Election Section

Fierce Debate Rages Over Monarch Migration Pattern By JOE EATON

Special to the Planet
Tuesday December 07, 2004

My thanks to Tom Butt for the reminder that the monarch butterflies have returned to their winter bivouac in the eucalyptus grove at UC’s Richmond Field Station. 

And they’re presumably also back in their other regular spots, up and down the coast from Mendocino to Baja: the cypresses at Pacific Grove, the trees at Former Natural Bridge State Beach near Santa Cruz, Morro Bay, Santa Barbara. I wonder if the roost at the Gill Tract is still there, or the one I saw long ago on Treasure Island when the Navy was in charge. 

Writing about monarchs feels a bit daunting. They’ve inspired some really gifted people: W. S. Merwin (the essay called “The Winter Palace”, in his collection The Ends of the Earth), Robert Michael Pyle (Chasing Monarchs), Sue Halpern (Four Wings and a Prayer). And it’s easy to see why. They’re beautiful, singly or en masse, and their migration is a mind-boggling phenomenon. How do they find their way each fall to the same scatter of coastal groves, or the same fir-covered Mexican mountains? And remember, the fall migrants—great-grandchildren, at least, of the previous winter’s generation—have never seen Mexico or Monterey. Is the route hardwired in their poppyseed-sized brains? The scientists still aren’t sure. 

A lot of what we thought we knew about the monarch migration is debatable, it turns out. Monarch scholars, as Halpern’s book shows, are a contentious lot, with strong opinions. UC Santa Barbara emeritus professor Adrian Wenner doesn’t believe their travels are true migrations like those of birds and bats. (Wenner, who I met a few years back on Santa Cruz Island, is an interesting combination of iconoclast and curmudgeon. He’s also convinced Von Frisch was wrong about the dance-language of bees.) He thinks what’s actually happening, on the West Coast at least, is an annual range expansion and contraction. Wenner’s field studies in Santa Barbara indicate local monarchs disperse from their roosts long before they could survive a crossing of the Santa Ynez Mountains, with some remaining near the coast year-round. 

Wenner’s is a minority view: other monarchists, like Lincoln Brower, the dean of monarch studies, regard the butterflies’ journey as a directed migration. But he acknowledges mysteries about origins and destinations. For years the ruling paradigm had monarchs west of the Rockies traveling to the California coast in winter, and those east of the Rockies migrating to Michoacan. Brower calls this notion of a butterfly Continental Divide—which has even been written up in Sunset—“a virtual canon in American natural history.” The problem, as he and Robert Pyle both concede, is that there’s little empirical support for it. And if you listen closely, you can hear the paradigms shifting. 

Only a handful of western monarchs from outside California have been recovered at coastal roosts. Two schoolteachers in Boise, Idaho, released locally hatched monarchs that were later found in California, and a butterfly that Pyle tagged in Washington State reached Santa Cruz. However, other Boise monarchs turned up in southern Utah, which would certainly be the long way around for a California-bound migrant. Pyle spent the fall of 1996 following monarchs through the Great Basin and saw most flying a southeasterly vector, as if headed for Mexico. 

Brower suspects there are large-scale interchanges between western and eastern monarch populations. Although some have claimed differences in wing length (related to longer distances traveled?), eastern and western monarchs are very similar genetically. Then there are the events of 1996. Western monarch numbers had nosedived in the early ‘90s, possibly due to a protozoan disease contracted from eastern butterflies released in the west. In the winter of 1995-96, observers reported an all-time low at the coastal roosts. But the monarchs had bounced back by the following summer. Conversely, eastern butterfly watchers saw far fewer northbound migrants than normal, with a 90 percent drop in some northern states.  

Brower and Sidney Gauthreaux, who studies migrant birds, have pointed out that wind patterns along the Gulf Coast shifted to the west in the spring of 96, causing a major western fallout of eastern warblers. Could the same thing have happened to the monarchs? And does the smaller western population rely on periodic infusions of fresh blood from the east? If so, the fate of the monarchs wintering in California could be bound to that of the oyamel fir forests of Mexico, under siege by loggers despite their nominally protected status.  

Things are dicey enough already for the western monarchs. The disease remains worrisome, prompting Brower, Pyle, and other conservationists to warn against the practice of releasing commercially-bred monarchs at weddings and funerals, “like biodegradable balloons.” Western monarchs have also chosen some highly desirable coastal real estate to roost in. And their preference for eucalyptus—75 percent of all California wintering sites, according to a recent survey, with Monterey pine a distant runner-up—can be problematic.  

Nobody is neutral about eucs. To native-plant advocates, they’re flammable megaweeds. But monarchists see their good side. Writing in Wings, the journal of the Xerces Society (a fine organization dedicated to the conservation of insects and other invertebrates), Pyle and Muir Woods ranger Mia Monroe make the case for eucalyptus as monarch habitat: “It is likely that, had ‘eucs’ not been introduced, the phenomenon of mass-wintering monarchs would not exist in California today; one of our highest goals should be to nurture tolerance for essential stands of eucalyptus—sometimes even refreshing them—until we can restore mature groves of native species.” 

Sounds reasonable to me. Monarchs have probably graced our coast for millennia. (The records in Monterey go back to the 1860s, prior to which, according to local monarch buff Lucia Shepardson, “nothing smaller than a bear would have attracted the…[early settlers’] attention.”) If the price of keeping them around is tolerating a few eucalyptus groves, it seems well worth it.