Recent Boston Globe "monarchs are threatened" article

[Patrick Foley]

Stan,

It is possible that the migration behavior is a ghost of selection past. 
If this is so, then without further environmental change, the Monarch's 
behavior is likely to change over time, and there is little to be done, 
and perhaps no good reason to do it. SO far I am agreeing with you.

But the highly polymorphic nature of the migratory trait (compare CA 
with Eastern US with Central America) argues against the theory that 
migration is a genetical fixe for the butterfly. It appears to be under 
selection, though the details are woefully less worked out than we would 
like.

Patrick

Stanley A. Gorodenski wrote:

> To play the devil's advocate, the implicit assumption is that there is 
> some selective advantage for the migratory behaviour. Although there 
> may be some selective advantage for it, the primary factors 
> maintaining it could be pleiotropy, and/or homeostasis, and/or 
> epistasis. It may be that what we revere as a unique and wondrous 
> phenomenon (a gift) may be an evolutionary "relic" that evolved due to 
> biogeographic and climatic changes and which has little adaptive value 
> now but only exists because, until now, selection against the 
> migratory behaviour has not been present. If this is the case, is 
> there any advantage (other than to maintain a Natural museum, so to 
> speak, of evolutionary outcomes) in preserving a trait (the migratory 
> behaviour) that has little or no selective advantage?
> Stan
>
>
> Patrick Foley wrote:
>
>> Paul,
>>
>> I believe you that there are lots of milkweeds in the Midwest.
>>
>> But predator-prey, pathogen-host and similar dynamics are often 
>> strongly affected by the density of the host/prey/plant. Epidemics, 
>> for example, can only get startred when a certain critical number of 
>> susceptibles is available. Predator-prey dynamics are famously unstable.
>>
>> How do you think the milkweed population sizes compare today to ten 
>> years ago? to 100 years ago? To 1000 years ago? Presumably the 
>> Monarch migratory behavior is a Pleistocene-Holocene phenomenon 
>> driven by great seasonal and perhaps climatological changes. If 
>> milkweeds today are 1/10 that of 1000 years ago, the migratory system 
>> may break down.
>>
>> Central American Danauus plexippus are apparently not migratory 
>> (DeVries 1987, Butterflies of Costa Rica), and it is worth recalling 
>> that the Milkweed and related families and their herbivores of the 
>> subfamily Danainae (150 spp) are mainly tropical (Ackery & 
>> Vane-Wright 1984, Milkweed Butterflies). Our Monarch is a rare gift, 
>> a tropical butterfly that has found a peculiar (unique in the view of 
>> some lepidopterists) life history that allows it exploit the 
>> seasonal, transient habitats of extratropical milkweeds. We are lucky 
>> to have the creature, and it may take luck, better knowledge and even 
>> some attention to the conservation of wintering and feeding sites to 
>> keep it around. Or not. But I would not err on the side  of hysteria, 
>> nor stolid indiference. I would err on the side of finding out more 
>> and keeping the system around while we do so.
>>
>> Patrick
>> patfoley at csus.edu
>>
>>
>> Patrick
>>
>> Paul Cherubini wrote:
>>
>>> Pat Foley wrote:
>>>
>>>  
>>>
>>>> You make an interesting point that migrations reappear in farflung
>>>> populations of the Monarch. That is well worth studying. Do you 
>>>> think it
>>>> is due to built-in genetically controlled tendencies? Do you think 
>>>> it is
>>>> a matter of immediate good decisionmaking by the butterflies? Is it a
>>>> vestige or an easily selectively reinforced set of traits? All good
>>>> questions.   
>>>
>>>
>>>
>>> At temperate latitudes (approx. 33-57
>>> degrees N or S of the equator) the relatively rapid day to day decline
>>> in daylength after about July 1  somehow triggers adult reproductive 
>>> diapause in monarchs and the associated "instinct" to "seek" a fall 
>>> / winter diapause site where the climate is consistently cool and 
>>> humid enough to maintain the diapause. So when monarchs became 
>>> inadvertently transported by man to farflung places like southern 
>>> Australia, the seasonal migration / overwintering phenomenon becomes 
>>> quickly established.
>>>
>>>  
>>>
>>>> But if the eastern NA Monarchs do not achieve sufficient
>>>> Darwinian fitness by their migration, the migration will sputter 
>>>> out. As
>>>> it may have in untold other species. And it will sputter out either
>>>> because the wintering sites or the feeding sites become compromised.
>>>>   
>>>
>>>
>>>
>>> Well the monarch migration phenomenon has a long established
>>> record of persisting even in cases of extremely small migratory 
>>> populations. For example, the monarch overwintering colonies in 
>>> Sydney, Australia contain only hundreds of monarchs rather than tens 
>>> of thousands as on the California coast or
>>> tens of millions as in central Mexico. And the monarch overwintering 
>>> colonies in Adelaide, Australia contain merely dozens of monarchs.
>>> Also, tagging has established that western and Midwestern
>>> USA fall migrant monarchs suffer relatively little mortality during 
>>> their long distance migrations.  Example: Approximately 1 in every 
>>> 50 fall migrants tagged in the Midwest USA are recaptured at the
>>> overwintering sites in Mexico regardless of whether they were tagged 
>>> way up north in Minnesota or way south in central Texas.  And in
>>> a 1990 study, 1 in  every 10 monarchs tagged in the state of 
>>> Washington was recaptured along the California coast vs. 1 in every 
>>> 11 tagged in Nevada or northeastern California.
>>>
>>>  
>>>
>>>> If the wintering site for the Eastern NA monarchs has no special
>>>> features, could you explain why the Monarchs don't overwinter in 
>>>> all the
>>>> closer (to the NA milkweeds) habitats on their routes?   
>>>
>>>
>>>
>>> The fall migrant monarchs thoughout the northern & central USA 
>>> migrate mostly south and southwest in September / October until they 
>>> reach geographical locations where the climate is consistently cool 
>>> and humid.   So monarchs arriving at the California coast in 
>>> September / October stop migrating while those arriving along the 
>>> south Atlantic coast, Gulf Coast and in Texas keep on migrating 
>>> because the climate is still so warm.  Considerable numbers of south 
>>> Atlantic coast monarchs enter Florida and don't live too long 
>>> because it's warm everywhere in Florida which breaks the 
>>> reproductive diapause and causes rapid aging.  A portion of the fall 
>>> migrants arriving along the Gulf Coast flies westward to Texas and 
>>> then southward into Mexico. Another portion simply heads out over 
>>> the waters
>>> of the Gulf of Mexico and drowns in the ocean at nightfall (except 
>>> for some
>>> that find temporary refuge on boats and oil drilling platforms).
>>>
>>> In other words, although a high percentage of western and Midwestern 
>>> monarchs survives the journey to the overwintering sites in 
>>> California and Mexico, only a small percentage of Atlantic coast 
>>> monarchs does so because fall migrant monarchs have limited 
>>> navigational capabilities.
>>>
>>>  
>>>
>>>> Paul, how are milkweed populations doing in the Midwestern US?   
>>>
>>>
>>>
>>> Milkweeds are virtually everywhere in the Midwest and almost every
>>> city and county of every midwestern State becomes practically 
>>> inundated with monarchs for a week or two every year during either 
>>> September or October depending on latitude.
>>>
>>> Paul Cherubini
>>>
>>>
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>>>  
>>>
>>
>
>



 
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