycorrhizal communities are particularly threatened by pollutants such as sulfur dioxide and NHx, probably due to forest soil acidification and nitrogen input, thereby rendering any potential mycorrhizal association less advantageous to the host plant community (Arnolds 1995). Elevated sulfur dioxide levels are known also to affect the phylloplane community, although some pigmented species may be less susceptible, thereby favoring their population growth (Magan 1993). The resulting impact on community interactions may have implications in the dynamics between pathogens and saprotrophs and the consequent natural limitation of plant diseases, and even on the subsequent decay rates of litter and hence nutrient cycling (Newsham et al. 1992). Similarly, high levels of radioactivity occurring around the Chernobyl site have produced altered fungal community structures (Zhdanova et al. 1994).
Considering the absolute and relative diversity, fundamental ecological importance, some host-specificity and -exclusivity, and established and potential biotechnological significance of fungi (Hawksworth and Colwell 1992; Oberwinkler 1992), their limited conservation seems unjustified and surprising. For example, only four fungal species are protected by UK law, comparing pitifully with over a hundred flowering plants (Marren 2001).
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