Every fall, starting around October, tens of thousands of monarch butterflies from across the West make their way to eucalyptus groves along the California coast. There, in a quasi-torpid state, they clump together in clusters, dangling from high branches like living chandeliers. Early in the new year, they once again take wing, sailing inland to lay their eggs on the emergent shoots of winter-dormant milkweeds.
This ranks among the world's greatest and most unusual migrations, for these are not the same individuals that overwintered here last year but rather their descendants. The migration has also become Exhibit A in the arguments mounted by pro-eucalyptus forces. While monarchs also roost in trees native to California – Monterey pines and cypresses, coast redwoods, even, on occasion, coast live oaks and western sycamores – there is no question that, in today's world, eucalyptus trees have become important to the butterfly's survival.
Monarchs, scientists think, make their way to the coast with the help of a genetically encoded navigation system. Among its components is a miniature sun-compass embedded in the insect's pinhead-size brain. The butterflies also likely make use of sensory clues, the sighting of a sun-filled gap within a cluster of trees, perhaps, or the presence of other butterflies.
What the monarchs are seeking, scientists say, are groups of trees that create the necessary suite of environmental conditions. The list starts with shelter from wind and rain, then moves on to include access to sunlight, shade, and water in the form of streams, ponds and morning dew. As a bonus, winter-flowering blue gums and red gums, along with native willows, give the butterflies a chance to replenish their dwindling fat stores with sips of calorie-packed nectar.
A monarch grove is a dynamic place. When one tree becomes too hot, too cold, or too exposed to the wind, the monarchs will relocate to another. That's why maintaining a range of options is important, says biologist Francis Villablanca, principal investigator for the Monarch Alert project at California Polytechnic State University. "Trees act as air conditioners, as umbrellas, as blankets and hot water bottles," he observes. From the perspective of a butterfly, trees of different species and different sizes probably offer a shopping mall's worth of thermal properties.
Frustrating to the communities that advertise them as tourist attractions, monarch groves seem to be ephemeral phenomena. Trees grow taller and denser, or they succumb to disease and fall. Over time, groves that once hosted hordes of these big, showy butterflies become too sunny, too shady or too exposed to storms that sweep in from the ocean. "That's why you have to manage these sites," says Cal Poly professor emeritus Kingston Leong, an expert on monarch grove restoration.
A number of grove restorations arenow underway, and more are needed. In some cases, replanting eucalyptus may be necessary, scientists say. Even more important may be ensuring
that groves harbor a broad diversity of trees. The deterioration of overwintering habitat is a likely factor in the decline of the monarch population, second only to the loss of native milkweeds that once grew in such wild profusion. West of the Rockies, the numbers are not quite as alarming as they are to the east, but the drop is still dramatic. Sixteen years ago, conservationists counted well over a million monarchs in California's coastal groves. Last year, there were fewer than 150,000.