Monitoring Results
How are Monarch Populations Doing?
Probably the most frequently-asked MLMP question is "How are monarchs doing this year?" The question was especially relevant in 2002, after millions of monarchs had died in the aftermath of a freak storm that brought rain, then snow and freezing temperatures to the Mexican overwintering sites.
Since insect population numbers often fluctuate a great deal from year to year, we really need many years of data to answer this question. After 7 years of the MLMP, we can start to make comparisons between years. One way of doing this is to look at yearly maximum second peak egg densities.
The maximum second peak egg density in a given year is simply the highest weekly per plant egg density in the second peak of the summer. For example, in the graph showing data from Wisconsin in 2001 (right), the highest second peak egg density was during the week of 7/22, and was about 0.25 eggs per plant.
We use this number because it reflects the number of monarchs that migrated south (earlier peaks produced monarchs that were not migratory).
The second graph shows maximum egg densities in the Upper Midwestern US from 1996 through 2003. We chose the upper Midwest for two reasons: first our highest concentration of volunteers monitor in this region, so the data are most reliable. Second, Wassenaar and Hobson (1998) collected monarchs overwintering in
This suggests that this region is most likely to reflect the overall population size. The graph illustrates how variable monarch numbers are when comparing across years. After the 2002 winter storm, the upper Midwestern monarch population was the second lowest it has been throughout the project, but still about twice as large as that in 1998, our lowest year. Data for 2003 suggest that there was not a strong recovery after one year; numbers are almost as low in 2003 as they were in 2002.
Clearly, we need data over more years to determine if the trajectory over time is flat, increasing, or decreasing. In addition, the more volunteers we have monitoring monarchs throughout their summer breeding range, the more accurate our understanding will be. We can use MLMP data to look for correlations between monarch numbers and weather, predator densities and other features of the environment, and then design experiments to test how environmental conditions affect monarch growth and survival.
References
Wassenaar, L.
I. and K. A. Hobson. 1998. Natal origins of migratory monarch
butterflies at wintering colonies in