An essential element of Red Books and Red Lists are the categories of threat. The first categories of threat were published in 1966 and were designed for a global assessment of mammals (IUCN 1966). Four particular categories were used: 1. Species about to become extinct, i.e. species that are under a serious threat of extinction and whose survival is already impossible without taking special measures for preservation; 2. Rare species, i.e. species that are not directly endangered but are found in such moderate numbers or on such limited territories that they may soon become extinct; 3. Species with declining numbers; 4. Indeterminate, i.e. species about which little is known but they are obviously threatened by extinction.
This system received wide recognition and was later used for the assessment of various taxonomic groups, as well as for the creation of regional Red Books and Red Lists. These first categories were used for almost 30 years. Their positive aspects amounted to their simplicity, moderate need of data and their wide recognition. Their defects consisted of their subjectivism and the lack of quantitative estimates; hence they were not in a position to adequately describe the degree of threat of the species. Apart from that, the assessments made by different organizations and for different regions were not comparable.
In 1989, the IUCN started the development of a new, more objective and quantitative approach to ensure a methodology for the assessment of the risk of extinction. As a result, a system of categories and criteria was proposed, published in 1994. Later they were revised and modified (IUCN 1994, 2001). The new system has several specific aims: to present an approach that is applicable in an identical way by different assessors; to increase objectivity by giving the users clear guidelines about how the different factors influencing the risk of extinction should be assessed; to present an approach that would facilitate the comparison between widely differing taxa; to give the users of the lists of endangered species a better understanding of the base on which the species were classified according to the degree of their threat.
Sometimes there are doubts that the quantitative essence of the criteria would be an obstacle to their application, because of insufficient data about the species subject to an assessment, a large part of which are rare, with small numbers and/or poorly and inconsistently studied within their area. It must be taken into account, however, that this is only perceived, as the borders given with the separate criteria are rather wide and exact figures on the size of the population, the speed of the reduction of the numbers or the territory of the area are not necessary. The criteria enable the use of projections or inference based on data about the habitat, closely related species, other species related to a given type of habitat, etc. Furthermore, it must be emphasized that it is enough to satisfy the requirements of only one of the criteria, not of all five. In many cases, some criteria will not be appropriate at all and will never be met, no matter how close a species comes to extinction. (IUCN 2005).
Initially, the categories and their definitions were developed for a global assessment of the risk of extinction. Global assessments, however, are not very practical for use at a regional level, for example, for the creation of national Red Books or Red Lists as a basis for concrete and adequate conservation programmes. An essential restriction of the application of global assessment at the regional level is the circumstance that the probability of extinction in part of the range of the taxon may be different from that within the whole range. To overcome these difficulties, a protocol was proposed for the employment of the global categories and criteria at the regional level (IUCN 2003).
During the preparation of this Red Data Book, the global categories and criteria of IUCN (2001) were used, as well as the guidelines for global categories and criteria (IUCN 2005), and the guidelines for the application of the global criteria at the regional level (IUCN 2003). On this basis, a procedure for the assessment of taxa (mainly species) was effected, proposed by a large circle of specialists. A first step in the procedure is the assessment of the national population of the taxon with respect to the standard criteria. After that the estimate is specified (increased or decreased), depending on the degree of binding with the populations from the surrounding countries. The estimate of the risk of extinction is decreased in cases of a possibility for immigration from the surrounding populations, as this has a favourable effect on the possibility for the survival of the national population. This “decreasing” of the estimate usually leads to a change of one degree in the category. When only a small part of an otherwise large population present in the country is also present in neighbouring countries, the decline is most often with more than one degree. Conversely, when there is clear evidence that the population in the country is in a demographic collapse and would not be able to survive without immigration from nearby populations, or when the surrounding populations are also declining, the category is increased. In the cases when there is no evidence of the role of surrounding populations on the risk of extinction of the regional population, global criteria are applied without changes. This is valid for most cases, as the most frequently endangered taxa have low numbers and a fragmented distribution, and the probability of isolation from the populations in the surrounding countries is large. Furthermore, with most endangered species the risk of extinction is considerable because of the destruction of habitats, and in that case the state of their populations could hardly be improved by the inflow of immigrants from outside. Adopting the contemporary criteria of IUCN cited above during the preparation of the current edition of the Red Data Book of the Republic of Bulgaria the taxa (species and subspecies) were classified into the following categories (see Appendix 1 for the terms used):