Size of the overwintering monarch populations - published data.

Chip Taylor chip at
Fri Oct 10 18:27:20 EDT 2003

Regarding data on the sizes of the overwintering monarch populations.

Data on the size of the monarch overwintering populations from 1993 
to the present can be found at These data 
were published in the 2001 Monarch Watch Season Summary. Measurement 
of some of the monarch colonies began in the late 70's but there was 
no attempt to travel to and measure all of the colonies until the 
winter of 1995/96. Some of these data were published previously in 
the Proceedings of the Morelia conference on monarchs in 1997. If 
anyone is interested in the text that accompanied these data, please 
respond to me personally.

The data are presented in hectares. The prior notion that there were 
10-12 million butterflies per hectare was shown to be false by the 
studies conducted by Brower et all following the mortality due to the 
winter storm in January 2002. The new estimates are much higher (> 50 
million) but there is uncertainty as to which value should be used to 
represent the mean number per hectare or whether this is even useful. 
We are exploring the possibility of using another approach to 
defining the size of the overwintering population that will overcome 
the variances from site to site in the density per hectare.

With regard to illegal logging at the overwintering sites, it 
continues on a substantial scale. The precise locations and extent of 
the deforestation is being documented in several ways and the 
evidence bearing on this will be presented in the future. Cerro 
Pelon, a protected area within the Federal Zone with up to three 
relatively small monarch colonies and probably the third most 
important area for overwintering monarchs, has been logged 
extensively over the last 3-4 years. Monarchs continue to use the 
overwintering sites on this mountain but may not be able to in the 
future if logging continues. Two sites, Altamirano and San Andres, 
were not protected by federal decrees and are now both badly 
degraded, the second partly by fire, with the result that monarchs 
seldom use the former in recent years and appear to have unstable 
colonies with poor survival in the latter.

Monarch Watch
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