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Mortality rate Print E-mail

In a free roaming herd of bison living in the Bia這wie瘸 Forest, the observed natural death rate is on the order of 3% of their population. Due to the lack of external natural selection factors, resulting in the elimination of certain specimens, it is possible to observe infrequent natural phenomena i.e. death due to old age. When the natural breeding process was initiated, the bison herds went through a period of extensive increase in the number of specimens between the years 1958 and 1970. In 1971, the total number of 200 specimens was exceeded and the further unlimited increase in the number of bison would be unfavourable for the local population and the forest itself. The situation from the beginning of the 20th century can be used here as an example. In the Bia這wie瘸 Forest, there were then 727 bison, 700 elks, 2100 deer, 4500 roe-deer, 1500 wild boars and 600 fallow deer. The number of deer increased in 1914 to 6800 specimens. The animals were destroying the young generation of trees, deterring thus natural forest renovation cycle.  General and health conditions of the bison were poor. Taking this experience into consideration, the moment the bison population exceeded 200 specimens in 1971, people started limiting the bison population by eliminating from the herd less valuable specimens e.g. sick, handicapped animals or specimens which had the habit of invading farming fields. Reduction in the number of healthy specimens was carried out taking special care not to destroy the structure of population, which was created at that time when the herd structure was not interfered with.

Free roaming bison populations should have the ability to function in natural conditions over an extended period of time, though at the same time should not put excessive pressure on the forest environment.  Therefore it is critical to establish an optimum, target size of each population.

Taking all these factors into consideration and based on the long-term observations of the local populations, it was established that in the Polish part of the Bia這wie瘸 Forest, the size of the local population should be approximately equal to 230 specimens. Constant monitoring of the population condition and its local environment should result in the correction of this level, both in terms of decrease as well as increase.

It is also worth remembering that the bison existence is still endangered. High level of consanguinity among all currently living bison can result in the decrease in the natural resistance and occurrence of genetic disorders.  Individual populations can also be faced with local environmental threats. In the Bia這wie瘸 Forest, such threats include decrease in the local food resources as well as periodic shortage of drinking water during long-term draught, competition from other hoofed animals (deer), conflicts with the forest management and farming, insect invasions as well as lack of possibility to migrate to other regions. Viral, bacterial and parasite sicknesses represent a constant threat to bison population, including also new sicknesses of unknown origin.

In the area of the Bia這wie瘸 Forest there is also a specifically cruel type of poaching – snaring. A snare is typically made from a loop made from several wires, the end of which is attached to a tree. Snares are always deployed along the popular animal paths. Bison become rather accidental victims of snares deployed typically for deer, wild boars or roe deer.  Every year there are at least few cases of bison injuries or even death caused by snares. The dangerous loops clench limbs, sometimes necks, hoofs or even heads. Thick steel wires, preventing the animals from freeing themselves, are as dangerous as the thin ones, which clench limbs resulting in permanent injuries.

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