Implications of summer breeding phenology on demography of monarch butterflies

Phenological changes have been widely documented in animal and plant responses to directional environmental change. However, predicting the consequences of these shifts for species interactions and population viability requires knowledge of vital rate responses to biotic and abiotic drivers. Here, we paired long-term phenology data documenting monarch butterfly abundance and occurrence of their milkweed hostplant with outdoor experiments in the central United States to ask how changes in spring arrival times to monarch breeding sites affect their development, survival, and within-season population growth. Monarch arrival times did not change across the 17 years of monitoring, but the peak abundance of monarchs, which occurred just prior to their fall migration, shifted 9 days later in 2019 as compared to 2003. Summer population growth declined from 2003 to 2019, significant in ~80% bootstrap calculations. Phenological changes in milkweed occurrence mirrored changes in monarch abundance, happening later through time. Our field experiment showed that early season larval survival was highest when the timing of hatching matched the average timing of the first natural monarch cohort; survival was lowest when egg hatching shifted 14 days earlier. The results of our study indicate that earlier arrival of adult monarchs to summer breeding habitat would be costly for monarchs-but field survey data show that arrival times have not changed to date. Instead, the local changes we observed in the timing of peak abundance occurred towards the end of the breeding season, not the onset. At present, we conclude that changes in early season phenology are not a threat to eastern North American monarchs living in the central United States, but drivers of breeding-season growth rates and changes in late-season phenology merit further study, both in the central United States and in other parts of the monarch's range.