Monarch (Danaus plexippus) Growth and Survival in a Changing World

Abstract

Eastern North American monarch butterflies (Danaus plexippus) are well known for being charismatic insects that undergo a yearly long-distance migration. With a world changing in both habitat and climate, and a monarch population in decline, my work attempts to better understand the impacts of these changes and to help better inform conservation efforts. Effective conservation of a migratory species requires an understanding of an organism at all areas throughout its migratory path. My dissertation seeks to understand both the thermal requirements of monarchs at novel overwintering sites along the Gulf Coast and the habitat requirements of monarchs throughout their breeding grounds. My work on cold tolerance enhances our understanding of the thermal impacts on immature monarchs that overwinter along the Gulf Coast, rather than the more utilized strategy of overwintering as adults in central Mexico. In my first chapter, I investigate the extreme temperature limits of immature monarchs by determining the supercooling points and lower lethal temperatures at different stages (including larval stadia). I then examine how cool temperatures influence immature monarchs, both directly and indirectly, as mediated through the impacts of cool temperatures on the monarch host plant, milkweed (Chapter 2). In addition to laboratory studies, my dissertation also uses citizen science data to examine the eastern North American monarch population. In Chapter 3, I use a novel combination of stage-specific citizen science sightings of immature monarchs along the Gulf Coast, along with a growing degree day model, to determine how long monarchs are predicted to remain in the area and to quantify the coldest temperatures that these monarchs are exposed to as immatures. Chapter 4 also uses citizen science data to estimate monarch survival and the habitat site characteristics that are associated with increased survival. Additionally, this work produced an estimate of the number of milkweeds needed to produce a migratory monarch. The insights that are gained from my dissertation are important to better understand the eastern North American population of monarchs, and combined with other research, can be used to help better inform conservation measures. This dissertation takes multiple approaches across two areas of habitat of monarchs, their northern breeding grounds and the Gulf Coast, through which they migrate through in the spring and fall, and in which they sometimes spend the winter. In a changing and increasingly fragmented world, this unifying approach looking at all aspects of a migratory species will be increasingly useful in conservation.