Using population dynamics of migratory monarch butterflies to inform conservation planning

Background/Question/Methods

Conserving migratory animals requires understanding population dynamics that link how individuals move, survive and reproduce throughout the annual cycle. Monarch butterflies (Danaus plexippus) in eastern North America are famous for their long-distance migration and unique life-history but they face multiple threats at different portions of the annual cycle that encompass portions of Mexico, the United States and Canada. Documented population declines are hypothesized to be driven by reduction in host plants due to land-use and agricultural practices on the breeding grounds and forest habitat loss and degradation on the overwintering grounds. It is currently unknown which life history stage, geographic region, and specific vital rate contribute the greatest to population growth, which is an important first step to address population declines. We considered one overwintering and three breeding regions and parameterized a stochastic density-dependent periodic projection matrix model for monarch butterflies with a two-cohort approach to differentiate reproductive non-migrants from non-reproductive migrants that can co-occur in the same region but have different physiological and demographic processes. We used transient elasticity analysis for population density to predict the vital rates, regions, seasons, life-stages and physiological states where conservation efforts would maximize population persistence given predicted climate change scenarios.

Results/Conclusions

We estimated larvae survival, lifetime fecundity and adult survival to be constant among breeding regions whereas movement patterns and survival during migration differed between the regions.  Monarch butterfly population growth was relatively robust with respect to weather-induced mass mortality events on the wintering grounds and the frequency of these mortality events is likely to decline with increasing temperatures on the wintering grounds as predicted by climate change models. In contrast, estimates of milkweed abundance differed across the breeding distribution dependent on land cover type and the proportion of agricultural crops. The widespread declines in host plants suggest an increasingly regulated population growth potential that may operate through density dependent larval competition. Overall, elasticity values varied seasonally, spatially and between life stages suggesting that no single vital rate or life-stage alone can be targeted to ensure a robust population. Our model will help guide which vital rate is the most appropriate target for action through management of milkweed abundance and forest canopy area to achieve long-term monarch butterfly persistence. This is the first attempt to link migratory connectivity with year-round population dynamics to inform how best to conserve migratory animals at continental scales.