The term population is used with a specific meaning in the IUCN criteria, different from the ordinary use in biology. Here population stands for the total number of the individuals of a given taxon. For functional reasons, mostly due to differences in life forms, the size of the populations is measured using the numbers of adult individuals only. In cases when some taxa are obligatorily dependent on other taxa for their whole life cycle or a part of it, biologically meaningful values must be used for the hosts.
The subpopulations are geographically (or in some other way) distinguishable groups in the population, between which there exists a small demographic or genetic exchange (usually one successfully migrating individual or a gamete yearly, or less often).
The number of adult individuals represents the number of individuals for which it has been established, or is supposed, that they can multiply. When this quantitative indicator is estimated, the following is to be taken into account:
The lifetime of the generation is the average age of the parent individuals of the present group of newly born individuals in the population. The lifetime of the generation, therefore, reflects the change of the ratios between the multiplying individuals in the population. The lifetime of the generation is longer than the age at which the first multiplication takes place and smaller than the age of the oldest individuals that are still multiplying (an exception is represented by taxa that multiply only once). When the duration of the generation changes in a state of threat, more natural values must be used for the lifetime of the generation, i.e., before the appearance of the threat.
The decline of the size of the population is a reduction of the number of adult individuals with at least a percentage registered in the relevant criteria for the indicated period of time, although the decline may not be a continuous tendency. The decline must not be considered as part of the fluctuation of the population, unless there is definite proof of this. The decrease of the number of individuals as a result of a natural fluctuation in the size of the population is, in principle, not considered to be a reduction.
The continuing decline is a recent, current or forecast decrease of the size of the population (it may be gradual, irregular or sporadic), that would last until relevant preventive measures are taken. In principle, fluctuations are not considered to be a continuing decline but also a continuing decline must not be regarded as a fluctuation, unless there is proof for this.
Extreme fluctuations are observed in many taxa in which the size of the population or its range change widely, rapidly and frequently, usually with variations larger than a tenfold increase or decline of the population.
The expression “strongly fragmented occurrence” pertains to situations in which the increased risk of extinction is due to the fact that most of the individuals are found in small and comparatively isolated subpopulations (in some cases this may be established from the information about the habitat). These small subpopulations may disappear, with a reduced probability for re-colonization.
The region of distribution is defined as the territory bounded by the shortest hypothetical continuing border that can be drawn so that it can embrace all known or supposed sites of the modern distribution of the taxon, excepting cases of vagrancy. This measure may preclude discontinuity or large disjunctions of the total area of the taxon (for example, large areas with habitats that are inappropriate for the taxon) (see also “occupied area”, art. 10). The area of distribution can often be measured with the help of the ‘minimal protruding polygon’ (the smallest polygon of which none of the internal angles exceeds 180° and that contains all the territories in which the taxon is found).
Occupied area is defined as the area of the ‘region of distribution’ that is occupied by the taxon, excluding cases of vagrancy. This measure reflects the fact that a given taxon usually does not occupy the whole area of its region of distribution, that may also contain inappropriate and non-occupied habitats. In some cases (for example, unchangeable colonial places for nesting, vitally significant places for feeding with migrating species), the occupied area is the smallest territory essential for the survival of the population of the taxon in any phase. The size of the occupied area depends on the scale with which the measurement is performed, hence a scale must be used that is appropriate, taking into account the relevant biological aspects of the taxon, the nature of the threatening factors, and the data available. To avoid inconsistence and divergence in measurements as a result of the application of different scales, standardization may be carried out if necessary, by using a correcting factor for the scale. It is hard to give precise instructions as to how standardization must be carried out, as the different taxa have different types of scale-area interrelationships.
The term habitat refers to a geographical or ecologically discernible territory in which a singular threatening factor may rapidly affect all individuals of a given taxon. The size of the habitat depends on the area on which a threatening factor is manifested and may include part of one or more subpopulations. When a given taxon is under the threat of more than one threatening factor, the habitat is defined by taking into account the most serious threat.
A quantitative analysis is any form of analysis through which the probability of the extinction of a given taxon is estimated on the basis of its life history, requirements with respect to the habitat, threats and certain manager’s options. Such a technology is the Population Viability Analysis (PVA). A quantitative analysis must be based on all available data. In situations in which there is limited information, the available data must be used in the best possible way, in order to assess the risk of extinction (for example, estimating the influence of accidental phenomena on the habitat). In the presentation of the results of a quantitative analysis, the allowances made (that must be appropriate and feasible), the data used, and the significance of the data or the quantitative model must be documented.