butterfly overwintering colonies are found in Mexico's oyamel fir forest,
a unique mountain habitat.
firs (Abies religiosa) grow only at high altitudes, between 2,400 and
- The distribution
of the oyamel fir forest in Mexico is extremely limited. It is shown
in black on the vegetation map of Mexico. Oyamel firs are confined
to high mountain massifs, which are few and far between.
- The oyamel
forest ecosystem is Mexico's most endangered forest-type. Only 2% of
the original forest remains.
- The cloud
cover that typically forms around the mountain tops helps provide the
moisture oyamels need, particularly during the dry season (Nov.-May).
- The oyamel
forest is a relic from a time when the earth was cooler and wetter.
As the earth warmed, the forest retreated up the mountainsides to retain
the cool, moist climate to which the trees are adapted.
at lower elevations are now too warm and dry to sustain the oyamel forest.
appear to be adapted, physically and behaviorally, to the same ecological
conditions as the trees. The monarchs seek out these high mountain habitats
for the same reason the Oyamel ended up there--it's cool and relatively
moist at high elevations when this region of Mexico is parched during
the dry season.
- The butterflies
are able to conserve their lipid reserves throughout the overwintering
season (which cooincides with the dry season) because the cool temperatures
slow their metabolism. At cool temperatures they burn less energy.
the butterflies are adapted to cool temperatures, if temperatures drop
to the mid-to low 20's (F) the butterflies begin to freeze to death.
Monarchs are essentially tropical butterflies and cannot tolerate sub-freezing
temperatures for very long.
- Two ecological
factors strongly influence the monarch's capacity to resist freezing:
1) becoming wet, and 2) being exposed to the clear night sky (and associated
- When monarchs
become wet, they tend to freeze at higher (warmer) temperatures than
they do when they are dry. In other words, they are at a higher risk
of freezing when they are wet.
following a winter storm in this region of Mexico, the skies generally
clear and the temperatures drop. Without a cloud cover, heat escapes
into the night sky and the ground and forest cool rapidly. (This is
called "radiational cooling.")
monarchs are protected by the forest canopy and also by one another.
- The thinning
of the forest is of concern to scientists because this changes the delicate
microclimate to which the butterflies have adapted.
J.B. & Brower, L.P (1996) Freeze-protection of overwintering monarch
butterflies in Mexico: critical role of the forest as a blanket and
an umbrella. Ecological Entomology, 21,, 107-116.
W.H. (pers communication)
- Education Standards