This winter In Mexico, two hundred million monarchs reversed recent population declines and occupied three and a half times the area of last year. All overwintering sites comprised 10 acres (about 4 hectares) this year, compared to 2.8 acres (1.13 hectares) in 2014. This encouraging population data from Mexico’s overwintering butterflies was reported Friday as the butterflies began their 3,100 mile spring migration north.
The number of monarchs in Mexico is extrapolated from the combined area of overwintering sites, assuming 50 million monarchs per hectare (2.471 acres). The overwintering data is the best estimate of how many monarchs live in the eastern US and southeastern Canada.
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The butterflies spent November 2015 until the end of February 2016 at high elevation mountains (9-10,000 feet) in Mexico. The dry season, which begins in October, is essential to the monarchs winter survival and helps prevent deaths from temperature extremes, high winds and heavy rain. The forest trees provide shelter and strong branches that support huge clusters of monarchs and the high elevation mountains trap moisture (fog) that prevents the butterflies from dessication.
As the dry season progresses, the monarchs move out of the clusters and seek water at springs and streams within the forest. They mate and load up on nectar. Warm dry temperatures and longer day length trigger spring migration. They need to leave before the rainy season begins and makes flight impossible. The butterflies’ northward route parallels the Sierra Madre mountain crest and they funnel into Texas, often the first point where female butterflies search for their milkweed host plants to lay eggs before they die.
Overwintering butterflies survive on stored body lipids (fats acquired during summer) by reducing their metabolic rate and don’t imbibe nectar until spring migration time. A colony of 50 million butterflies (one hectare of occupied overwintering habitat) contains as much fat as 1,500 pounds of butter. The distance a female butterfly can migrate in spring depends on the lipid levels in her body. Varied levels among individual butterflies help scatter the migrating females to different locales (those with more lipids can fly further). This dispersion ensures some will find milkweed host plants for eggs and larvae that are the next generation continuing the journey to their summer ranges. It takes an average of two new generations (not counting the first migrating females) to reach the far northern summer ranges. Two more generations usually develop in the summer ranges with the last of these being the long-lived super generation that migrates south beginning August 15th and starts the spring migration back north.
Logging in the Biosphere and adjacent forest is the major human threat to overwintering monarchs in Mexico. Inclement weather and climate change also pose threats but are more difficult to address. A study released in August 2015 found logging in the Biosphere increased 284 percent during the 2014-2015 period, compared to the prior year. Between December 2015 to January 2016, 25 acres of monarch habitat was found illegally logged in a critical area within the core zone. The logging activity, discovered by comparing satellite maps of the areas, showed a large clear-cut and several smaller logged areas near known overwintering roost sites.
The overwintering monarchs in Mexico are considered “Critically Vulnerable/Imperiled” because nearly the entire eastern population overwinters at 19 sites (significantly fewer sites in many years) in a narrow habitat belt. Together all these overwintering sites comprise a small area ranging from 18 acres (1996-97) to 0.67 acres (2013-2014). In the 1990’s a billion or more monarchs overwintered in Mexico. Fewer than five acres have been occupied annually since 2008-2009. A storm or other inclement weather here between late October to March has the potential to eliminate the population that migrates back to the eastern US in spring, thus the “Critically Vulnerable/Imperiled” rating. California’s overwintering population is rated as “Vulnerable/Imperiled,” a lesser level of threat than Mexico’s, because there are more sites scattered throughout a larger area, although development and other threats are high
All monarchs in North America are one species, Danaus plexippus, and genetically one population even though their lives don’t overlap. Monarchs from west of the Rockies migrate to California’s coastal belt to overwinter. The life history, threats and other details about western monarchs are much less understood than the eastern subpopulation. Population data for this year from the California overwintering monarchs, plus information on why butterflies choose these two regions (Mexico and California) for their winter vacations was reported in Sugar up, mate and fly away.
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