Size of the Monarch Colony in Mexico 2010

[Mona Miller]

This was posted to dplex list serve by Chip Taylor:

"Last evening I received the WWFMexico report on the number and sizes of the monarch overwintering colonies in Mexcio. I wish to thank Eduardo Rendon for providing this information. The report (in Spanish) can be found at: http://www.wwf.org.mx/wwfmex/publicacio ... =reps&p=bm.

The news is not good. The total area occupied by monarchs at the overwintering sites in December was 1.92 hectares. Only 7 colonies were found. The three largest colonies El Capulin (Cerro Pelon) 0.53ha, El Rosario 0.50ha, and Cerro Prieto (Chincua) 0.47ha constitute 78% of the total area. The totals for both hectares and numbers of colonies are at an all time low. Good records of the numbers of colonies and area occupied go back to 1992 and there is less complete data for most years going back to the late 1970s and numbers this year appear to be lower than observed for any year since the overwintering colonies became known to science in 1975. The lowest previous total, 2.19 hectares, was reported in 2004. This decline continues a trend that started in the late 1990s. In the decade of the 90s the mean area occupied by monarch colonies was close to 9 hectares. The mean for the last 10 years, through the 09 migration, is now below 5 hectares per year and the three lowest monarch overwintering populations were reported in this decade.

In a posting to the Monarch Watch Blog on the 20th of October (http://monarchwatch.org/blog/2009/10/mo ... -status-5/) I summarized the conditions monarchs confronted during the breeding season and predicted that: "the total hectares will be in the range of 2 to 3.5 when all colonies are measured in December." More recently, based on the relative success of taggers this fall, I was anticipating that the total hectares would be closer to 3 hectares. Unfortunately, the final number is below my most pessimistic expectation.

As many of you know my predictions are based on a combination of interpretations of the effects of temperature on monarch populations, observations reported to us throughout the breeding season and early in the migration and experience. Of these, temperatures are the best predictor.

Without going into great detail and wishing not to repeat the October report, here is a brief summary of the reasons for the low overwintering numbers.

1.) High temperatures in Texas in March and early April limited production of first generation monarchs. It is these monarchs that recolonize the northern breeding range and fewer monarchs moving north/northeast out of Texas from late April to June impacts the rest of the breeding season.
2.) Conditions were less than ideal for the first generation monarchs as they moved north in May and early June.
3.) Upon arrival in Minnesota monarchs encountered drought conditions that limited reproductive success of first generation in that area.
4.) As the summer progressed, cool and cool, rainy conditions prevailed in many areas, limiting reproduction and slowing development of larvae.
5.) Colder than normal condition prevailed for most of the western two thirds of the northern breeding area from mid June into early September.

In many respects the conditions during the monarch breeding season in 2009 were a repeat of the conditions seen in 2004 that contributed to the previous low overwintering population number of 2.19 hectares.

In spite of the recent cold snap that reached into Mexico, there have been no indications of weather related mortality at the overwintering sites. Let's hope that normal winter conditions prevail during the next 7-8 weeks. Even if there should be some mortality, our experience with the disaster of 2002, in which an estimated 80% of the population died as the result of a January storm, showed that, if at least 1 hectare of monarchs survives to move north and, IF they encounter normal conditions as they move north through Mexico and in Texas, the population can recover.

I have a pdf of the WWFMx report and will be glad to send it to those who are interested."

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