Size of the overwintering monarch populations - published data.

John Shuey jshuey at TNC.ORG
Mon Oct 13 11:24:02 EDT 2003

I have to admit that I haven't been watching this thread very closely (to
much personal crap for my taste) - but it seems like there is a fundamental
error of logic permeating the discussion in this thread.

My take on the question is this:  Is the monarch migration phenomenon likely
to go extinct because of disruption/disturbance of forest in Mexico?

The evidence everyone keeps looking at is the size occupied by the
"returning" monarch each fall.

To me this is like studying oranges to talk about apples.

The size of the returning fall migration is the end product of summer
weather, habitat, and luck in the US and Canada.  Environmental trends in
the states and provinces control the number of monarchs that return to
Mexico.  Remember that even monarchs are insects (yes - even monarchs) - and
fluctuations of 1-2 (or more) orders of magnitude in population size during
the breeding season SHOULD BE EXPECTED.  So the difference between
overwintering populations covering 12 hectares versus 2 hectares has more to
do with summer in northern North America than winter in the Mexican
highlands.  Granted, the number of monarchs leaving the Mexican highlands
each spring influences the number returning each fall, but variable
reproductive success in the north would mask that initial influence in most

The critical measure of phenomenon extinction seems likely to be the percent
survival of monarchs during the overwintering phase.  So that if the
%-survival is decreasing over time (mortality is positively correlated with
forest loss/thinning) - then you have would have a disturbing trend.  (and
this is what Brower et al have been talking about in most of the
literature - not the absolute size of the population that returns to Mexico,
but how much of those butterflies survive to head back north).

My take on the concern is that if %-mortality becomes very high, then in one
of the down reproductive years, essentially the entire overwintering
population could be wiped out, erasing the phenomenon (but not the species).

Some quantitative measure of percent mortality over many years would be
required to get at this basic question - is anyone working on that angle?


John Shuey
Director of Conservation Science
Indiana Office of The Nature Conservancy


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