Mystery of the missing monarch butterflies
http://dervishcom.blogspot.com/2011/06/mystery-of-missing-monarch-butterflies.html
What is going on with monarch butterflies? Populations have been falling for 17 years at their wintering sites in Mexico, but a new study suggests they are holding steady in the eastern US.
Monarch butterflies (Danaus plexippus) famously migrate south to Mexico in the autumn, returning to the US and Canada in the spring. Earlier this year Ernest Williams of Hamilton College in Clinton, New York, and colleagues showed that winter populations in Mexico hit an all-time low in 2009-10.
But now Andy Davis of the University of Georgia in Athens has looked at records of migrating monarchs from two sites in New Jersey and Michigan, spanning 15 and 19 years respectively. During the migration, volunteers go out three times a day and record the number of butterflies they see. Davis found no evidence of a decline at either site.
Davis says many butterflies may die on the way to Mexico, explaining the discrepancy. Also, monarchs can rebound from bad winters by breeding rapidly the following spring.
Williams says there is a simple explanation for Davis's results: the US census sites are in the wrong places. Most monarchs come from central states like Nebraska, where their habitat and food are in decline.
New Jersey and Michigan are on the edge of their range, and habitats there are undamaged. A census from the core of the migration route would reveal a different picture, he says.
