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Contents:
1) Welcome!
4) "Genetic Memory" of the Overwintering Sites?
5) Monarch Population Crashes and Recoveries
6) How to Unsubscribe from this Update
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1) Welcome to Monarch Watch's Update List!
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This month's update is short and sweet as we are frantically trying to finalize our trip to Mexico. There are still lots of hoops to jump through but if all goes well, we should be able to give you a preliminary report of the Adopt-a-Classroom / Tag Recovery trip in the March update.
We would like to thank all of you who have made contributions to our Adopt-a-Classroom program to aid the schools in Mexico and to those of you who have contributed to the Tag Recovery Fund.
Wish us luck and stay tuned! ;-)
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3) Status of the Population - by Chip Taylor
Last month I emphasized that all dire predictions about the size of the overwintering population were wrong and that preliminary indications were that the population was neither as low as the all time low (2.83 hectares) nor as high as the population measured last winter (9.35 hectares). This year Marco Bernal, Bill Calvert, Isabel Ramirez, Jose Maria Suarez, and Lincoln Brower measured the colonies. Marco Bernal is the new director of the Monarch Reserve. The total area for all colonies was 7.54 hectares. This is lower than the average of 9.6 hectares for all sites from 1993-2001 and is the fourth lowest population observed over the last 10 years, other low populations being 97-98 (5.77), 98-99 (5.56) and 00-01 (2.83). (The total for 93-94 was 6.23 but adjusting for the colonies not measured that year gives a total of 7.75 hectares. The rational for this adjustment is given in the upcoming Season Summary).
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Table:
Location / Trees per Hectare / Basal Area / Hectares with Monarchs
San Andres / 331.44 / 3.08 / .1116
El Rosario / 63.53* / 4.72 / 4.3313
Chincua / 580.20 / 43.31 / 1.5908
La Mesa / -- / -- / .0921
Pelon - G / -- / -- / .2578
Pelon - D / -- / -- / .6177
Oxtotilpan / 302.69 / 19.80 / .0838
Palomas / 504.90 / 38.61 / .1116
Piedra Herrada / 226.77 / 14.49 / .1672
Mil Cumbres / -- / -- / .1747
Total hectares with monarchs: 7.54 ha**
*This figure appears to be in error. At the time of measurement, the colony at El Rosario was located in a relatively mature but thin stand of oyamel fir trees but the figure given here is too low to be representative of this forest.
** In a statement released to the press on February 12th by the Mexican Commission for Protected Areas, The World Wildlife Fund Mexico, and the Monarch Sanctuary Foundation, the total is given as 8 hectares. No explanation is provided for the increase but El Rosario has been difficult to measure this year due to the structure of the forest and the scattered nature of the colony. This would appear to make the final number for El Rosario to be 4.89 hectares. If 8 hectares is accepted as the final number, this years’ overwintering population is the 5th lowest in the last 10 years.
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Last year after the massive winter kill I tried to reconcile the differences in the apparent densities of monarchs at Chincua with those of El Rosario by comparing the tree densities at each location. However, when I converted the number of trees with monarchs at each site to the number of trees per hectare, I came up with about 350 trees per hectare for both sites. This was true of all other locations as well and I concluded that the numbers of trees covered with monarchs recorded for each site had been estimated based on the assumption that the average tree density was 350 per hectare. The forests differ substantially among the sites (see Table above) and if the sites are to be compared, tree density and mean size of the trees has to be established. This year an attempt has been made to count the trees and to calculate the mean basal area of the trees at the most important sites. The tree measurement used was the diameter at breast height (dbh), a standard measure of tree size used in forestry. The mean dbh was multiplied by the tree count per hectare to give an estimate of the cover provided by the forest. For example, a forest with a density of 500 trees per hectare and mean dbh of .4 meters per tree yields a basal area coverage of 20.0. Young forests have high tree densities and low basal areas and mature forests have fewer trees with higher basal coverage. A priori, we might speculate that mature forests would provide more cover and protection for monarchs and that the monarch densities at such sites might be higher. However, this relationship has yet to be established.
Winter is the dry season in central Mexico, but this has been a wetter winter than usual. The high moisture levels should be beneficial to the monarchs, as they should keep the dust down on the trails in the sanctuaries and should allow the farmers in the mountains to plant earlier than usual. It may even keep some of the loggers out of the forests. Rainfall is only a concern if soaking rains are followed by periods of freezing weather. This was the scenario that produced the massive winter kill in January of 2002.
Valentine’s Day was just last week, and (coincidentally I’m sure) this signals the time that mating activity increases in the colonies. Generally, it is the small, worn, males with little fat body that begin to mate first. These males tend to mate with some of the largest females, but, as I pointed out in the 2000 Season Summary, this seems to be due to the lower maneuverability of the large females rather than some manifestation of mate selection.
A portion of the overwintering population should begin to head north in the third week of February (weather permitting) with a few of them reaching Texas in the first week of March. The major portion of the overwintering population leaves the colonies in the first two weeks of March.
I wish to thank Bill Calvert for explaining the procedures used for measuring colonies this year.
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4) "Genetic Memory" of the Overwintering Sites? - by Chip Taylor
Do monarchs have a "genetic memory" of the overwintering sites in Mexico?
One of the ongoing debates among some honey bee biologists is whether honey bees have a cognitive memory (i.e. a spatial memory) of two or more sites that allows them to fly to a new location, to take short cuts, or, to take a new route to a familiar site. As humans, we are able to visualize images of sites we have visited and to recall the routes taken to reach these sites. In addition, we can estimate the spatial relationship of several sites to each other and to plot courses along routes we’ve never traveled to reach particular sites and even to reach sites we have never visited. We do this often as we move about our cities to shop. It isn’t possible for monarchs to have a cognitive memory of the overwintering sites, or the route to reach them, since they have never been there. Each of the monarchs making the fall journey is three to five generations removed from the population of monarchs that made the trip the previous fall. If there is no individual or collective memory, how do monarchs arrive at the specific overwintering area - indeed frequently the same sites on mountains - in Mexico year after year? One of the ideas offered to explain this phenomenon is "genetic memory" - but, what is that? Can DNA contain a code for the migration route and the overwintering locations in Mexico? No, it can’t. DNA codes for the production of proteins; so, if the term genetic memory is used, we have to explain how the production of proteins could lead to the complex set of behaviors that result in specific flight paths and the choice of overwintering sites. I’d rather not use the term - I don’t think it applies and even if it can be used in a more inclusive sense, the use of the word "memory" is misleading.
So, how might we explain how monarchs reach the overwintering sites? What sort of default system is involved and what is the role of DNA? The production of proteins coded by the DNA results in complex behaviors and sensory systems that can detect a large number of environmental parameters. The latter include celestial information such as the position of the sun, day length, wind speed and direction, temperature, humidity, odors, the ability to detect longitude and perhaps latitude, and many more. In the fall, newly emerged adults acquire environmental information for some indeterminate period (an induction period). The information acquired through the sensory system results in a multitude of behavioral changes that culminate in directional migratory behavior that is appropriate for the monarchs’ region of origin. The implication here is that the environmental signals induce a cascade of reactions through the sensory system that activates and inactivates numerous genes and that these genes produce the proteins and subsequent biosynthesis that give rise to the appropriate behavioral changes. This is the simple version, but it leads to the view that if we knew what the butterflies perceived and which signals served as triggers that resulted in changes in behavior, we could predict the general path of a fall migration and the regions in which they would overwinter. In other words, if monarchs were introduced to a new continent, and we knew their requirements, we could predict the direction they would fly and where they would overwinter. This applies to Australia and the introduced monarchs in that country show patterns of migratory behavior and overwintering similar to that seen in the western states and California. But, this is another story; let's get back to the default system.
In the broadest sense, the DNA of the monarchs is interacting with the changing environmental signals (the proximate climatic signals, biotic signals and even physiography of the landscape) in a manner that has the effect of leading or guiding them to the general region of the overwintering sites in the fall and back to the breeding habitats in the spring. There is no genetic memory for the overwintering sites per se. The migration stalls at the overwintering sites (19.4 N) because the rate of change of the environmental signals cueing the migration becomes exceedingly small at this latitude in November. Of course, all these responses to environmental signals have been selected for, so, in a general sense, the DNA has been fine-tuned and might be said to have a "memory" for the environments encountered throughout the migration. However, this is a stretch and I would prefer to leave the term genetic memory out of the discussion because it doesn’t add to our understanding. To understand the migration, we need to focus on the process questions. What are the environmental signals, how does the butterfly perceive and process this information and how do the perceived signals lead to physiological, biochemical and behavioral changes which initiate, maintain, stop, and then (months later) restart the migration in the opposite direction?
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5) Monarch Population Crashes and Recoveries - by Chip Taylor
The monarch population has crashed and recovered in dramatic fashion several times in the last 4 years. The size of the overwintering population in 1999-2000, 9.05 hectares, was near normal. In the following winter, 2000-2001, the total area was only 2.83 hectares, the all-time low for the wintering population early in the season. This was an unprecedented crash and it occurred during the summer. The explanation appears to be drought. In the spring a drought extended from central Mexico into Texas, and later, in early and mid summer, it included the mid-west. These conditions were followed by extremely dry weather and reduced flowering along much of the migratory route in September and October. Overall, this was the driest year in the last 11 seasons and monarchs did not do well under these conditions.
During the winter of 2000-2001, there were two events that resulted in significant mortality. Early in the winter, the colony at San Andres, a severely degraded and poorly protected site, experienced temperatures low enough to kill most of the monarchs in that colony. Similarly, in the first week of March, high winds, rain, snow, and ice reportedly killed most of the monarchs at Cerro Pelon and some of the other eastern colonies. The numbers of monarchs killed at Chincua and El Rosario at this time were reported to be relatively modest (7%). (The mortality of the monarchs at San Andres was originally attributed to pesticides used by loggers, but this was subsequently shown not to be the case). As a result of these episodes of catastrophic mortality, the initial population of 2.83 hectares was much diminished by the end of the winter. Unfortunately, there was no late winter measurement of the colonies. However, it is likely that no more than 2 hectares of butterflies, remained. We had never seen such a low population, so the prospects for the next season did not look good.
The monarchs bounced back the following summer and in the winter of 2001-2002 the population measured 9.35 hectares. This was an extraordinary recovery, one that demonstrated the remarkable ability of the monarch population to increase under favorable conditions. All was well with the wintering population until the storm of 12-16 January 2002. This storm, which is described in detail on the web site, the Season Summary and in press accounts, contributed to the deaths of 75-80% of the monarchs at Chincua and El Rosario. The storm was widespread, and, if this level of mortality occurred at all colonies, the population was once again reduced to approximately 2 hectares of surviving butterflies. Once again the monarch population recovered in the breeding season and this winter the population has been estimated to be 8 hectares. The monarchs have been fortunate. Two bad winters have been followed by two reasonably good breeding seasons in which the populations have been able to increase to nearly average numbers. Imagine the outcome if the low overwintering populations of the last two years encountered spring, summer, and fall conditions similar to those in 2000. The subsequent overwintering population would be extremely small.
We haven't been tracking monarchs long enough to be able to estimate the likelihood of low overwintering populations being followed by harsh, unfavorable, summers, but this will happen; the question is when.
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