Monarchs and temperature

[Chip Taylor]

Woody: Thank you for your thoughtful contribution to the discussion 
of climate change on monarchs. I have been meaning to look into the 
long term climate projections for central Mexico but haven't done so. 
Your comments are on target and very relevant even now.  Consequences 
of forest degradation at the overwintering sites in Mexico which have 
not received attention are increases in mean as well as maximum and 
minimum temperatures. Slight changes could have a subtle, yet 
significant, impact on mortality of the overwintering monarchs for 
the very reasons you mention. None of the current monitoring appears 
to be sensitive enough to pick up on this possibility.

In addition to lipid reserves, the butterflies need water, even if it 
is only the condensation which forms on an adjacent butterfly. 
Degradation of the forest results a drying up of the area and it is 
probable that the dew point is reached less often. Each day large 
numbers of butterflies "die without cause". Most of these have ample 
lipid reserves leading to the hypothesis that some of these deaths 
occur when the butterflies lack sufficient water to metabolize the 
lipids. In this connection, it is interesting to note that the 
population declined significantly after the hot and extremely dry El 
Nino winter of 1997. In late winter of 97 , possibly due to the lack 
of water in the usual areas on the mountainside, the monarchs moved 
down the mountains into areas where they had not been seen in recent 
memory. The population may have also declined in part because of the 
extreme drought in northern Mexico and Texas at the time the 
butterflies were moving north from late Feb to April. All of this is 
not simple. The average lipid masses of the arriving butterflies 
probably varies from year to year due to the availability of nectar 
during the fall, especially over the last 600 miles of the journey. 
Monarchs are one of the few organisms that gain in mass during the 
migration and this gain is likely to be critical to wintering success.

Chip

>In the exchange between Chip Taylor and Paul Cherubini, the possible impacts
>of global warming on Monarch populations have been brought up mainly in
>connection with the effects on host plant range. It might also be worth asking
>how a long-term warming trend might affect the Monarch overwintering strategy
>in Mexico. Ron Gatrelle's note about chilling butterflies for transportation
>goes right to the core of that strategy-- the Monarchs essentially put
>themselves "into the refrigerator" to conserve lipid reserves, which have to
>last about three months. The suite of observed behaviors at the overwintering
>sites, as well as the conditions offered by the sites themselves,are all part
>of balancing a fragile energy budget, and success depends upon staying cold,
>but not TOO cold (see Masters, Malcolm and Brower, 1988, Ecology 69:458). The
>energetic margin of error is small enough that many run out of lipid reserves
>anyway-- those that began their migration a little too early or late (Gibo and
>McCurdy 1995, I think-- haven't got it handy) arrive with less fuel in the
>tank. The point is that this is a complex and energetically delicate game
>plan, and that gradual increases in overwintering site temperatures night
>increase the average "idling speed" and therefore fuel consumption of the
>butterflies. Maybe they could compensate behaviorally, maybe not. I think it's
>worth asking, though, whether a long-term warming trend is a greater threat
>down the road than the occasional exceptional winters Brower has reported.
>
>Woody Woods
>--
>*********************************************************
>William A. Woods Jr.
>Department of Biology
>University of Massachusetts Boston
>100 Morrissey Blvd                      Lab: 617-287-6642
>Boston, MA 02125                        Fax: 617-287-6650
>*********************************************************

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