Monarchs and temperature

Kondla, Norbert FOR:EX Norbert.Kondla at
Thu Oct 12 14:37:28 EDT 2000

Very interesting.  Apologies for likely mangling the Latin from my stone-age
school days but this is a cause/effect discussion that warrants
consideration of the old logic pitfall of "post hoc, ergo propter hoc" which
means "after this, therefore because of this".  The effect is documented;
the cause is open to interpretation and debate.  In the absence of any
personal information that would cause me to propose a different cause; I can
only observe that both interesting observations by Chip and Paul are
plausible explanations (=causes) for the observed effect.

-----Original Message-----
From: Paul Cherubini [mailto:cherubini at]
Sent: Wednesday, October 11, 2000 8:47 PM
To: leps-l at
Subject: Re: Monarchs and temperature

Chip Taylor wrote:

> 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 [selective logging] 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 [monarch] population declined significantly after
> the hot and extremely dry El Nino winter of 1997-98.

Yes, it's true that monarch sightings reported to Journey North 
were down by almost 50% in the spring of 1998 vs the spring
of 1997: 

                             Sightings Reported 
                       Spring 1998     Spring 1997  

March                       48                    76       
April                         50                  109


Dr. Taylor suggested this decline was related to the hot dry 
El Nino winter of 1997-98 and possibly exacerbated 
by forest thinning which he claims "results in a drying up of
the area and it is probable that the dew point is reached less

However, a key point that was not mentioned is that the size 
of the monarch overwintering population itself during
the winter of 1997-98 was substantially lower than in 
1996-97, hence one would expect the spring migration of
1998 would turn out to be substantially smaller than that 
observed in the spring of 1997:

Size of Five Overwintering colonies (hectares)

                                   Winter         Winter

Colony                      1997-98      1996-97  Fir Tree Density

Chincua                         0.73            7.09    251 trees/acre
El Rosario                     2.12            7.61    102 trees/acre
3 other colonies             0.33            3.17 

Totals:                            3.18         17.87


Data copied from:

It is also ironic that during the hot and extremely dry winter
of 1997-98, a far greater proportion of the butterflies "decided"
to overwinter at the El Rosario colony than the Chincua colony,
even though the tree density at El Rosario is less than half as
dense as at Chincua. 

Paul Cherubini

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