Recent Boston Globe "monarchs are threatened" article

[Patrick Foley]

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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