Monarch Population Status

Each December Eduardo Rendón from the World Wildlife Fund Mexico and a team of biologists search for and measure each monarch overwintering colony – those within the Monarch Butterfly Biosphere Reserve (MBBR) and those outside this area as well. Eduardo has kindly forwarded the report based on their findings and the commentary that follows is based on this summary. The report (in Spanish) can be found on the WWF Mexico site.

E. Rendón Salinas, C. A. Valera-Bermejo, S. Rodríguez-Mejía, Y F. Martínez-Meza. Monitoreo de las colonias de hibernación de la mariposa Monarca: superficie forestal de ocupación en diciembre de 2009. WWF y Reserva de la Biosfera Mariposa Monarca. 8 pp.

The news is not good.

Only 7 colonies were found and the total area occupied by monarchs at the overwintering sites in December was 1.92 hectares (ha). The three largest colonies (El Capulin (Cerro Pelon) at 0.53ha, El Rosario at 0.50ha, and Cerro Prieto (Chincua) at 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. The 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 2009 migration, is now below 5 hectares per year; the three lowest monarch overwintering populations were reported in this decade.

In the Monarch Population Status posting to the Monarch Watch Blog on the 20th of October 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 my 18 years of experience in following the fall migration closely. 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 for the winter of 2009-2010.

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 conditions 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 this winter, our experience with the disaster of 2002, in which an estimated 80% of the population died as the result of a January storm, suggests that the monarchs can recover. The number of monarchs surviving is critical. IF at least 1 hectare of monarchs survives to move north and IF they encounter normal conditions as they move north through Mexico and Texas, the population can rebound in one breeding season.