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"Knowledge of European bison ecology is mainly based on data obtained from the Białowieża Primeval Forest (BPF), but also from Prioksko-Terrasnyjj reserve and Cejjskijj zakaznik. Data concerning the functioning of a medium-sized population (50-70 individuals) in Borecka Forest and a small population (10-18 individuals) in Knyszyńska Forest have been used for comparison. In Białowieża Primeval Forest, European bison have always been treated in a specific way and are subject to special protection. However, their role in the ecosystem has to be considered in relation to other ungulates. All five elements of the ungulates community, characteristic for continental Europe - European bison (Bison bonasus), red deer (Cervus elaphus), roe deer (Capreolus capreolus), moose (Alces alces) and wild boar (Sus scrofa) - should be preserved in Białowieża Forest. The problem is how to establish adequate proportions between the herbivore species and determine what numbers are optimal for the forest habitat conditions.

(Relationship between European bison and its available habitat)

During the initial stages of reintroduction, all free-ranging European bison populations occupied small ranges that gradually enlarged until the number of animals in the population reached the optimal level. In no forest complex have European bison used the whole area (cf. Tables 9.1 and 9.2). In BPF, European bison occupy about 60% of the area (Krasiński et al. 1999). European bison select the most favourable forest types for their ranging area (Korochkina 1973; Krasiński 1978a, 1983; Bunevich and Kochko 1988; Kazmin and Smirnov 1992). For long periods, they inhabited limited ranges, with high densities (e.g., Białowieża Forest). However, sometimes the area of a population was enlarged, (Krasińska and Krasiński 1994), or European bison were transported to other unoccupied territories (Bunevich 1989, 1994). Recent distributions in Białowieża Forest practically cover the whole forest complex (Figure 10.1). Deciduous forest types are the most suitable habitats for European bison. In BPF they mainly forage in fresh and moist deciduous forests and then in mixed coniferous forests (Krasińska et al. 1987; Krasiński and Krasińska 1994). Forest complexes with a mosaic-like forest type arrangement (Białowieża and Borecka Forests, Poland) are most favourable. In fresh deciduous forest, European bison find food throughout the vegetative season. In the Caucasus region, European bison prefer foothill forests; in summer, they feed on alpine meadows (Kazmin and Smirnov 1992; Kazmin et al. 1992). However, considerable plasticity of European bison with regard to food means they also forage in habitats where coniferous forests predominate (e.g., the Belarusian part of the BPF) (Krasiński et al. 1994a, 1999). All European bison populations inhabit ranges that include open areas, such as, mown meadows, deforested feeding glades covered with grass, clear cuts and young plantations up to 10 years old (Dzięciołowski 1991; Krasińska and Krasiński 1994; Krasiński et al. 1994a, 1999). The attraction of open areas results from the fact that exploited meadows and glades provide ungulates with much more food than the same area of the forest herb layer and food is more easily available there (Korochkina and Bunevich 1980; Kazmin et al., 1992). In Lithuania, free-ranging European bison spend most of their time in open, half-open areas and forest fragments in agrocenoses and meadows (Balčiauskas 1999). Bearing in mind the historical distribution, this species may not soley survive in a zone of deciduous forest. Little information is available on the populations inhabiting the Caucasus Mountains (Russia) or the Carpathians (Poland, Ukraine). Therefore, it is necessary to conduct systematic studies on the ecology of free-ranging populations in other regions, and particularly European bison of the Lowland-Caucasian line. The forest herb layer also provides food for other ungulates, constituting approximately 30% of roe deer diet and 40% of red deer diet (Dzięciołowski et al. 1975). However, among the ungulates inhabiting the BPF, only red deer are viewed as a potential food competitor with the European bison. Therefore, the management plan for European bison should also incorporate the feeding needs of other ungulates living in the same forest complex. European bison habitat should be properly managed, with the formation of watering places, cultivated meadows or feeding glades for the use of other ungulates. European bison pressure on the forest could be decreased considerably by creating properly managed feeding glades and forest meadows of an adequate size.

Studies on the European bison's feeding habits were conducted mainly in Białowieża Forest, Prioksko-Terrasnyjj reserve and Cejjskijj zakaznik. Borowski and Kossak (1972) have shown that the European bison’s diet in the Polish part of Białowieża Forest includes 131 plant species, with 27 species of trees and shrubs, 14 species of grasses and sedges and 96 species of dicotyledonous forbs (unrecognized grass species were treated jointly as "grass"). In the total diet, trees and shrubs constitute 33%, while grasses, sedges and herbs feature at 67%. Among trees and shrubs Carpinus betulus, Salix caprea, Fraxinus excelsior and Betula pubescens are prefered. Favourite grasses and sedges include Calamagrostis arundinaceae, Carex sylvatica and Carex hirta; Dicotyledonous forbs - Aegopodium podagraria, Urtica dioica, Ranunculus lanuginosus and Cirsium oleraceum. The trees most barked are Quercus robur, Carpinus betulus, Fraxinus excelsior and Picea abies. A favourite food of European bison is acorn; however, its yield only occurs in the forest every few years. Analysis of the rumen contents has confirmed that the European bison's basic diet contains grasses, sedges and herbs, constituting 90% of rumen capacity, while trees and shrubs only constitute 7-13% (Gębczyńska et al. 1991). Other investigators note that the basic part of the European bison's diet includes more than 50 species of grasses and about 10 species of trees and shrubs; species preference can be different depending on the regions (Zablotskaya 1957; Korochkina 1969, 1972; Kazmin and Smirnov 1992; Kazmin et al. 1992). European bison living in anthropogenic landscapes (as in Lithuania) feed mostly on grass and agricultural crops. Browse usage is restricted mainly to the non-vegetative period of the year (Balčiauskas 1999). In the North Caucasus Mountains, bison are living in the forest but during the summer period forage on sub-alpine meadows (Kazmin et al. 1992). As ruminants, European bison have adapted to use a variety of vegetable food. High food demand means that European bison roam the forest continuously. It has been established experimentally that calves up to 1 year of age eat 8.5kg of fresh food every day, the young (2-3 years old) 19.5-28.5kg and adults 23-32kg. This food contains a considerable amount of leaves and browse (40%) (Gębczyńska and Krasińska 1972). According to other authors, a hybrid adult bull of the North American and European bison in the Caucasus eats 30-60kg of fresh food daily (Kalugin 1968; Aleksandrov and Golgovskaya 1965). Daily food consumption in European bison, living in enclosures in the Prioksko-Terrasnyjj Reserve, was between 25-50kg of fresh matter (grass, hay and willow branches) daily, depending on the age of animals and the kind of food (Kholodova and Belousova 1989). A food requirement, defined as dry matter of food eaten, was from 15-22g per 1kg of animal body mass (Kholodova and Belousova 1993). In Białowieża Forest, it was calculated that a herd of European bison is able to consume 0.9% of herb layer biomass from coniferous and deciduous forests during the vegetative season. This value can slightly decrease in the spring and particularly in autumn if the consumption of herbs in alder woods is added (Krasińska et al.1997). Basic herbage food is seasonally (summer) supplemented with a small amount (up to 10% of diet) of woody plants (Borowski and Kossak 1972; Gębczyńska et al. 1991). In winter, the portion of woody plants can be higher if no supplementary feeding is offered (Korochkina 1969; Kazmin and Smirnov 1992). Natural food is well utilized by European bison, this is confirmed by the high digestibility of its components; 51–61 % of dry matter (Kowalczyk et al. 1976; Kholodova and Belousova 1989). The ability to digest lignin at a higher rate than cattle indicates a specific adaptation of European bison to forest conditions (Gębczyńska et al. 1974). In Poland and Belarus, all free-ranging herds, large, medium-sized and small, have been traditionally fed with hay in winter since their formation (Korochkina 1974; Krasiński 1978a, 1983; Krasiński and Krasińska 1994). Winter supplementary feeding limits natural mortality of European bison, but at the same time leads to a few-months concentration around the feeding places, which may affect European bison health. We believe that winter feeding of European bison in BPF should be continued. However in other sites of bison reintroduction, with larger areas of meadows or cultivated fields (like in Lithuania, Balačiauskas 1999), it may not be necessary. Constant observation of animals’ condition and forest damage is recommended to ensure an immediate response to an unfavourable situation.

European bison's daily activity rhythm is polyphase and thus typical of other ruminants; phases of foraging alternate with resting spent mostly on rumination. In the summer period, the main phases of European bison daily activity rhythms are highly synchronized in the group, thus confirming the consolidation of the herd as a structural unit of the population. Common feeding in the group allows utilization of the European bison's food strategy based on active feeding during movement. In the vegetative season, European bison spend approximately 60% of their daily activity on feeding, 30% on resting, and the remaining 10% on roaming without feeding. A reverse situation can be observed in winter, when European bison are additionally fed with hay and spend about 30% of their daily activity feeding, 60% resting and the roaming time is the same (Caboń-Raczyńska et al. 1983, 1987). It has been found that European bison from mixed groups share their feeding activity in the vegetative season feeding on herb layer plants (95%), browsing (3%) and debarking (2%) (Caboń-Raczyńska et al. 1987). Debarking is seasonal, being most intensive at the turn of winter and spring, in BPF conditions in April (18% of feeding activity). Drinking in the snow-free period is not regular in the daily activity rhythm of European bison. Those living in mixed groups normally use permanent water reservoirs or watercourses (small rivers and streams). Solitary bulls frequently drink water from road pools. During winter, European bison also use snow-water, crumble ice on streams, or tread frozen soil in alder woods to get to water.

According to European Bison Pedigree Book data, bulls living in reserves begin to mature sexually in the second year of life (Zablotsky 1949; Jaczewski 1958). However, histological studies of the testes and epididymes have revealed that European bison from free-ranging populations and reserves begin to mature sexually at the third year of life. Bulls aged 4-12 are characterized by fully developed spermatogenesis and are able to produce mature spermatozoa (Czykier et al. 1999). Young bulls from free-ranging populations, aged 4-6 years, are sexually mature, but do not take part in reproduction for behavioural reasons; they are not allowed to cover cows by older bulls (Krasiński 1967; Krasiński and Raczyński 1967; Krasińska and Krasiński 1995). The breeding period in males in a free-ranging population is short, lasting from the 6th to 12th year of life and later it is limited due to attenuated spermatogenic process (Czykier et al. 1999). Cows usually reach sexual maturity in the third year of life, giving birth to their first calf in the fourth year. In a free-ranging population, approximately 20% of females give birth to the first calf in the third year of life, but frequently (36.5%) at the age of five or six (Krasiński and Raczyński 1967). Females can give birth until the end of life, although the upper limit accepted for cows from free-ranging populations is about 18-20 years of age (Krasiński 1978b; Balčiauskas 1999). The rutting season in free-ranging populations continues from August to October. The gestation period of a cow lasts for 264 days on average (254-277) (Krasiński and Raczyński 1967) or 265.7 days (256-279) (Jaczewski 1958) and 267.4 days (259-279, n=21) (Kiseleva 1969). A female European bison usually gives birth to one calf at a time; twins are sporadically observed in captive breeding. Parturition, lasting from 1h 30min to 2h 11min, has only been observed in reserves. Cows calve lying down and immediately after giving birth they begin to lick neonates intensively. The first standing of a calf takes place 22–45 minutes following birth, and the first suckling occurs within the first hour of the calf’s life (Daleszczyk and Krasiński 2001). For the period of parturition in a free-ranging population, the cow leaves the herd to return with a calf after a few days. The calving period in a free-ranging herd lasts from May to July; however, late parturitions can happen (August–October) (Krasiński and Raczyński 1967; Krasiński 1978a,b; Balčiauskas 1999). Long-lasting observations of European bison in Białowieża Primeval Forest and Cejjskijj zakaznik revealed that the sex ratio at birth did not differ significantly from 1:1. However, in some years deviations are observed. The reproductive potential of the population is expressed by the coefficient of births (the ratio of the number of calves born to the population size) and the coefficient of fecundity (the ratio of the number of calves born to the number of cows capable of reproduction). In large and medium-sized European bison populations, the mean coefficient of births ranges between 14 and 17% in multi-year cycles (the minimum 5%, the maximum 35%) (Krasiński and Raczyński 1967; Krasiński 1978a; Krasiński et al. 1994a, 1999; Krasiński and Krasińska 1992, 1994). The coefficient of fecundity has been estimated only for the Białowieża Forest population, being on average, 50% in the Polish population and 40% in the Belarusian population in a multi-year cycle. This indicates that almost half of the females capable of reproduction in free-ranging herds give birth to calves every year (Krasiński 1978a; Krasiński et al. 1994a, 1999). The highest mean coefficient of births (22.4%) and coefficient of fecundity (70.3%) were in the first years when the population increased rapidly (1958–1966) (Krasiński and Raczyński 1967; Krasiński 1978a). In populations living in the Caucasus Mountains the coefficient of fecundity varied from 22–62% (Kazmin 1989). Both mean coefficients of reproduction were higher in European bison living in enclosures compared to those in free-ranging populations (Raczyński 1975; Pucek 1984). Bulls from captive breeding reach the age of 20, while those from free-ranging populations do not live longer than 14-16 years. Cows bred in captivity live up to the age of 28, while the oldest marked cow from a free-ranging herd in the Białowieża Forest lived for 24 years (Krasiński 1978a). The mean body mass of European bison males at birth is 27.6kg, being higher than in females (24.4kg), but the difference is insignificant. In males, body size (mass and measurements) increases proportionally with age up to six years. In females, the highest increase in body mass is observed in the first year of life and at the age of 3–5 the growth rate becomes slower than in males and starts declining at the age of five. The mean body mass of European bison males from Białowieża reserves aged six years and older is 747.1kg (n=25), females 460.2kg (n=19), while the mean body mass of European bison living in a free-ranging population is 634.1kg (n=79) and 423.7kg (n=76) respectively (Krasińska and Krasiński 2002). The highest rate of increase in body measurements occurs in the first year of life. Later the increase is slower and declines at the age of 5-6. The rate of increase in body measurements is higher with age in males than in females. Body measurements are correlated significantly with body mass. The maximum body measurements of six year-old bulls, and older, living in reserves and in free-ranging herds in BPF are: withers height – 188 cm, body length – 300 cm, oblique body length – 270 cm, heart girth – 280 cm; in adult cows, 167 cm, 270 cm, 172 cm and 246 cm respectively. The hump formed by spinal processes of the thoracic vertebrae surrounded by powerful muscles gives adult European bison an impressive appearance. The hump of cows is less developed. Sexual dimorphism expressed by body mass and measurements develops gradually during the postnatal period, becomes pronounced at the age of three and is maintained until the end of life. The physical development of European bison ends at the age of five years in females and at the age of six years in males (Krasińska and Krasiński 2002).

In the first 10–20 years after reintroduction to Białowieża Forest, the size and structure of large and medium-sized European bison populations developed without human interference. The established population structure formed is believed to ensure normal development. Bulls (four years old and older) constitute 25% of the population, cows (four years old and older) 35%, the young of both sexes (2–3 years old) 25% and calves represent 15% on average (Krasiński et al. 1994a, 1999). The European bison is a gregarious animal. Mixed groups and bull groups are the basic units observed. Mixed groups contain cows, the young aged 2–3, calves and temporarily adult bulls. The average size of mixed groups is environment-dependent. As a rule, groups consist on average of 8–13 animals in different populations (Krasiński and Krasińska 1992, 1994; Krasiński et al. 1994b, 1999). In BPF the mixed group size ranges 2–92, with groups of 20 being the most common (65–85%). Sometimes, European bison foraging in open areas (mown or mountain meadows and deforested grassland glades) form larger groups, amounting to 23 individuals on average (2–140) (Bunevich and Kochko 1988; Krasińska et al. 1997; Kazmin and Smirnov 1992). Groups of bulls in all populations are small and contain two animals on average (1–11). More than half of the males lead a solitary life (Krasiński and Krasińska 1994; Krasiński et al. 1994a). Groups of European bison are not family units. The size and structure of mixed groups change, some of the changes being seasonal (calving, joining of bulls in the rutting period), others for behavioural reasons. Groups meet frequently, combine and then quickly split exchanging some of the individuals. The bonds between the young are the least permanent; young bulls exchange most frequently (Krasińska et al. 1987). European bison movements are connected mainly with feeding activity and ensure optimum use of food supply. Habitat utilization by European bison depends on group size and structure, habitat preferences, and rotational exploitation of the environment, which prevents overgrazing (Krasińska et al. 1987). In winter, the majority of European bison gather around feeding sites and form large mixed aggregations and smaller bull groups. Depending on the population size, there are one or more winter aggregations of different sizes. The largest mixed aggregation of 100 European bison is observed in BPF (Krasiński et al. 1999). In all populations, some bulls take advantage of extra winter-feeding in a limited way. The size of bull home ranges is correlated with their age. In BPF the average home range of younger bulls (5–6 years old) is 44 km2, being significantly smaller than that of older bulls over six-years old (84.3 km2). Bulls inhabiting the forest periphery occupy the largest home ranges (136.5 and 151.6 km2). The maximum cow home ranges cover approximately 100 km2. Winter home ranges of bulls in BPF are larger than those of cows (10.7 km2 and 7.9 km2 respectively), and are correlated with duration of permanent snow cover and mean winter temperature. Low temperature and long-lasting snow cover delimit European bison mobility in winter. In the snow-free period the mean size of a bull home range is 69.5 km2 and does not differ significantly from that of cows (68.8 km2) (Krasińska et al. 2000). European bison home ranges are not defended and overlap greatly. Central parts of their ranges are the most intensively used. Small core areas are sited around watering places and meadows (Krasiński et al. 1999). The core areas of Lithuanian populations are about 20 km2; however, animals frequently visited territory of between 100–200 km2 (Balčiauskas 1999). This data should be taken into consideration when planning European bison reintroductions. An area of 200 km2 seems to be sufficient for a population of 50–70 individuals; in smaller forest complexes, conflicts with agriculture may arise