Size of the overwintering monarch populations - published dat a.

Mark Walker MWalker at
Mon Oct 13 23:35:46 EDT 2003

John Shuey wrote:

> 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
> years.
> 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).

I don't agree with this at all.  By your own admission we're talking about
indicators for extinction here, not the health of a particular overwintering
site.  Why would %mortality at one location be a better extinction warning
indicator than the trending of overall health and population size of the
overall returning migration?  While it is agreed that a high mortality at a
site where so many Monarchs are concentrated would indeed be an issue of
concern, the proof of whether or not the insect can sustain such a blow
would be determined by the overall effects on the following year's
population.  Even Brower and Taylor would agree with this, as they've cited
this expected result as part of their annual predictions.

In my view, until someone demonstrates a strong correlation between high
mortality and a resulting depleted Monarch population, we cannot presume any
cause and effect.  In the meantime, I will continue to support Monarch
overwintering site habitat protection and monitoring - because it is
prudent.  But I am yet to be convinced that freezing butterflies in Mexico
necessarily equate to the extinction of this obviously robust butterfly.

Mark Walker.


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