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It is the season at Monarch Watch when the mailbox is full of tagging datasheets. Many of the datasheets are arriving with returned tags along with short notes lamenting the lack of monarchs this past fall and requests for an explanation as to why the fall 2008 monarch population was so low.
The lack of monarchs was apparent all through the season and not just in the fall. On July 18th, again at the urging of many concerned Monarch Watchers, I posed my midseason assessment of the status of the population (“Where Are The Monarchs?”). My interpretation at that time was that the conditions during the period from late April to early June, when the first generation monarchs were moving north from the breeding areas in Texas and other southern states, limited the number of monarchs reaching the summer breeding range. Specifically, I said:
May, the moving month for first generation monarchs, was cold – throughout the entire northern breeding range. It was also a period of frequent storms and heavy rains, particularly during the second half of May. Early June also saw heavy rains, especially in the east north central and central portions of the country.” Cold and rain were thought to a) limit dispersal, b) reduce egg laying, and c) increase mortality of adults resulting in a reduction of the potential fecundity of this generation. After assessing all of the factors my conclusion was that “unless you are all missing something out there, the number of reproducing adults over the next three weeks will be low, to be followed by a relatively small migratory generation.
Unfortunately, our collective observations were correct and a relatively small fall population has come to pass.
One issue that I raised in the July assessment was that the number of first generation monarchs coming out of Texas and other areas of the South might have been lower than predicted by temperatures and observations of the numbers of larvae found on milkweeds. Drought, fire ants, herbicides (especially those applied to the milkweed rich pastures in central Texas), and loss of habitat could certainly reduce the numbers of first generation monarchs. The quality and quantity of monarch habitats as well as land management practices need to be assessed for the South. Indeed, we need a national assessment of monarch/milkweed habitats throughout all breeding areas to better understand what needs to be done to conserve the monarch migration. I will be writing more on this topic in the coming months.
As the observations of the migration came in via email, Dplex-L, the Monarch Forums, and other sources, I tried to arrive at a guess as to the size of the overwintering population in Mexico. Based on previous experience it was obvious to me the overwintering population was certain to be less than five hectares. But how much less? The previous models I’ve developed were not too helpful, since we have only seen one other year (1997) when the April-May conditions seemed to limit the summer breeding population. So, I’ve just had to make a flat out guess, albeit one based on my memory of migrations over the last 16 years, and my guess is that the overwintering population will be 3-4 hectares with an expectation that the final number will be closer to the low end of this range.
An early December report in a Mexican newspaper indicated that authorities expected the overwintering population to be 4.8 hectares. However, it was not clear whether this figure reflected actual measurements of the colonies or was an estimate. As you may remember, the overwintering colonies are measured twice in December and it is the measures taken in the second half of the month that are used to calculate the final estimate for the year. Two measures of each colony are taken because many butterflies are on the move early in the month and the colonies are still in the process of consolidating. Once the colonies are well established, cooler weather tends to cause the colonies to contract and the area occupied by monarchs diminishes by 10-20%. So, the expectation of 4.8 hectares reported to Mexican newspapers could well be reduced if it was, in fact, based on early December measures of the colonies.
Tagging Datasheets – a Reminder
If you have not sent in your tagging data sheets, PLEASE DO SO NOW before these data are lost. We will be heading for Mexico later this winter to buy tags. Each year we return having bought many tags without data due to the fact that some of our participants have not returned datasheets. Although most of the money for the recovered tags comes out of my pocket, I’m most concerned about the lost data. Every recovery provides valuable information and you may never learn if the monarchs you, or your group, caught and tagged were recovered in Mexico, so please send us those datasheets! Thank you for your interest and participation in our Monarch Tagging Program.