Advances in understanding the long-term population decline of monarch butterflies

Abstract

Monarch butterflies are an icon of nature: spectacular in form, known for their unfathomable annual migration, and frequent visitors in our backyards (Fig. 1). It is no wonder they are a darling among invertebrates. And what has now captured our attention is the striking and precipitous decline of monarch populations over the past two decades. So much of the decline in biodiversity, part of the current mass extinction, seems abstract to us—the polar bear floating on an iceberg in the arctic, or the slash and burn of tropical rain forests. But the decline of monarch butterflies has been observed like a “silent spring,” with biologists and casual observers alike noticing the missing butterflies from so many of our recent summers, especially in the northeastern and midwestern United States. Two new studies published in PNAS add fresh analyses, considerably new data, and novel approaches to tackle the monarch mystery (1, 2). Fig. 1. ( Top ) The annual migratory cycle of eastern North American monarch butterflies. Generation 1 occurs in the US Gulf Coast states centered on Texas (March–April), and generations 2 to 4 occur in the Northeast and Midwest (May–September). The fourth migratory generation lives for 8 mo, including overwintering in Mexico and returning to the Gulf Coast states, and does not rely on milkweed between September and February. ( Bottom ) The population decline of monarch butterflies in the highlands of Central Mexico (generation 4, estimated during overwintering) (data collected by World Wildlife Fund Mexico). The circled years are those investigated by Saunders et al. (1). Although concerns about monarch conservation have been voiced at least since 1977 (a year after their overwintering grounds were discovered by citizen scientists, in collaboration with Canadian biologists Fred and Nora Urquhart), two key moments in the monarch’s rise to public prominence came in 1999 and … [↵][1]1Email: agrawal{at}cornell.edu. [1]: #xref-corresp-1-1