IUCN Red List Guidelines - The IUCN Red List of Threatened Species

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Red List Guidelines 30 Index of abundance 100 10 1 100 10 1 100 10 1 100 10 1 100 10 1 Time (a) (c) (e) (g) (i) 100 10 1 100 10 1 100 10 1 100 10 4.8 Severely fragmented (criterion B) 1 (b) (f) (h) (d) Figure 4.1. Fluctuations without directional change in population size (a-d), population reductions or declines without fluctuations (e-f), population reductions in combination with fluctuations (g-i). “The phrase ‘severely fragmented’ refers to the situation in which increased extinction risks to the taxon results from the fact that most of its individuals are found in small and relatively isolated subpopulations (in certain circumstances this may be inferred from habitat information). These small subpopulations may go extinct, with a reduced probability of recolonization.” (IUCN 2001) Fragmentation must be assessed at a scale that is appropriate to biological isolation in the taxon under consideration. In general, taxa with highly mobile adult life stages or with a large production of small mobile diaspores are considered more widely dispersed, and hence not so vulnerable to isolation through fragmentation of their habitats. Taxa that produce only small numbers of diaspores (or none at all), or only large ones, are less efficient at long distance dispersal and therefore more easily isolated. If natural habitats have been
Red List Guidelines 31 fragmented (e.g., old growth forests and rich fens), this can be used as direct evidence for fragmentation for taxa with poor dispersal ability. In cases where data are available on (i) the distribution of area of occupancy (i.e., detailed maps of occupied habitat), (ii) some aspect of the dispersal ability of the taxon, (e.g., average dispersal distance), and (iii) average population density in occupied habitat (e.g., information on territory size, home range size, etc.), then the following criterion can be used to decide whether there is severe fragmentation: A taxon can be considered to be severely fragmented if most (>50%) of its total area of occupancy is in habitat patches that are (1) smaller than would be required to support a viable population, and (2) separated from other habitat patches by a large distance For (1), the area for a viable population should be based on rudimentary estimates of population density, and on the ecology of the taxon. For example, for many vertebrates, patches that can support fewer than a hundred individuals may be considered too small to be viable. For (2), the degree of isolation of patches should be based on dispersal distance of the taxon. For example, patches that are isolated by distances several times greater than the (long-term) average dispersal distance of the taxon may be considered isolated. Note that the existence (or even a large number) of small and isolated patches is not sufficient to consider the taxon severely fragmented. For meeting this criterion, more than half of the individuals (or, more than half of the occupied habitat area) must be in small and isolated patches. For many taxa, the information on population density and dispersal distance can be based on other similar taxa. Biologically informed values can be set by the assessors for large taxonomic groups (families or even orders) or for other groupings of taxa based on their biology. For example in bryophytes, information on the effects of isolation of subpopulations is often lacking. It is recommended that in most circumstances, a minimum distance greater than 50 km between subpopulations of taxa without spore dispersal can indicate severe fragmentation, and a distance of between 100 km and 1,000 km for taxa with spores (Hallingbäck et al. 2000). The definition of severe fragmentation is based on the distribution of subpopulations. This is often confused with the concept of "location" (see section 4.11), but is independent of it. A taxon may be severely fragmented, yet all the isolated subpopulations may be threatened by the same major factor (single location), or each subpopulation may be threatened by a different factor (many locations). 4.9 Extent of occurrence (criteria A and B) “Extent of occurrence is defined as the area contained within the shortest continuous imaginary boundary which can be drawn to encompass all the known, inferred or projected sites of present occurrence of a taxon, excluding cases of vagrancy. This measure may exclude discontinuities or disjunctions within the overall distributions of taxa (e.g., large areas of obviously unsuitable habitat) [but see 'area of occupancy', section 4.10 below]. Extent of occurrence can often be measured by a minimum convex polygon (the smallest
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Red List Guidelines 87 14. Appendix
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IUCN Red List Guidelines - The IUCN Red List of Threatened Species

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