Linking the continental migratory cycle of the monarch butterfly to understand its population decline

Abstract

Threats to several of the world's great animal migrations necessitate a research agenda focused on identifying drivers of their population dynamics. The monarch butterfly is an iconic species whose continental migratory population in eastern North America has been declining precipitously. Recent analyses have linked the monarch decline to reduced abundance of milkweed host plants in the USA caused by increased use of genetically modified herbicide‐resistant crops. To identify the most sensitive stages in the monarch's annual multi‐generational migration, and to test the milkweed limitation hypothesis, we analyzed 22 years of citizen science records from four monitoring programs across North America. We analyzed the relationships between butterfly population indices at successive stages of the annual migratory cycle to assess demographic connections and to address the roles of migrant population size versus temporal trends that reflect changes in habitat or resource quality. We find a sharp annual population decline in the first breeding generation in the southern USA, driven by the progressively smaller numbers of spring migrants from the overwintering grounds in Mexico. Monarch populations then build regionally during the summer generations. Contrary to the milkweed limitation hypothesis, we did not find statistically significant temporal trends in stage‐to‐stage population relationships in the mid‐western or northeastern USA. In contrast, there are statistically significant negative temporal trends at the overwintering grounds in Mexico, suggesting that monarch success during the fall migration and re‐establishment strongly contributes to the butterfly decline. Lack of milkweed, the only host plant for monarch butterfly caterpillars, is unlikely to be driving the monarch's population decline. Conservation efforts therefore require additional focus on the later phases in the monarch's annual migratory cycle. We hypothesize that lack of nectar sources, habitat fragmentation, continued degradation at the overwintering sites, or other threats to successful fall migration are critical limiting factors for declining monarchs.

Effects of phenological synchronization on caterpillar early-instar survival under a changing climate

“…The main threats causing monarch population declines appear linked to the disappearance of milkweed plants linked to intensive agricultural practices, especially in the US corn belt (Flockhart et al 2015;Stenoien et al 2016), although research also suggests declines occurring during migration or on overwintering grounds (Inamine et al 2016). Summer breeding success does not appear to be affected by changing climate (Stenoien et al 2016), but monarch populations could be impacted by climate change via increases in extreme weather events during overwintering or reduced rainfall and concomitant decrease in milkweed abundance in the southern spring breeding grounds (Oberhauser et al 2017).…”

Science‐Policy‐Practice Interfaces: Emergent knowledge and monarch butterfly conservation

“…Dyer and Forister (2016) argued that the dispute pointed to unanswered scientific questions, and monarch scientists need to focus attention on what is causing the conflicting results of population trends through application of well-known and trustworthy scientific methods. Inamine et al (2016) took up this challenge through study of the relationship between regions in the monarchs' multigenerational migratory cycle. The study confirms that there is a long-term decline in the population at the overwintering grounds in Mexico, and there is not a decline in the population during the summer breeding generations.…”

“…Inamine et al (2016) took up this challenge through study of the relationship between regions in the monarchs' multigenerational migratory cycle. With these results Inamine et al (2016) (Flockhart et al, 2014, p. 158), educated guesses have been turned into estimations of reality. Thus, despite decline in Mexico, the population is able to rebound during the summer in the USA and Canada.…”

“…Planting tropical milkweed (Asclepias curassavica), which does not naturally senesce in the autumn like native milkweeds (e.g. Looking ahead, the findings by Inamine et al (2016) that the cause of the monarch decline in Mexico occurs during the fall migration, after the migrating generation has become independent of milkweed, may destabilize scientific and popular understanding of the link between milkweed and monarch population trends. This change in resource availability has implications for migrating and overwintering populations (Satterfield et al, 2015).…”

Public Knowledge of Monarchs and Support for Butterfly Conservation

“…Due to this migration, the vast majority of the United States public has the opportunity to interact with this species at some point during the year. However, due to over-wintering site habitat loss as well as the loss and contamination of food resources (milkweed) and habitat in its summer range [37][38][39][40], the decline of monarch populations has been of increasing concern [41][42][43]. Concerns over monarch decline have caused the US Fish and Wildlife to undertake an effort to evaluate its threatened status in accordance with the Endangered Species Act [44][45][46] in addition to private efforts to conserve and create suitable habitat [47,48].…”