- 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. Bog areas could also naturally limit bison distribution.
- 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. Reconstructed ranges have recently declined in some parts of the previous range (e.g., Caucasus Mountains).
- As a consequence of passing a dramatic bottleneck, 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). It is interesting that 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 (Kobrynczuk 1985), and possibly lowers the resistance of bison to disease and pathologies.
- The genetic contribution of founders is uneven, highly dominated by one pair, and changing very little in the species' entire gene pool throughout the decades of its restitution (Olech 1989). In the last few years, a decrease in the founders' contribution and genes retention was observed in those founders specific to the Lowland-Caucasian line (Belousova 1999; Olech 1999). This means continuing loss of genetic variability in the species. There are very serious worries about the further reduction in genetic variability through losses represented by very rare founder's genes.
- Because of the 'second' bottleneck between 1940 and 1945, the founder's Y-chromosomes are not equally spread throughout the recent world population of European bison. Lowland line animals have copies of the same Y-chromosome from the founder No. 45 "Plebejer". The Y-chromosome of founder No. 100 "Kaukasus" can be found in Bieszczady and in some captive groups. The Y-chromosome of founders No. 15 "Begrunder" and No. 147 "Bismarck" were lost in the breeding process of 1945 to 1997 (Sipko et al. 1999).
- At the beginning of restitution (1924) the contribution of the Lowland line to the world population approximated 70%, today this stands at 42% due to mixing of L and LC lines. In enclosed breeding centres, the Lowland-Caucasian line predominates, constituting 75%. On the other hand, in free-ranging herds the proportion of Lowland line to Lowland-Caucasian line is almost equal, 57%:43%. The further mixing of both lines has led to losses of founder genes specific to the Lowland-Caucasian line.
- The impetus for reintroduction into the wild seems to have slowed down recently due to a lack of suitable habitat or limited economic possibilities within particular countries. As a result, numbers and other demographic characteristics of the global European bison population are increasing rather slowly (for example Sipko et al. 1999).
- Inappropriate (traditional) forms of management (based on zootechnical practices, rather than forest ecosystems ecology), along with supplementary feeding during winter, slow down the adaptation process of European bison into contemporary woodlands. Artificial woodlands are not appropriate for European bison. Such practices do not lead to the naturalization of bison within large herbivore communities and within modern European forest ecosystems.
- Possibilities of mixing free European bison populations in some regions of reconstructed range with hybrids of European and American bison, as well as with pure prairie bison, introduced for farming/ranching in several European countries.
- 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. Last century, cases of epizooty were noted among bison in Bialowieza Forest. It is known that European bison exhibit a particular sensitivity to foot-and-mouth disease (Aphte epizooticae), appearing in the Forest nearly each year at the beginning of the 20th century and causing about 5% mortality (Wróblewski 1927). Half a century ago, foot-and-mouth disease caused the deaths of 35 bison in reserves in the south of Poland in the years 1953 to 1954 (Jaczewski 1960; Podgurniak 1967). Cases of tuberculosis were registered recently (1996) in Bieszczady Mountains (Poland) (Zórawski and Lipiec 1997).
- The most important disease, however, 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 Bialowieza Forest (Kita et al. 1995; Piusinski et al. 1997; Jakob et al. 2000); although similar symptoms had been reported earlier (Korochkina and Kochko 1983b) in Russia and Ukraine (Shabailo and Pererva 1989; Krasochko et al. 1997). This disease was also sporadically observed in other regions of Poland, such as Goluchów, Puszcza Borecka, and Bieszczady. At the end of the 1990s, similar symptoms were observed in five young European bison from Bayerisher Wald National Park, Germany (Wolf et al. 2000). Despite many years of study, its pathogenesis has not yet been elucidated. Bulls with these symptoms do not exhibit changes in the general physiological mechanism as indicated in the long-term studies of 30 physiological indices (Gill 1989, 1992a, 1992b, 1999; Wolk and Józefczak 1988). Generally the non-specific immunity of the species is very low (Gill 1995), however, in Bialowieza Primeval Forest it was recently found that several biochemical indices in blood signifantly differed from those observed some 20 years ago. This is believed to be related to an increased intensity of pathological changes observed in this population (Wolk and Krasinska, in prep.). Some authors believe that genetic factors may predispose bison to the disease, due to reduced resistance. Winter concentration and associated environmental pollution are likely sources of bacteria which are transmitted from soil to organism and then found in the affected tissues.
- Parasitic diseases are a serious threat to bison health in the present population. Besides parasites which are specific to European bison, 11 additional parasites have been found in recent years, all being characteristic of Cervidae (Drózdz 1961; Drózdz et al. 1989, 1994). New parasites may still be found (Drózdz et al. 2000).
- 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. World population numbers have decreased, with some populations seriously decimated and others becoming extinct in recent years.
- Several administrative bodies responsible for managing the same population may create serious threats for bison populations. Due to their conflicts of interest different bodies observe different aims (e.g., forest administration unit, national park /or reserve, and agricultural land). Therefore, the management of European bison populations should be the responsibility of one administrative body.
- The legal status of the species is not clearly established, particularly with regard to its position as a protected species, management, and conservation procedures, such as international animal transfers, monitoring and the controversial issue of elimination and hunting.
The effects of restitution generally show a positive picture of the species' rescue from extinction. However, stating that the European bison is completely safe would be rather premature. A thorough and more critical analysis of the current state of the species reveals serious threats stemming mainly from its genetic structure and from its management. The risk of extinction to the species, both in captivity and in the wild, is still very high. There are many reasons for this: