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Showshoe hares and moose interact to change browse

Posted: March 14, 2013 - 2:22pm
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A snowshoe hare beginning to girdle a four-inch diameter willow tree.
A snowshoe hare beginning to girdle a four-inch diameter willow tree.

Snowshoe hares, especially during years of high populations, like those that have occurred here over the last couple of years, can have significant effects on vegetation that both hares and moose feed upon during winter. The interaction and competition between wintering hares and moose was first documented on the Kenai Peninsula by Ernest P. Walker, a game warden and member of the Alaska Game Commission, in a report titled “The Shortage of Feed for Moose during the Winter 1923-24 on the Kenai Alaska Wintering Grounds”. He stated that “the present time is near the peak of the cycle of rabbit abundance and the rabbits are competitors of the moose for the winter feed ... together [they] consumed practically all food above snow to as high as the moose could reach”.

In the mid-1970s, during another high in the snowshoe hare cycle, John Oldemeyer, who was studying the effects of moose and hares on browse production for his doctorate at the Alaska Department of Fish and Game’s Moose Research Center reported that “during the four years, snowshoe hares browsed a greater percentage of tagged plants than did moose ... 70 percent of the willow and 58.7 percent of the aspen plants were browsed [by hares and moose] during winter”.

During the peak winter (1973-74) of their population cycle, hares had browsed nearly 90 percent of the birch production, and there were many more dead than live aspen and willow on his study areas. Oldemeyer cited an earlier study of tagged browse plants on what was then the Kenai National Moose Range that revealed that 75 percent of willow, 65 percent of aspen and 44 percent of birch had died after 12 years of heavy moose browsing.

When snowshoe hares deplete their preferred sources of food in the winter — the small terminal twigs of willow, birch, and aspen — they, like moose, sometimes begin gnawing on the bark of willow and aspen in late winter and early spring because the bark of these species apparently becomes digestible as nutrients move through the bark’s cambium layer. If the hares (or moose) completely remove the bark around the circumference of the shrub or tree, the plant above the girdle dies. At high population densities, hares kill many small shrubs, and moose may kill some of the larger willows thus making them unavailable as food for future hare and moose populations.

Fortunately, there is usually an increase in plant growth following declining hare populations. Some of the larger shrubs and trees sprout new stems below the girdles or from the root stock, but this takes time and the delay itself affects the future winter food supply for hares and moose.

When repetitively browsed, the new birch twigs produce structures called glandular trichomes which contain a resin rich in triterpene papyriferic acid, a toxic compound that deters browsing by snowshoe hares. Captive hares rapidly starved to death if given only twigs rich in papyriferic acid to eat. Although the toxicity of papyriferic acid on Alaskan moose or their rumen microbes has yet to be experimentally demonstrated, moose in Scandinavia preferred clones of birch with small current annual shoots and few resin droplets.

In interior Alaska, studies along the Tanana River have shown that the combined browsing pressure of hares and moose in high moose density areas lowered the above-ground biomass of preferred willow species, increased the proportion of dead stems, and favored later-successional species such as alder and cottonwood. However, alder contains pinosylvans which are toxic and deter feeding by hares and moose, and cottonwood, especially resprouted growth, contains phenolic glycosides, which also deter browsing by mammalian herbivores. Spruce also contain contains pinosylvans and complex mixtures of terpenes that are toxic to rumen microbes in moose. This is probably why moose do not feed on spruce and both hares and moose usually avoid eating alders. However, when food stressed, hares will sometimes eat alders, chew the bark of alders, and eat spruce from mature stems when large spruce trees fall to the ground.

Since predators of snowshoe hares, such as lynx and coyotes, and of moose, such as wolves, influence the number of herbivores, their absence or reduction also indirectly influence the quantity, quality and composition of vegetation. In the Yukon Territory, a long-term study conducted as part of the Kluane Project showed that when lynx and coyotes were excluded from experimental enclosures, there was a 2.4-fold increase in hare density; when food was added to the exclosures, there was a 14.4-fold increase in hare density. These results indicate that the combined effects of predation on hares and the food supply drive the snowshoe hare cycle.

Not surprisingly, fewer herbivores can also affect vegetation dynamics. A computer model for the Tanana River floodplain of interior Alaska, published in the journal Ecological Modelling in 2012, demonstrated that wolves, by keeping moose populations down, help slow down plant succession from low-toxicity deciduous woody vegetation toward high-toxicity evergreens like spruce. The model showed that reductions in wolf densities could accelerate the normal transition from deciduous vegetation to spruce by more than 15 years!

Discussing his 1970s research on the Kenai National Wildlife Refuge, which recognized the significant interaction of browsing between hares and moose, and stressed the importance of fires to produce new vegetation, Dr. Oldemeyer stated “if willow and aspen are less tolerant of browsing than birch, as indicated [during his study], then moose population management should be directed at retaining these species in productive condition. However, because of uncontrollable factors such as cyclic snowshoe hare populations and the effects of varying winter conditions on year-to-year moose distributions, maintaining a productive browse population cannot be achieved solely through herd management alone”.

Ted Bailey is a retired Kenai National Wildlife Refuge wildlife biologist who has lived on the Kenai Peninsula for over 36 years. He maintains a keen interest in the Kenai Peninsula’s wildlife and natural history. ‘Like’ the Refuge on Facebook to stay connected with upcoming events, news and recreation updates at www.facebook.com/kenainationalwildliferefuge.

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