What is an Individual

In all macrofungi an individual is to be understood as a mycelium. In the field it is easy to observe fruit bodies, but not mycelia; hence, it is often difficult to decide how many mycelia there are in a given area. One piece of wood may host several individuals belonging to the same species (e.g. Kauserud and Schumacher, 2002, 2003), but in other cases a mycelium may become very large and inhabit several hectares (Smith et al., 1992). Old mycelia may be fragmented and split up into

Box 1. IUCN Red-List Criteria Applied to Fungi. Criterion A: Population decrease

The criterion concerning population decrease basically deals with the percentage decrease in population size over a period of three generations (within a range of 10—100 years).

The population decrease may either be an observed decrease in recent years or an expected decrease in the coming years, i.e. caused by an expected loss of habitat quality. The threat category obtained by using criterion A depends on the rate of decrease in the species and, to some extent, on whether the process is understood and reversible. However, it is independent of the population size. Species must have decreased or expected to decrease by at least 30% within three generations (or 10 years). Criterion A is accordingly most relevant for common fungi with a long generation time where the quality or quantity of habitats has decreased significantly or is expected to decrease significantly in the near future. Examples may be where increasing nitrogen deposition is threatening vulnerable habitats, or where changes in land use result in a dramatic change in habitat quality and quantity. If a species is decreasing and already rare, it will normally qualify as more threatened when evaluated under criterion C.

Criterion B: Decrease in geographical range

Species with a small area of occurrence may qualify as threatened under criterion B if at least two of the following are met: (a) the distribution is strongly fragmented; (b) a decrease takes place in distribution/habitat quality/population size; and (c) there is a fluctuation in distribution/population size. The actual threat category obtained by using criterion B depends on the area of occurrence and on how many localities the species is known from.

Criterion B was designed to identify populations that are severely fragmented, undergoing a form of continuing decline and/or are subject to extreme fluctuations (IUCN, 2003). Fragmentation of populations may well constitute a problem for fungi, but fungal spores may cross long distances, and the problem is likely to be smaller than for organisms with a more limited dispersal potential.

For some fungi it has, however, been shown that area of distribution has decreased. The best example is fungi restricted to nutrient-poor soil, which have disappeared from areas with a nitrogen deposit exceeding the critical load for the species in question. This has been documented for several mycorrhizal species, including species of Hydnellum, Phellodon, Bankera, Sarcodon, Tricholoma (Arnolds, 1989, 1991; Otto, 1992; Vesterholt et al., 2000), but is also likely to be relevant for saprotrophic basidiomycetes that occur on naturally nutrient-poor soils. For such species criterion B is relevant if the regional geographic range of the species in question is restricted. Criterion C: Small population size and decline

This criterion is justified by the fact that small populations are more sensitive to decline than larger populations. Like criterion A, criterion C covers species which are declining and species that are expected to decline in the near future. Species with less than 10,000 mature individuals qualify for one of the threatened categories, provided there is an estimated continuing decrease in the number of mature individuals of at least 10% over a period of one to three generations (max. 100 years). Species may also qualify if a smaller or slower decline is taking place, if the species is also restricted to very few, very small or highly fluctuating subpopulations.

Criterion C can be applied to strongly or moderately decreasing fungi which are already rare. This decrease may be inferred, either because there has been an actual decrease in population size, or because an observed or expected loss of habitat quality is assumed to have a negative influence on the number of mature individuals. A general loss of habitat quality is observable for many fungi. One example is grassland species that are threatened by eutrophication and management changes in many parts of Europe, which also qualify for a threat category under criterion B, if the regional population size is small.

Criterion D: Very small or restricted population size

Criterion D covers species with very small populations of mature individuals, because species with small populations have a risk of extinction as result of unforeseen incidents and inbreeding. There is no requirement for a species to be declining. Species with less than 1,000 mature individuals qualify for one of the threat categories, the actual threat category depending on the number of mature individuals. Also species with larger population sizes, known from only up to five localities, or with an area of occurrence below 20 km , may also be red-listed. Criterion D is relevant for all types of rare fungi, but it is most important for species that do not qualify using other criteria, i.e. very rare species that are not known to be decreasing. Species with less than 1,000 mature individuals qualify to a threat category irrespective of the size of the area considered.

Criteria A—D are quantitative measures, and the different criteria may point to different threat categories (Table 1). The actual status of the species is then determined by the most severe of the threat categories (see IUCN, 2001, 2003).

several ramets. Somatic incompatibility and molecular methods may help to delimit and quantify individual mycelia (e.g. Boddy and Rayner, 1983; Kirby et al., 1990; Kauserud and Schumacher, 2002, 2003), but for monitoring, the number of mycelia occurring on a locality still has to be estimated based on counting groups of sporocarps. In Sweden, each mycelium has been assumed to consist of 10 ramets (counting as individuals) in ground-living and 2 ramets in saproxylic species (Gardenfors, 2005), but this approach has not been followed in Denmark, partly because we have assumed that ramets of a mycelium with restricted spatial distribution (<10 m2) are all liable to be wiped out if a negative factor affects a habitat.

With some of the IUCN criteria, the number of mature individuals is relevant. It can be assumed that a fungus is mature when it starts producing fruit bodies, and that it remains mature as long as it is capable of doing so. In general fungi that can be observed in the field, must be considered as mature.

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