Moose-Wolf-Vegetation Management Plan/EIS Background Information

Aerial View of Washington Harbor
Aerial View of Washington Harbor

NPS Photo/Paul Brown

Isle Royale National Park (Isle Royale) is an island archipelago in the northwestern portion of Lake Superior consisting almost entirely of designated wilderness. Isle Royale exhibits a unique biogeography. Organisms that live on islands have more dynamic population trends and are more often subjected to extinction events with colonization and immigration occurrences depending on island size, distance to the mainland, length of isolation (time), chance events, habitat suitability and human activity, to name a few influencing factors. In other words, things come and go from islands; local extirpation is natural, as is establishment and re-establishment of new populations.

 
Wolf
Wolf

NPS Photo

Wolves are the only predator of moose on the island, and they affect moose abundance and distribution across the island. Wolves were first documented near Isle Royale in the 1940s when they were observed on the ice between the Sibley Peninsula of Ontario and Isle Royale. It is largely accepted that wild wolves arrived on Isle Royale by crossing an ice bridge during winter between 1948 and 1950, and that they founded the Isle Royale population. However, the 1948-50 arrival of wolves was followed in 1951-52 by the release of four pen-raised wolves from the Detroit Zoo, two of which were subsequently removed from the wild. Researchers have raised questions regarding the contribution of those that potentially persisted into the population.

Like many mainland wolf populations, the population at Isle Royale has fluctuated widely over time even though wolves have always been protected and never hunted or subjected to control efforts. The long-term average wolf population on the island is 22. Wolf population variation on the island has been primarily driven by the availability of older moose and calves, which are most vulnerable to predation. The vast majority of wolf mortalities on Isle Royale are due to natural processes, primarily wolves killing other wolves and starvation. Furthermore, genetic inbreeding, a natural process in small, isolated populations, has led to physical deformities and has likely resulted in low productivity and survival. The future presence of wolves on the island is in doubt unless new wolves emigrate or are introduced.

 
moose11
Bull Moose

NPS Photo/Paul Brown

Moose first arrived on Isle Royale in the early 1900s and their population has fluctuated dramatically (from below 500 to over 3,000) over the past century. Moose are known to have important effects on island vegetation. For example, in the past the population has increased dramatically and then declined following severe over-browsing of island vegetation. This boom and bust cycle has occurred in both the presence and absence of wolves; however, the magnitude of the fluctuation is likely to increase without wolves. More recently, the moose population has again been on the rise and is currently estimated to be 1,250. Although the population is likely to increase in the short-term in the presence of a very low wolf population, it is unclear how sensitive moose on Isle Royale will be to the changing climate. Heat stress and the availability of preferred vegetation are issues if temperatures continue to warm. Mainland moose populations have been in decline and it is unclear if the moose on Isle Royale will follow this same trend in the future.

 
Treetops
Treetops

NPS Photo/Paul Brown

Vegetation at Isle Royale is also changing. The park lies within a temperate-boreal forest transition zone where temperate tree species are near their northern range limits and boreal trees are near their southern range limits. Recent trends suggest the beginning of a shift from boreal to temperate vegetation. Since moose favor some boreal tree species such as balsam fir for food in the winter, this change may alter the available moose forage in the future.

Last updated: July 20, 2015

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Houghton, MI 49931

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