Interpreting surveys to estimate the size of the monarch butterfly population: Pitfalls and prospects

Abstract

To assess the change in the size of the eastern North American monarch butterfly summer population, studies have used long-term data sets of counts of adult butterflies or eggs per milkweed stem. Despite the observed decline in the monarch population as measured at overwintering sites in Mexico, these studies found no decline in summer counts in the Midwest, the core of the summer breeding range, leading to a suggestion that the cause of the monarch population decline is not the loss of Midwest agricultural milkweeds but increased mortality during the fall migration. Using these counts to estimate population size, however, does not account for the shift of monarch activity from agricultural fields to non-agricultural sites over the past 20 years, as a result of the loss of agricultural milkweeds due to the near-ubiquitous use of glyphosate herbicides. We present the counter-hypotheses that the proportion of the monarch population present in non-agricultural habitats, where counts are made, has increased and that counts reflect both population size and the proportion of the population observed. We use data on the historical change in the proportion of milkweeds, and thus monarch activity, in agricultural fields and non-agricultural habitats to show why using counts can produce misleading conclusions about population size. We then separate out the shifting proportion effect from the counts to estimate the population size and show that these corrected summer monarch counts show a decline over time and are correlated with the size of the overwintering population. In addition, we present evidence against the hypothesis of increased mortality during migration. The milkweed limitation hypothesis for monarch decline remains supported and conservation efforts focusing on adding milkweeds to the landscape in the summer breeding region have a sound scientific basis.

Why are monarch butterflies declining in the West? Understanding the importance of multiple correlated drivers

“…, Pleasants et al. ). We urge ecologists interested in monarch conservation to complement broad‐scale monitoring studies with targeted observations and small‐scale experiments to understand the demographic mechanisms behind landscape‐scale patterns.…”

Monarch butterfly and milkweed declines substantially predate the use of genetically modified crops

“…The blue vertical line indicates the point at which half of all corn, soybeans, and cotton are GM varieties. 1B: indicates (1) the discovery of the monarch overwintering grounds in Mexico; (2) the introduction of GM crops; (3) the winter population census at the Mexican overwintering grounds 20 ; (4) the summer NABA census of adults (available from 6 ); (5) the summer MLMP census of eggs and larvae (available from 12 ); (6) the summer census of Iowa A. syriaca abundance (available from 12 ).…”

“…Over the past 25 years, this migratory population of the monarch has experienced a drastic decline, as much as 80%, as measured at the overwintering area in Mexico 4, 11 . Surveys of both immature and adult stages suggest a decline at the breeding grounds as well 12 . Previous work has identified the decline of milkweed (Asclepias spp.…”

“…The best evidence for this is that the decline of monarch butterflies appears to predate the use of GM crops. The monarch population size has been recorded in the overwintering grounds since 1993 11 , and the population decline is thought to be either linear or exponential over this period 2, 12 . However, herbicide-resistant crops were not introduced until 1996, and initially accounted for only 2% of US cropland (Figure S4).…”

Monarch butterfly declines reported in Boyle et al. (2019) are biased by unexamined changes in museum collections over time

“…Does this mean that Monarch abundance is increasing ( Figure 1B)? I argue that these data are insufficient to model abundance, as the trend in museum specimens contradicts the evidence, from systematic monitoring, of a decline in the summer range (Pleasants et al 2017)…”