Potential impacts of climate change on monarch butterflies, Danaus plexippus

Abstract

Monarch butterflies are believed to be vulnerable to the effects of climate change. Ecological niche models predict current over-wintering sites in Mexico will be unsuitable for monarchs in 40 to 50 years. Climate is likely to be equally or more important during the summer. Here, I seek to predict the consequences of climate change for monarchs during the summer breeding season and fall migration and determine their ability to cope with these changes. Using ecological niche modeling, I predicted a marked northward range shift throughout the summer breeding season, necessitating movement before each summer generation and a longer, faster migration. It is unclear whether monarchs can track geographic changes to their ecological niche, and if not, they stand to loose up to half their available breeding habitat. If monarchs can withstand temperatures hotter than their current ecological niche, their range shifts may not be as drastic. I assessed the lethal and sub-lethal impacts of exposure to temperatures above their predicted ecological niche. Daytime temperatures of 38 and 40oC caused a slight increase in mortality, as well as difficulty pupating, smaller adult size and a longer development time. Daytime temperatures of 42oC caused substantially more mortality and no larvae survived exposure to 44oC. These results suggest that the ecological niche models are accurate and monarchs could lose a substantial amount of available breeding habitat. To assess the monarch butterfly’s potential to cope with a changing climate and adjust its migration, I aimed to identify any level of plasticity currently present in the migration. An introduced species of milkweed, Asclepias curassavica, present in Texas, may be changing the monarchs’ breeding habits during its fall migration. Data from the Monarch Larva Monitoring Project and field studies I conducted indicate that female monarchs prefer the introduced milkweed to native milkweed particularly during the fall migration. If monarchs are already changing their migration in response to this environmental change, there may be the variability in monarch migratory and reproductive behavior that could allow this species to adjust to future climate change.