Size of the overwintering monarch populations - published data.
Stanley A. Gorodenski
stan_gorodenski at asualumni.org
Mon Oct 13 18:47:35 EDT 2003
Your comments have started me thinking again. Something that has not
been discussed, or if it has not much, is the population genetics aspect
of the Monarch. This _may_ be a very important consideration for
research than just following the trend of the size of the over wintering
In 1995 Lande and Lynch, et. al., published papers showing that in
theory the effective population size to prevent extinction of a local
population may be 10 time or more higher than what had been previously
theorized. The Franklin-Soule number of 500 that has been widely used as
a general guide for conservation purposes may be much too low.
There are two aspects of the Monarch life history that is related to
this effective population size consideration: the size of the Monarch
population at the end of the year before the migration takes place, and
the size of the returning population that reproduces. With respect to
the latter, just for discussion purposes, assume 20 million Monarchs
arrive to the Mexico over wintering location each year. This is
approximately equal to the 2000-2001 “Millions of Butterflies” in Paul’s
graph he recently provided. There are a number of unknowns regarding the
effective population size 20 million represents. We do not know how many
of these butterflies survive their return to the northern latitudes in
the spring, and how many of these actually reproduce. Other unknowns
that affect effective population size are things such as unequal sex
ratio and the variance of reproductive success. Just for the sake of
discussion, knowing that we do not know all these things, assume that 20
million represents an effective population size of one million (1/20th).
We also do not know what the effective population size for the Monarch
is to prevent extinction, but assume it is 5,000 (i.e., 10 times the
Franklin-Soule number). Assume also that 400 local populations exist in
the northern latitudes, defined either by geographic isolation or
neighborhood size considerations. One million divided by 400 gives an
effective population size of 2,500 which is considerably less than
5,000. What this means is that over time the entire Monarch population
(that migrates to Mexico) will go to extinction. The process will not be
immediate because it takes generations for the accumulation of
detrimental mutations to take place, and this is the insidious part. The
rapid recovery in a following year due to a freeze out (like the two
that recently occurred) may lead one to erroneously conclude that
nothing is wrong.
All the above assumed one thing. This is that each of the returning
Monarchs return to the same locality (there are 400 that I assumed) from
which they came. On the other hand, if the over wintering Mexico
Monarchs represent one huge panmictic (I am using this term loosely. I
know the over wintering population while in Mexico is not in a
reproductive phase) population that randomly spreads out over the 400
localities upon returning, then all the above is not in the least an
important consideration. What do we know about the return distribution
of Monarchs? Are there any tag counts of returning butterflies to
determine if they return to the same area, or generally the same area? I
recall reading on this list of an indication from some research that
some geographic differentiation was taking place. Not knowing more about
this research, it could indicate Monarchs are returning pretty close to
the areas from which they emigrated. Thus, determining the minimum over
wintering population size to prevent extinction may be just as or more
important than determining whether a trend exists in the data.
John Shuey wrote:
>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?
>Director of Conservation Science
>Indiana Office of The Nature Conservancy
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