Size of the overwintering monarch populations - published dat a.

Woody Woods woody.woods at
Tue Oct 14 09:43:57 EDT 2003

John's and Mark's exchange below-- I left them in because I think they are
worth rereading side by side-- points out how simple this isn't!

A thorough explanation of a population's decline or increase over time isn't
an easy matter to manage-- think of the things that need to be done:

First, it must be established whether the population is increasing or
declining, and for insects that alone can be challenging,for reasons John
points out-- only two eggs of the hundreds laid by a female adult need
reproduce for the population to remain stable, but in some generations far
more than that will. On top of that, some insect populations show regular
extreme population cycles, making shorter-term studies misleading.
Neotropical species I have studied have shown considerable changes in
relative abundance from one year to the next.

Second, assuming a population is convincingly shown to be declining, the
particular point of the organism's life-history where mortality is on the
rise must be identified. For the eastern migrating Monarch population,
multivoltine as it is and using a succession of habitats from one generation
to the next, that's not easy! We know as much as we do about the Mexican
overwintering sites because they are small-- the habitat of the other
generations of that population are a bit larger!

Third, assuming it is possible to pin population decline on a particular
part of a life history, it is then necessary to find out what factor,
natural or otherwise, has changed in a way that explains the effect. This
means identifying the factor or factors causing INDIVIDUALS to fail to
reproduce, and it can be a long process with sometimes surprising results--
Andrew Blaustein's amphibian work is a good example.

But-- in the meantime keeping an eye on human impacts, which move at a
blinding pace compared to non-human impacts, simply makes sense. As Mark
says, supporting monitoring and protection at the overwintering site is just
prudent-- while we seek to learn more. As to really learning the entire
story-- it's a moving target, after all, and there will always be gaps in
our knowledge...


William A. Woods Jr.
Department of Biology
University of Massachusetts Boston
100 Morrissey Blvd
Boston, MA 02125

Lab: 617-287-6642
Fax: 617-287-6650

> From: Mark Walker <MWalker at>
> Reply-To: MWalker at
> Date: Mon, 13 Oct 2003 23:35:46 -0400
> To: "'jshuey at'" <jshuey at>, leps-l at
> Subject: RE: Size of the overwintering monarch populations - published dat a.
> 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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