The more you know about monarch butterflies, the more extraordinary they seem. Their life cycle — the adaptive web of behaviors they have evolved — is almost unbelievably complex. They migrate en masse (this month, as a rule) from all across the Midwest and Northeast to just a few high altitude sites in Mexico, where they winter, sheltering in profusion beneath a canopy of fir trees, from whose trunks they draw the residual heat they need to stay alive. Winter past, they migrate thousands of miles northward, where they lay their eggs on milkweed plants, the only plant a monarch caterpillar can eat.

The complexity of their life cycle is mirrored by the complexity of the threats they face. For the past 15 years, scientists have been watching monarch numbers plummet, as much as 81 percent between 1999 and 2010. They reached nearly catastrophic lows in the winter of 2009-2010 and have barely recovered since.

One recent study suggests that the long-term survival of the species may be in doubt. A few weeks ago, one of the scientists devoted to studying monarchs, Ernest Williams at Hamilton College, summarized for me the threats that have been reported in recent studies.

Nearly every link in the monarchs’ chain of being, he said, is at risk. Illegal logging in Mexico has reduced their winter habitat — an already vanishingly small area, which is itself being altered by the warming climate. Ecotourists who come to witness the congregation of so many butterflies disturb the creatures they have come to see. But perhaps most damaging is the demise of milkweed.