Linking the continental migratory cycle of the monarch butterfly to understand its population decline
For hundreds of years the annual continental migration of the monarch butterfly (Danaus plexippus) has captured the imagination of scientists and non-scientists. Although monarchs have a history of >10-fold annual population fluctuations, in 2011, a 17-year study revealed a strong and precipitous decline of monarch populations at the overwintering sites in Mexico. In contrast, two independent fall census programs that enumerate monarchs returning from the northern USA did not show a decline over the same period. Given their predictable migratory behavior, a stronger connection was expected between fall migrating monarchs and the population in Mexico.
To understand the monarch population dynamics, we characterized monarch population transitions across the annual migratory cycle using a set of databases covering 22 years, including citizen science data curated by the North American Butterfly Association (NABA), building on previous work by L. Ries and others. We used these data to link the cycle from overwintering abundances, to summer breeding population estimates, and finally to fall migrating butterfly censuses. With these population estimates, we perform model selection to evaluate hypotheses about trends in monarch population dynamics relevant to the observed decline.
Our approach captures the dynamics of the monarchs’ continental migration, with a spring wave of population increase and decrease in the southern USA, followed by a similar but extended pattern in the Midwest and Northeast. We also observe a fall wave of butterflies returning through the South to the overwintering grounds. This supports the reliability of the NABA dataset for assessing large-scale population dynamics.
Across each annual cycle, from winter in Mexico to summer breeding grounds, we found that population size at a stage reflected population size at the previous stage, with at most equivocal evidence of small temporal trends in these relationships. However, this was not true for fall migration and the subsequent overwintering population. The failure of northern censuses to predict subsequent numbers in the south is different from all previous stages and violates our basic expectations.
Our analysis indicates that small overwintering populations build up during the summer, and hence that larval resources (e.g., milkweed) do not appear to be a key limitation to population growth during the spring and summer generations. We hypothesize that a crucial factor in the overall monarch decline occurs during the adults’ fall migration or re-establishment in Mexico.