UC Davis

Varying temperature and precipitation regimes under global change have the potential to disrupt organisms’ phenology and increase their exposure to adverse environmental conditions. Disruptions in phenology can lead to phenological mismatch, where interacting species become temporally misaligned due to different responses to changing phenological cues. Specific life history traits, like activity during the hottest part of the summer, obligatory relationships with small taxonomic groups of other species, and temperature-sensitive phenological events like migration and diapause, likely lead to greater sensitivity to climate change and phenological mismatch.Western monarch butterflies (Danaus plexippus), found west of the Rocky Mountains in North America, offer an ideal study system for studying the effects of climate shifts on phenology. Monarchs interact with milkweeds (Asclepias spp.), their obligate host plant. Their breeding activity during mid-summer leaves them vulnerable to increased thermal stress during a crucial life history phase. Western monarchs migrate every year between coastal overwintering sites in California and inland breeding areas in California, Nevada, and Idaho. The scale of this migration creates opportunities for phenological mismatch across their range. Finally, western monarchs overwinter as adults in diapause, a state of suspended reproduction that is highly temperature-sensitive. All of these traits make monarch biology vulnerable to the effects of climate change on phenology and exposure to stressful environmental conditions.In the first chapter of my dissertation, I investigate how year-to-year changes in temperature and precipitation alter the phenology of milkweeds in the western US. Warmer temperatures and drier rain-years lead to advanced phenology across species and phenophases, with significant effects of species’ traits and local adaptation. In my second chapter, I study how extreme heat effects the survival of monarch eggs and caterpillars developing in the field. I find support for strong increases in mortality above thermal thresholds in the mid- and high-30’s (°C). Finally, in my last chapter I investigate how changes in winter temperature at monarch overwintering sites change the timing of departure for the spring migration, both year-to-year and over the past century. Warmer winters correspond to earlier departure timing, and I find evidence that departure times have advanced ~20 days over the past 100 years. Together, these results offer a picture of how the landscape of western monarchs is shifting under climate change.