European bison became a fugitive species before eventual extinction

Image: Royal Natural History Volume 2 - Public Domain

Image: Royal Natural History Volume 2 - Public Domain

In prehistoric times bison preferred to inhabit open spaces, not only forests. This is the conclusion that Polish and German scientists have come to after testing the oldest European bison (Bison bonasus) bones.

Their discovery, published in the journal PLOS ONE (open access), is important for the conservation and management of modern populations of the species, previously restored in forest environments.

The analysis of the isotopic composition of the bones showed that bison preferred open areas, and their diet was mixed, containing grassy vegetation, stems of trees and shrubs, as well as lichens – determined the researchers led by Dr. Rafał Kowalczyk from the Mammal Research Institute PAS in Białowieża and Prof. Herve Bocherens from the University of Tübingen.

Wild population restored

The European bison is the largest land mammal in Europe. After extinction in the wild in the early twentieth century, its wild populations have been restored from specimens preserved in closed herds. The world population of the species in the wild state is currently 3 thousand animals, distributed in more than thirty forest areas of Central and Eastern Europe.

There are signs, however, that the forest environment is not optimal for bison, because during winter it does not provide enough food for the large herbivores. Therefore, to survive in times of harsh and snowy winter, bison needs feeding by humans. In the absence or under-feeding, the animals migrate to the open areas, where they cause severe damage to agricultural crops. Open environment – meadows, river valleys – provide large amounts of food throughout the year, and therefore can assure the bison survival without intensive human care” – noted director of the Mammal Research Institute PAS, Prof. Kowalczyk.

Environment change

However, most of the species conservation programs assume that the bison is adapted to living in forest environments, unlike it’s cousin – American bison and extinct thousands of years ago steppe bison (Bison priscus) – closely associated with grassland. “The question is whether in the past, forests were the environment preferred by bison, or rather the environment in which the bison were forced to live, because their optimal habitat disappeared or had been occupied by man with the development of agriculture?” – noted Prof. Kowalczyk.

To answer this question, researchers from the Mammal Research Institute PAS and the University of Tübingen studied the pattern of environment usage and diet based on Early Holocene bison bones found in northern Germany, Denmark and southern Sweden.

Bone age was determined at 10-12 thousand years. During that time, after the retreat of the glacier, climate warmed and woody species gradually replaced glacial, grass-shrub tundra and steppe-tundra.

To find out which environment bison used at the time and which groups of plants were their food, the researchers measured the content of stable carbon and nitrogen isotopes in their bones. The isotopic composition of carbon allows to distinguish between animals foraging in the forest and those that graze in the open air, and nitrogen isotope content allows to determine the type of food consumed by the herbivore, because its amount is different in the grass than in the leaves of trees and shrubs, herbaceous plants and lichens.

The authors later compared the results of these studies with the composition of isotopes in the bones of other large herbivores: herbivorous aurochs and elk (that specialized in gnawing shoots), living in the same area during the same period. “As it turned out, at the beginning of the Holocene bison lived in open environments, feeding on stems of trees and shrubs, as well as grassy food. Their diet also included lichens, as in contemporary reindeer, closely associated with tundra environment. Bison, being a species with less specialized diet, could cope in difficult winter conditions and avoid direct food and habitat competition with specialized elk and aurochs” – noted Prof. Kowalczyk.

A fugitive species

Open living environment of the early Holocene bison indicate that the species was not originally adapted to live in dense forests and preferred open and mixed environment. Along with the progressive loss of open environments (as a result of global warming, forest expansion and human occupation), bison retreated to the forest and became a fugitive species. Density and survival of bison in the forests were probably lower than in open areas. This led to a gradual fragmentation of its distribution and extinction of species in Europe.

The study results are very important in the context of the conservation strategy and population management of an endangered species – the researchers emphasised. In their opinion, we should stop seeing bison as a typically forest species. “The environment, which can ensure the survival of the bison and reduce its dependence on man is a mosaic of forest and open areas. These areas should be selected as places to create new wild bison populations” – emphasised Prof. Kowalczyk.

PAP – Science and Scholarship in Poland