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The March of the Monarch

Photo: Courtesy of Lincoln P. Brower, Sweet Briar College  FIGHTING FOR SURVIVAL: Overwintering monarch butterflies on the ground after a snow and rain storm in the Sierra Chincua, January 20, 1983. Many of these survived because the temperature didn't plunge below freezing following the storm. Across my dreams with nets of wonder I chase the bright elusive butterfly of love --Bob Lind Jan. 11, 2002 started a bad weekend for monarch butterflies. Late that night, an unusually powerful

By | October 14, 2002

Photo: Courtesy of Lincoln P. Brower, Sweet Briar College
 FIGHTING FOR SURVIVAL: Overwintering monarch butterflies on the ground after a snow and rain storm in the Sierra Chincua, January 20, 1983. Many of these survived because the temperature didn't plunge below freezing following the storm.

Across my dreams with nets of wonder
I chase the bright elusive butterfly of love

--Bob Lind

Jan. 11, 2002 started a bad weekend for monarch butterflies. Late that night, an unusually powerful storm invaded the butterflies' overwintering grounds in the Oyamel fir forests west of Mexico City, which are normally dry at that time of the year. According to Monarch Watch at the University of Kansas in Lawrence, the butterflies "form dense clusters on the trees, with densities of 10 million or more butterflies per hectare," distributed over several different colonies. That can add up to 100-500 million butterflies south of the US border. As cold rain changed to snow and the temperature dropped into the 20s that night, the inactive, hibernating monarchs dropped from the trees by the bushelful.

Monarch Watch director Orly 'Chip' Taylor, who visited the forest shortly afterward, reported that at one location, "the entire area was covered by a carpet of dead monarchs, in piles up to eight inches or more in depth." It looked like a forest floor covered in autumn leaves. Taylor's observations confirmed widely reported comments by butterfly guru Lincoln P. Brower of Sweet Briar College in Virginia. Brower estimated that 70% to 80% of the butterflies had died in some colonies. Only 27% to 37% of the area inhabited by butterflies remained, according to Monarch Watch. That kind of mortality is "unprecedented in the known history of monarch overwintering sites," the organization noted in a February 2002 E-mail alert.

Catastrophic kills aren't new to monarchs, but this one came right after monarch populations soared last summer following a disastrous, drought-ridden season two years ago. Needless to say, the mood of monarch watchers swung just as wildly, from giddy optimism to fearful disappointment. Last season's resurgence came despite an all time record low 4-5 million females returning north into Texas in the spring of 2001. By fall, the population exceeded 200 million. That's because several generations of monarchs breed each summer, and every female can produce 400 or more eggs. Sharply reduced fire ant numbers, also due to drought, helped fuel butterfly survival. As monarchs began their return north again this spring, researchers wondered if they could make another strong comeback.

TRACKING THE NUMBERS By April, Monarch Watch was cautiously optimistic, reporting that "the number [of butterflies] reported in the first three weeks of the season exceeded those for this period in each of the previous four years." The organization tracked the migration with a network of observers throughout Texas. Said Taylor, "It really looks like the return migration is pretty good ... and the population will make a reasonable recovery." Still, weather conditions didn't seem as favorable for reproduction as before. While nectar- producing flowers to feed lovesick adults, and milkweeds for their babies, were ample, lots more hungry fire ants awaited the new brood of caterpillars.

By May, though, the news was not as good. Monarch migration into the Midwest lagged, while the butterflies that did make it looked faded and tattered. While flower nectar and milkweed were still ample, Taylor opined, "It's still too early to determine whether the population is in the process of recovery. Last year at this time, large numbers of first-generation monarchs were moving into the Midwest from Texas. There is no sign of such an influx so far this year."

Monarchs reached northern breeding grounds in June--later than usual because cold and rainy weather slowed them down. More worrisome, Monarch Watch estimated that only half the number of butterflies showed up compared with the same period last year. The news still was not good in Lawrence, either. "I have seen only seven monarchs this spring and this is certainly my all-time low for this far into the season," said Taylor. He hadn't seen any eggs, either, meaning not many caterpillars in the new generation.

Observers reported monarchs in most of their northern breeding areas, including Canada, by early July--again a little late--but eggs and larvae on milkweed leaves were sparse. Since it is this generation and butterflies appearing shortly thereafter that start the migration back south, the outlook was grim. "The number will be significantly lower than last year," predicted Taylor.

By the end of the month, Brower admitted, "I don't think we really know what the monarch status is at this point in time. All indications are that they're down, but how far down and where they may be rebounding" are unclear. Brower too has heard that the monarch population is low in Kansas, but one of his students working on the Door County peninsula in Wisconsin reports, "They're in pretty good shape there."

As fall migration took flight, the die seemed cast. According to Taylor in the September Monarch Watch update, "the reports from around the country suggest the migration will be even smaller than I anticipated. ... The only sizable populations are those in Minnesota, Wisconsin, and Iowa, and these are lower than last year."


Photo: Courtesy of Lincoln P. Brower, Sweet Briar College
 BURIED: Chris Kisiel, Lincoln Brower's research assistant, buried beneath dead monarch butterflies killed by the catastrophic winter storm in January 2002. The photograph was taken on March 3, 2002, in the Sierra Campanario Conejos overwintering colony which was very hard hit by the storm. Piles of dead monarchs beneath the fir trees were more than two feet deep in places.

ON THE ROLLER COASTER With all the ups and downs in monarch numbers, scientists still don't have a firm handle on why monarch populations vary. Monarch Watch thinks "it is the overwintering sites that hold the key to the continuation of the migration in eastern North America." Mexican forest preserves act like a buffer by tempering large swings in temperature and moisture. That's why encroachment from logging poses a threat. "Degradation of the forests at colony sites is likely to break down this protection, leading to higher mortality and greater vulnerability of the clustered butterflies to occasional snow or freezing rain," says the organization. "Monarchs require relatively intact forests to successfully overwinter. The challenge is to maintain the integrity of these forests when the economic conditions are such that local landowners view the trees as a source of income." In other words, habitat destruction in Mexico is the main threat to monarchs. Anything else is a blip on the radar screen.

That view received strong support recently from an international team led by Brower. By analyzing successive aerial photographs of a 42,000-hectare area taken between 1971 and 1999, the researchers documented a 44% degradation in conserved forest.1 The largest patch dropped from more than 27,000 hectares to just a little over 5,800 hectares. Between 1984 and 1999, the annual rate of loss jumped to 2.4%. At that rate, Brower and colleagues estimated that less than 4,500 hectares of forest would remain in 50 years. Though the forest is supposedly protected by government decree, it is not being preserved. The team warned that "the grandeur of the monarch butterfly overwintering phenomenon in this tiny area of Mexico is too great a cultural and biological treasure for this rampantly destructive process to continue."

Photo: Courtesy of Lincoln P. Brower, Sweet Briar College
 GETTING WET: Male monarch butterfly with water droplets on wings after a winter snow and rain in the Sierra Chincua, January 1981. Similar wetting in the January 2002 storm resulted in millions of individual monarchs losing their natural protection against freezing. When the temperature dropped to about -4°C nearly 80% of the butterflies were killed.

Taylor thinks "the Brower paper is first rate." While "a number of nonprofessionals with an antiscience approach to conservation" criticized it, Taylor responds that "one can quibble about the amount of loss until all the forest is gone, but there can be no doubt it is going."

Still, Taylor admits that there is a lot to learn about monarch population dynamics. Scientists have not accounted for how weather, climate, and biological factors like predators and parasites interact to affect butterfly numbers from year to year. Weather has an impact on both monarchs and predatory insects, including fire ants. Though relatively few monarchs flew out of Mexico last year, Texas fire ant populations were also down due to drought, so the migrants flourished. Since it may take several favorable seasons for predators to recover after a crash, firm predictions of how monarchs will do in any given year are difficult to make. "It will take a number of years to establish the cause-and-effect relationships that govern monarch populations," says Taylor, "yet it will be done." He concludes, "This kind of knowledge is essential for the conservation of the migration and to distinguish between natural and anthropogenic effects on monarch populations."

Brower is philosophical about the current situation. "The good news is the monarch is a resilient creature. The no news is it's premature to make a judgment on what's going to happen. The real judgment call will be in September when they begin migrating south, and in early October as they're migrating down through Texas into Mexico. It's then that we'll really have a good idea how badly the big freeze in Mexico resulted in knocking back the monarchs."

Barry A. Palevitz (palevitz@dogwood.botany.uga.edu) is a contributing editor.

1. L.P. Brower et al., "Quantitative changes in forest quality in a principal overwintering area of the monarch butterfly in Mexico, 1971-1999," Conservation Biology, 16:346-59, 2002.
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