The Aliens Among Us by Leslie Anthony

Author:Leslie Anthony
Language: eng
Format: epub
ISBN: 9780300231618
Publisher: Yale University Press

Moose Crossing

While invasive moose made a mess of Newfoundland, the species was paradoxically experiencing steep declines elsewhere in North America. What was happening varied according to location, but even in areas of relative abundance or natural increase, signs pointed to imminent collapse. By 2013, alarming declines were seen in all provinces from Quebec westward, and several northern U.S. states. Although the answer to “Why?” wasn’t entirely clear, like most other animals in decline it appeared to be a variety of factors—disease, predation, and climate change chief among them. And you didn’t need to look far into that troika to find an odd connection: propelled by climate change and other factors, deer were overpopulating and expanding their range everywhere, spreading disease to moose and increasing the abundance of predators affecting them.

In Minnesota, for instance, 40 percent of adult moose were dying from brain worm (Parelaphostrongylus tenuis), a tiny parasite transmitted by deer. An additional 20 percent were dying from higher-than-normal winter tick loads, and a final 20 percent from a combination of stressors. In Canada’s Jasper National Park, moose decline was attributed to the deadly giant liver fluke (Fascioloides magna), more wolves, and continued—albeit reduced—collisions on the railways and highways traversing the park. Another emerging problem was decreasing calf survival. In Minnesota’s Grand Portage Trust Lands, for instance, 90 percent of moose calves died each year—75 percent lost to predation during the first weeks of life, the remaining 15 percent to subsequent health issues.

Though brain worm doesn’t affect its deer carriers (it lives benignly in the connective tissue around a deer’s brain and spinal cord), the parasite has a typically complex life cycle: the worms release eggs that hatch in the deer’s lungs; after being coughed up, swallowed, and excreted, the larvae find homes in snails and slugs. By accidentally consuming these mollusks while grazing, 90 percent of all deer are infected in their first two years of life. In areas of deer abundance, moose pick up the worm, too. But brain worm isn’t happy in moose: it just burrows around looking for a whitetail deer brain that it never finds. Not only is the moose a dead-end host, it also usually ends up dead. (On Lake Superior’s Isle Royale—free of deer and thus brain worm—the trend is opposite: collapse of the wolf population from inbreeding sent moose numbers soaring. As on predator-free Newfoundland and Anticosti, the moose irruption now threatens to exceed carrying capacity by wiping out its main food, balsam fir, another example of why predators are critical to ecosystem health.)

Ditto for winter tick (Dermacentor albipictus), another devastating pest moved around by fellow cervids like deer and elk. Larval ticks climb onto moose in the fall and feed themselves into adulthood over the winter. Moose with particularly heavy loads—over 100,000 ticks were counted on one decidedly miserable animal—simply can’t eat enough to replace the blood loss, and end up cannibalizing their own muscle tissue for protein before, inevitably, dying.

Despite proximal parasite problems, however, the underlying cause of moose declines

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