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Where Have All the Monarchs Gone?
Dr. Chip Taylor of Monarch Watch shared his thoughts, carefully reviewing
each step along the monarch's way--from the fall migration to their return this spring.
"Our ability to explain the dynamics of monarch populations is still very limited," said Taylor. "It
has been a quiet spring--but a normal spring, unlike the spring of 1997," he added. "Although the numbers
of spring monarchs appear to be lower this year, their arrival times at new locations are near the long term averages."
Did last fall's large numbers of monarchs make it to Mexico?
"Perhaps many of the Monarchs didn't make it to Mexico or died shortly after their arrival. The opinions were
mixed, some said the populations were down, others offered the opinion that the numbers were normal. No one offered
that the populations were above normal, and nearly all agreed that there were fewer Monarchs than in the fall/winter
of 1996. Is it possible that a large proportion of the monarchs simply didn't survive the journey to the overwintering
sites? Another little mystery. Again, this appears to be an aspect of the population and seasonal cycle that is
difficult to interpret from what we see at other locations."
Did they die shortly after their arrival?
"Perhaps many of the Monarchs didn't make it to Mexico or died shortly after their arrival. I visited El Rosario
on 14 November. The numbers of monarchs in the air and on the trees were spectacular but, as a biologist, I couldn't
take my eyes off the numbers of dead and dying already scattered on the forest floor. How amazing, I thought. How
strange to have the biological drive to fly all the way to Mexico only to die within days of arrival. The dying
butterflies were thin and appeared to be low on fat reserves. I supposed it was normal for some butterflies to
arrive in poor condition but were the numbers of arriving butterflies with low fat reserves greater than normal
in 1997? I don't know. There is much to learn. Our ability to explain the dynamics of monarch populations is still
Did this winter's extreme cold reduce their numbers?
"The weather during the winter, particularly during December and January, can have a major impact on the monarchs
populations. In mid December 1997, a major cold front pushed into central Mexico causing severe crop damage and
killing the native vegetation in many areas. Although there were no reports of widespread deaths of monarchs due
to this cold snap, dead monarchs, up to two feet deep, were seen under some Oyamel trees near the top of the ridge
at Chincua. The remainder of the Chincua population, on the southerly and southwestern slopes of the mountain,
survived this cold period."
What was the effect of the unusually dry El Nino weather pattern of 97-98?
"Although it is normally dry during the winters in central Mexico, rainfall usually occurs in the mountains,
and days and nights are often cloudy. The El Nino weather pattern of 97-98 was unusually dry. Rain was scarce and
cloud cover was minimal throughout the winter. Lack of cloud cover at night resulted in overnight temperatures
that were at or below freezing at ground level night after night. In February we saw evidence in several locations
at El Rosario that Monarchs caught in the open or on the ground at the end of the day had probably frozen to death.
Cold mornings limit the ability of the monarch to fly to sources of water and water became increasingly difficult
for the butterflies to find as the winter progressed. At El Rosario, the lack of water contributed to an unusual
redistribution of the monarchs late in the winter. In late February and March, a large portion of the colony moved
downhill to a source of moisture and trees on the property of the adjacent Angangueo ejido. Much of the tourist
traffic destined for El Rosario was diverted to Angangueo to the consternation of many of the El Rosario guides
and vendors. This appears to be the first time in memory that the colony has resettled in Angangueo."
Were the monarchs in poor condition at the over-wintering sites?
"Winter can be a stressful period and the monarchs that survive are often in poor condition. The condition
of the Monarchs at the end of winter probably determines their ability to remigrate in the spring. Was this a more
stressful winter than normal? We don't know. When I visited El Rosario in mid February, the Monarchs seemed to
be in good condition. The wings did not seem to be especially worn and the butterflies did not appear to be thin
as they do when the fat body is depleted. David Marriott of the Monarch Program, who had a longer opportunity to
evaluate the condition of the population in late February and early March, reported that the monarchs appeared
to be in poor condition."
Was this spring's 600 mile trip north across Mexico unusually dry?
"After leaving the overwintering sites, the Monarchs zigzag 600 miles of more through the mountains of Mexico
before they reach Texas. Although there is some egg laying on the milkweeds they encounter in the mountains, it
is not clear whether offspring reared in Mexico subsequently move northward. Monarchs are rare or absent from most
of northern Mexico during the summer months. This spring the mountains were unusually dry but whether this limited
the availability of nectar for the northward migrants is not clear. Even under dry conditions many herbaceous and
shrubby plants bloom during the last two weeks of March when the monarchs are moving through the area."