Wood Bison Restoration in Alaska
Herd Size and Growth
The wood bison management plan (PDF 1,322 kB) developed with input from the public calls for herd growth, both in numbers and range. Biologists think that 400-500 animals is a minimum number for long-term conservation of genetic diversity. The Lower Innoko/Yukon River area has plenty of suitable habitat to support thousands of wood bison. Biologists will monitor the bison to make sure they don't exceed carrying capacity of the habitat as the herd grows. It is difficult to predict 50 years into the future, but biologists expect that wildlife managers may want to stabilize the population somewhere close to 5,000 wood bison. How they are managed will be driven by public input, so biology may not be the most important factor in their ultimate population number.
In the first year following release, there were 19 known bison mortalities. Nine fell through thawing ice a month after release, and 10 died of unknown causes, most likely from the stress of making the transition for captive to wild. We found no evidence of predation, human caused death, or infectious disease. The force of natural selection chose the strongest animals to survive in the wild. Natural selection has and will continue to make the future generations of wood bison in Alaska stronger.
As long as the number of bison born every year outnumbers deaths, the herd will grow. Breeding takes place in late July and August, and wild-bred calves are generally born April through June, with a few exceptions. Some bison will die every year due to accidents and other factors. During spring break up, some bison will likely break through thinning ice and drown. Drowning is the number one cause of death of wild wood bison populations in Canada. Some bison may also die in severe winters or when snow is deep or persistent. Predators are not likely to take bison during the early years. Predators near herds established in Canada took many years to learn how to kill bison. The Delta herd of plains bison did not lose an animal to predators for almost three decades.
Our models of future population size usually predict a population of:
- 250 by 2025
- 500 by 2035
- 1000 by 2045
Most models suggest that harvest could start around year 10. The management plan (PDF 1,322 kB) suggests that harvest should start when there is a harvestable surplus of at least 20 animals. The population should be around 250 at that time. Catastrophic mortality events will obviously throw these projections off (e.g., 30 cows falling through the ice on a lake).These models are based on growth observed in the many reintroduced Canadian populations over the last several decades. We simply don't know yet how well bison will respond in this new habitat. We have high hopes, but we can only wait and see. As the bison herd increases, things like winter severity, frequency of flooding, and harvest rate will have an influence on the growth of the population. Once hunting has begun, population growth can be adjusted up or down by changing harvest strategies, guided by public input.