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

Abstract

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.