A comparison of summer, fall and winter estimates of monarch population size before and after milkweed eradication from crop fields in North America

Abstract

Measures of the area occupied by overwintering monarchs in México since the mid‐1990s show a decline. Summer surveys of monarchs, however, do not show a similar decline. This discrepancy has led to the proposition that summer monarch numbers are actually stable and that increasing mortality during migration has led to declining overwintering numbers. A competing hypothesis is that this discrepancy is due to a sampling bias in the summer counts and that the summer population has declined because of the eradication of milkweed habitat from crop fields that occurred in the late 1990s through mid‐2000s. We posit that the sampling bias occurred during the period when milkweeds were declining in crop fields and resulted from not sampling in the crop fields. We examined three measures of the size of the population made in the fall during migration and four summer survey measures and compared them to the overwintering measures. Counts of migrating monarchs are not expected to have this sampling bias since migrants come from all habitats, including crop fields. During the period of milkweed eradication, counts of the migrating population were correlated with the size of the overwintering population, whereas summer survey counts were not. After milkweed eradication from crop fields, all population measures were correlated with one another. These results indicate that during the pre‐eradication period, summer counts were not an accurate measure of summer population size. Population trends observed at the overwintering site reflect factors, principally milkweed loss, that affect summer population size.

Dramatic Recent Declines in the Size of Monarch Butterfly (Danaus plexippus) Roosts During Fall Migration

“…Debate over the conservation status of monarch butterflies has largely focused on how to weigh two very different sets of population trends, with clear declines at overwintering sites in Mexico (3,4,8) but possible relative stability in the summer breeding range (despite clear summer declines in regions where use of the herbicide glyphosate is particularly high, for example) (12). This disparity in population trends for winter versus summer monarchs has led to confusion and controversy over the conservation status of monarchs, perhaps best exemplified by the recent listing, and then delisting, of monarchs as "endangered" by IUCN (45).…”

“…decline at the overwintering sites (3)(4)(5)(6)(7)(8), and these declines have driven efforts by government and non-government agencies in the U.S. and Canada to move monarchs to endangered status (9)(10)(11). Yet, other monitoring datasets from the summer breeding range suggest little to no change in monarch population size (12,13), supported by recent genomic analyses finding no evidence for a recent genetic bottleneck that would be consistent with population decline (14).…”

Changes in landscape and climate in Mexico and Texas reveal small effects on migratory habitat of monarch butterflies (Danaus plexippus)

“…Taylor et al 27 , based on monarch tagging data, argued migration success has not declined, but concerns exist about this conclusion 28 . Others have argued the lack of observed summer trends in the breeding region stems primarily from a sampling design that does not account for loss of monarchs in agricultural lands 24 , 29 – 31 .…”