All time low numbers of monarchs overwintering in Mexico

New report by WWF indicates the number of overwintering monarchs is at an all time low

Dr. Lincoln Brower's comments on the WWF overwintering colony monitoring report for the 2009-2010 overwintering season, 25 January 2010:

On 16 January 2010, World Wildlife Fund-Mexico posted the annual December census of the total colony area occupied by overwintering monarch butterflies in Mexico for the 2009-2010 season (see figure below, and WWF report, in Spanish, here). The total area occupied by butterflies this season is 1.92 ha (4.74 acres), the lowest on record. Systematic monitoring over the past 16 overwintering seasons recorded a maximum of 21.0 ha hectares in 1996-1997, and prior to this year, a minimum of 2.19 ha, with an overall average of 7.44 ha per year (see figure below). This year is particularly distressing, because of 16 areas known to have overwintering sites in the past, nine have no monarch colonies, three have less than tenth of a hectare each, and only three have colonies occupying more that 0.50 hectare. Reasons for the current historically all time low are very likely a combination of reduced survival during last year's winter (2008-2009) and limited breeding success in the US and Canada over the spring and summer of 2009.

The likely reduced winter survival during December 2008 to March 2009 is almost certainly a consequence of illegal logging over the past decade that has either destroyed overwintering forests or degraded them so that they can no longer provide adequate microclimatic protection for the butterflies. Note in the graph that the last six years have been below the 7.44 average, corresponding with the increased intensity of illegal logging during this period. Logging encroachments range from petty tree theft to the clear cutting of hundreds of hectares. Encroachments have occurred on and adjacent to the La Mesa, Chincua, Pelon, Rosario, Contepec and other overwintering areas. For example, between 2004 and 2008, 450 hectares (1,110 acres) of the Oyamel forest that hosted the Lomas de Aparacio colonies were destroyed (reference here). This represents about 3.3% of the 13,552 hectare core zone of the entire Monarch Butterfly Biosphere Reserve. The reported fourth largest colony this year is on Chivati-Huacal and is only 0.29 ha.  I visited this site last year and the remnant colony had settled in a very degraded forest bordering on the edge of the previously clear cut and then burned Oyamel forest.

It is likely that the microclimate protection of all the colonies is becoming increasingly precarious. We can only hope that a major winter storm does not impact this year's small colonies between now and the end of March. If we have another killer storm such as occurred in January 2002 (Brower et al. 2004; see reference below), it could degrade the migratory phenomenon to a possibly unrecoverable level.

An article about the monarch overwintering colony locations in Mexico, including a map, in pdf format can be found here.

Brower, L.P., Kust, D.R., Rendon-Salinas, E., Serrano, E.G., Kust, K.R., Miller, J., Fernandez del Rey, C., & Pape, K. (2004). Catastrophic winter storm mortality of monarch butterflies in Mexico during January 2002. In The Monarch Butterfly. Biology and Conservation (eds. K.S. Oberhauser & M.J. Solensky), pp. 151-166. Cornell University Press, Ithaca.