Habitat degradation and fragmentation due to agricultural activity, forest logging, and unlimited hunting and poaching were the primary reasons for the decrease and extinction of European bison populations. Pucek (1991, 1994) has summarized the history of their extinction. Among the primary reasons for the rapid decrease of the European bison population in Bia?owie?a Primeval Forest at the beginning of 19th century was the over-population of deer species, and the drastic reduction of natural food resources for herbivores which followed (Wróblewski 1927). During the period of World War I and the Russian Revolution of 1917, conflict and heavy poaching exacted a severe toll on remaining populations (Pucek et al. 2004).

Conflict and political instability continues to be a threat to the species in the Caucasus, where reintroduced free-living herds have suffered very severe losses (leading to extinctions) in recent years (Pucek et al. 2004). Other current threats include lack of appropriate habitat, fragmentation of populations (and concomitant loss of genetic diversity), inbreeding depression, disease, hybridisation, and poaching. There is little space for a large herbivore such as the European bison in Europe?s contemporary ecosystems, especially in the west. The most significant limit for the enlargement of European bison populations is human population density; forestry and agricultural activity is not a limiting factor. Fragmentation and isolation of free-ranging (and captive) herds result in little or no exchange of genetic material. Small isolated populations quickly lose their genetic heterogeneity and are more vulnerable to extinction (Franklin 1980). As yet, the opportunity to reconstruct a more compact geographic range to facilitate migration of bison between herds does not exist. As a consequence of passing a dramatic bottleneck (the current population descends from just 12 founder animals), the gene pool is limited and animals are highly inbred. The average inbreeding coefficient is very high compared to other large mammals, and is equal to 44% in the Lowland line and 26% in the Lowland-Caucasian line for individuals with a full pedigree (Olech 1998). The negative effects of inbreeding, manifested in the decline in reproduction rate, are more strongly pronounced in the Lowland-Caucasian line than in the Lowland line (Olech 1987, 1989, 1998). Inbreeding exerts a harmful effect on skeleton growth, particularly in females (Kobry?czuk 1985), and possibly lowers the resistance of bison to disease and pathologies.

Diseases appearing in European bison populations can bring serious threats to the whole species. It is not certain whether the species has always shown a weak resistance to disease or if immunity has declined, due to limited genetic heterogeneity. The most important disease affects the male reproductive organs and is manifested in the inflammation of the penis and prepuce, leading to diphtheroid-necrotic lesions, diagnosed as balanoposthitis. This disease was discovered at the beginning of the 1980s in Bia?owie?a Forest (Kita et al. 1995, Piusi?ski et al. 1997, Jakob et al. 2000); although similar symptoms had been reported earlier (Korochkina and Kochko 1982) in Russia and Ukraine (Krasochko et al. 1997). Despite many years of study, its pathogenesis has not yet been elucidated. Other diseases that are potentially major threats to herds include foot-and-mouth disease Aphte epizooticae (to which the species is known to be sensitive) (Podgurniak 1967), and tuberculosis (?órawski and Lipiec 1997, Welz et al. 2005).

A particular problem concerning the management of extant populations of European bison is the existence of hybrid herds, especially European × American bison hybrids living in the Caucasus. Two free-living hybrid herds have been established in the Caucasus Mountains, in close proximity to reintroduced free-living herds of the pure blood Lowland-Caucasian line. There are fears that all these animals will crossbreed, creating a mixture of various genotypes. According to Russian authors, the distances between herds are not so great, but the configuration of mountain ridges and valleys make it impossible for contact between them. There are also two small semi-free herds of European and American bison hybrids in Toksove Forest Park (St Petersburg) and the Mordovia Wildlife Reserve (Pucek et al. 2004). Finally, poaching as a result of administrative disorders and a failure to enforce nature conservancy law threatens free-living herds of European bison in many countries.