Size of the overwintering monarch populations - published data.

patrick foley patfoley at
Mon Oct 13 18:41:54 EDT 2003


Franklin's magic number of 500 is bogus. His paper  (championed by 
Soule, was simply wrong. Soule knows it (I told him so), and Franklin 
never published again on the subject. See my 1992 Evolution paper for 
discussion. Lande (who is usually very good) knows the magic number 500 
is bogus, but unaccountably feeds the flames with his paper that you 
mention. I wrote a rebuttal to Conservation Biology, but they (having 
just cited the number favorably 3 times in one issue including the Lande 
paper), felt that my point was already well understood and did not need 

Lynch and Lande have an interesting potential phenomenon in mutational 
meltdown. It is essentially a form of Muller's ratchet, The mechanism 
which may explain the maintenance of sexual outcrossing.

There is no clear cut minimum effective population size that leads to 
inability to evolve or to mutational meltdown. And Monarchs are a very 
unlikely candidate anyhow due to the large overall population size and 
wide outbreeding.

The population genetics question that seems most pointed to me is, when 
do the less migratory genotypes begin to beat out the less migratory 
types? When that happens, the migration starts to decline.

patfoley at

Stanley A. Gorodenski wrote:

> 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 
> Mexican population.
> 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.
> Stan
> 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
>> 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).
>> 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
>> _________________
>> John Shuey
>> Director of Conservation Science
>> Indiana Office of The Nature Conservancy
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