The size of the overwintering monarch population in Mexico is usually released in late January or February. Early reporting helps all of us plan for the season ahead. Unfortunately, for reasons that are not clear, this report wasn’t released to the press until the 15th of March – at the end of the season and a week after the first monarchs from Mexico had made their appearance in Texas. The total for all colonies, as reported to the press, is 2.89 hectares. There is good news and bad news in this number. The good news is that the population was larger, by almost a hectare, than I expected. The bad news is that this number represents the 4th lowest total for the monarch colonies recorded since the winter of 1994-1995. Further, this population represents a continuation of a trend – the 8th consecutive population below the long term average.
The figure below represents the total number of hectares of monarchs at the overwintering sites in Mexico from 1994 through the winter of 2011-2012*. Two averages are indicated – the long-term average that incorporates measures for all years (X=6.99) and an average for 2004 to present (x=4.16).
It is apparent that there has been a significant decline in the overwintering monarch population since 2003. This decline is related to the adoption of herbicide tolerant row crops that were first introduced in 1996. As pointed out in previous communications, the milkweeds in these row crops were shown in 2000 to produce more monarchs per unit area than other monarch habitats. By 2004 the adoption of corn and soybeans genetically modified to resist glyphosate exceeded 51%. The adoption rate by 2010 was 81% (see figure**). Milkweeds are now scarce in this formerly productive habitat. Overall, the amount of habitat lost due to the adoption of these crops may exceed 100 million acres.
These losses combined with the annual loss of habitat of 2 million acres, the overuse of herbicides and mowing, the reduction of conservation reserve program (CRP) acreage and the increasing conversion of pastures and grasslands to corn and soybeans for the production of biofuels point to a continuing decline in monarch numbers in the years ahead.
As I say in the header of our website: “To assure a future for monarchs, conservation and restoration of milkweeds needs to become a national priority.”
The question I’m wrestling with is: how can we make this happen? How can the monarch community engage and inform the public and the decision makers of the necessity of restoring milkweeds to save the monarch migration. Our Monarch Waystation and the Bring Back the Monarchs programs, or their facsimile, need to be broadly adopted. We need a national campaign to restore habitats for monarchs.
* The areas of the monarch overwintering colonies are measured several times during the winter. Because the sizes of the colonies increase as the season progresses from mid November to mid December and vary with temperatures, the areas obtained in mid December, a period with relatively low temperatures and little flight, are used to represent the population. Over the period from 1994 to the present, the assessment of colony sizes has been conducted by two organizations, the Monarch Butterfly Biosphere Reserve (MBBR) and World Wildlife Fund Mexico (WWFM). The MBBR teams were led by Eligio Garcia Serrano. Since 2004, Eduardo Rendon has led the monitoring teams supported by WWFM. The monitoring by these teams over the years is much appreciated. Without their hard work our understanding of the factors that determine monarch numbers from year to year would be poor indeed.
** Thanks to Janis Lentz for creating this figure.Filed under Mexico, Monarch Conservation, Monarch Migration, Monarch Population Status | 8 Comments »