Deforestation is the leading cause to monarch mortality

Earlier this month, Lincoln Brower, Dan Slayback, Robert Simmon, and Isabel Ramirez published NASA photos showing deforestation within the Monarch Butterfly Biosphere Reserve in Mexico. At the time of publication, we (Monarch Watch) were in Mexico buying recovered tags as part of our program to assess the dynamics of the monarch migration. We visited four monarch colonies and witnessed first-hand the impact of deforestation. The rapid degradation of monarch habitats in both Mexico and the United States is now too compelling to ignore. We must act. In this communication, I’ll summarize my views on the effects of habitat loss in Mexico and the United States and will reflect on what needs to be done to conserve the monarch migration. Impact of Deforestation on Monarchs Deforestation is already contributing to the mortality of overwintering monarchs. In the winter of 2006-2007 a small monarch colony formed at a site known locally as Aseradero on the northwest aspect of Cerro Pelon, one of the most deforested areas in the region. The trees at this location were relatively small (4-10 meters) and offered little protection from winds and cold weather. A large proportion of the monarchs died at this site when the temperature dropped below freezing in January 2007. No surviving monarchs were present when we visited this location in March 2007 (see “New Tree Nursery Near Cerro Pelon”). The degradation of the forest surrounding the Aseradero colony can be seen in satellite photos of the area (see monarchwatch.org/mx/aseradero.jpg). This year at Cerro Pelon, the El Capulin colony formed in a relatively degraded forest near the top of a ridge. Monarch colonies typically move during the winter and tend to progress down southwesterly-facing slopes. In this case, the monarchs moved progressively down-slope to the northwest across a badly degraded area and settled in the Las Canoas arroyo where we saw them on the 6th of March. There is one large tree in the arroyo and it was covered with monarchs (see monarchwatch.org/mx/capulin.jpg and monarchwatch.org/mx/capulin-tree.jpg). The surrounding trees were also covered with monarchs but there was little in the way of canopy to protect the butterflies from freezing temperatures. I was shocked to see how exposed this colony was at this time but now realize that it has been exposed through most of the winter, resulting in the death of millions of monarchs. Typically, 10-20% of the monarchs at each colony die of various causes during the winter, though rainfall followed by freezing temperatures can be devastating as it was in January of 2002 and January and February of 2004. Excessive mortality due to exposure throughout the winter is not the norm for monarch colonies. Evidence of greater mortality for the El Capulin colony this year comes from eyewitness accounts and is also reflected in the number of tag recoveries. In years without catastrophic mortality due to winter storms, we recover an average of twenty-two tags from the residents of communities on Cerro Pelon. The tags are usually found on dead butterflies beneath the clusters and the numbers recovered from most of the colonies appear to be proportional to the overwintering mortality. Ninety-two tags were recovered at Cerro Pelon this season – more than four times the typical rate of recovery. Deforestation The rate of deforestation in the Monarch Biosphere Reserve exceeds the rate of natural forest replacement and the current efforts at reforestation. It is simply not sustainable and must be reduced. Estimates of the rate of deforestation vary depending on the time period, the authorities involved, and perhaps the methodologies used to make the estimates. Nevertheless, it is clear that the average annual rate of deforestation (2-5%) is devastating the monarch overwintering habitat and destroying the forests that sustain the water supplies supporting the people in the region. Nearly all of the old growth forests with >200 year-old oyamel fir trees are gone from the monarch reserve and for decades monarchs have overwintered in areas with trees ranging from 40 to 80 years of age. In the last two years, possibly due to the lack of more mature forests, monarchs have attempted to overwinter at three sites at which most of the trees were less than 15 years old. Such forests do not protect the monarchs from periodic freezes and monarch mortality is known to have been high at two of these sites.