Size of the overwintering monarch populations - published dat a.
agrkovich at tmpeng.com
Tue Oct 14 11:08:36 EDT 2003
For the record, leaving in a previous post or two is not a bad idea...it
makes, as Woody pointed out, it easier to follow the thread...It's doing
what I have a habit of doing...that's leaving in a string of posts... that
should be discouraged...
> -----Original Message-----
> From: Woody Woods [SMTP:woody.woods at umb.edu]
> Sent: Tuesday, October 14, 2003 9:44 AM
> To: MWalker at gensym.com; 'jshuey at tnc.org'; leps-l at lists.yale.edu
> Subject: Re: Size of the overwintering monarch populations -
> published dat a.
> 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
> 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
> 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
> 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
> 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 gensym.com>
> > Reply-To: MWalker at gensym.com
> > Date: Mon, 13 Oct 2003 23:35:46 -0400
> > To: "'jshuey at tnc.org'" <jshuey at tnc.org>, leps-l at lists.yale.edu
> > 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
> >> 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
> >> to
> >> do with summer in northern North America than winter in the Mexican
> >> highlands. Granted, the number of monarchs leaving the Mexican
> >> 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.
> >> 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
> > indicators for extinction here, not the health of a particular
> > site. Why would %mortality at one location be a better extinction
> > 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
> > 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
> > 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
> > necessarily equate to the extinction of this obviously robust butterfly.
> > Mark Walker.
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