For such large creatures, moose are a relatively hidden species. They are hard to count. They live where humans do not like to go, along the boggy fringes of northern lakes and rivers where the sound of summer mosquitoes fills the air. They thrive in cold weather, and their well-being depends on sharp, cold autumns and late springs, which protect them from their most important predator: not the wolf, but the winter tick.

Moose are dying off across North America for a number of reasons, most of which can be linked to a warming climate and an eroding winter. Long, warm autumns and early, wet springs benefit winter ticks, which can cluster on moose in unbelievable numbers, causing anemia, loss of appetite, hair loss from rubbing — weakening the animals at the onset of winter, just when they need their strength most.

In the Upper Midwest, moose are at risk from liver fluke, a parasitic disease, and brain worm, which can afflict the nervous system. In British Columbia, they have lost protective cover thanks to the die-back of white pine forests caused by an epidemic of pine bark beetles. The epidemic, largely attributed to climate change, has also robbed grizzly bears of the seeds they depend on for winter food.

The collapse in moose numbers — one Minnesota population has fallen from 4,000 animals to fewer than 100 — is something scientists can track but otherwise can do nothing about. It is typical of the kind of shifts that a warming climate is causing, tipping the balance in ways that favor some species and do grievous harm to others. It is the sign of an entire ecosystem caught up in changes largely brought about by human activity.