Monarchs and temperature
Paul Cherubini
cherubini at mindspring.com
Wed Oct 11 23:47:00 EDT 2000
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
often"
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:
http://www.learner.org/jnorth/tm/monarch/PopulationMexico.html
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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