Why We Still Tag Monarchs
Given the obvious success of the tag recovery program, one might ask: why do we continue with the tagging program? Don’t we have enough recoveries to learn all there is to learn about the migration? The simple answer to the questions is that we learn something new each year. When we started this program in 1992, only 99 monarchs tagged by Fred and Nora Urquhart and their associates had been recovered in Mexico. The number of recoveries from Mexico now exceeds 6800 with approximately 75% of those coming from the winters of 2002 and 2004 when severe January storms killed an estimated 75% and 70% of the population respectively. Yes, in general terms, we know where the monarchs come from that overwinter in Mexico but there are many details that are still not clear. One of the puzzles is why the recoveries are not a linear function of distance. In other words, given that the distance from St. Paul, MN to El Rosario (1791 miles) why does Don Davis have to tag more than 2.5 times as many butterflies for each recovery when the distance from Toronto, Canada to El Rosario (2053 miles) is only 1.1 times greater?
We have learned a great deal about the migration to date from all of your tagging efforts. The tagging data have revealed that there is a pattern to the timing and pace of the migration. When we initiated this program there were no data on the time course of the migration. It appeared to be primarily driven by the weather. We now know that the migration is intrinsically driven by an interaction of the monarchs with the changing celestial conditions in the fall such that the pace of the migration across the latitudes is remarkably predictable. This pattern is so robust that it allows us to anticipate the arrival of the wave of southerly moving monarchs at each latitude. We are presently engaged in an extensive study of all the tagging data to date and anticipate that additional insights concerning the migration will result from this analysis.
One of our goals for the tagging program was to use this mark and recapture effort to derive estimates for the size of the fall migratory population, the amount of mortality during the migration and the size of the overwintering population. To be able to arrive at such estimates, traditional mark and recapture methods require that the number recaptured or viewed (in the case of dead butterflies on a forest floor) is known. In other words, we need specific measures of the number of untagged to tagged butterflies for each population estimate. Because the ratio of untagged to tagged may be 10s of thousands to one, establishing a true measure of the population using this method has eluded us. We’ve tried to estimate the number “viewed” in the winter population for each recovery and to use the total hectares occupied by overwintering monarchs, and the various estimates of the number of butterflies per hectare suggested by various studies, but none of these methods has yielded a satisfactory estimate of the total population. Estimates of the number of monarchs per hectare varies from 10 to over 50 million - too broad to be of much use particularly since the measurements of hectares varies (usually declining by 30% or more) from December to January at the same site. It remains that we are going to have to devise a way to establish the ratio of untagged to tagged monarchs to arrive at a reasonable estimate of the number of overwintering monarchs. This is doable but it will take some technological innovations to accomplish this objective. At present, we are limited to collecting thousands of dead monarchs from the overwintering sites and scanning them visually for tags. This is too time consuming as we discovered first hand when it took 4 of us many hours over 4 days to scan 40,000 dead monarchs for tags. We are confident that once we have developed and perfected the technique we have in mind, we will be able to arrive at consistent estimates of the number of untagged to tagged monarchs that can be used to arrive at more accurate population estimates.
There are two other reasons for continuing the tagging program. The involvement of thousands of taggers has created a veritable army of observers in the field. The reports from this large cohort of collaborators provide us with insights on the dynamics of the migration and the size and quality of the population each season. Perhaps of even greater importance is the fact that this program brings at least 100,000 people into intimate contact with one of the worlds most remarkable natural events each fall. Once people become familiar with monarchs, the value of preserving the monarch migration in eastern North America becomes apparent. Saving the monarch migration is possible but the threats to the migration posed by a variety of human activities, e.g., illegal logging at the overwintering sites in Mexico, are such that the public in Canada, the United States and Mexico will have to lobby their politicians to enact and fund measures that assure the preservation of the migration.