Evidence for a growing population of eastern migratory monarch butterflies is currently insufficient

Abstract

The eastern migratory population of monarch butterflies has experienced a multi-10 decadal decline, but a recent increase in abundance (to 6.05 ha in winter 2018) has led some 11 observers to question whether the population has reversed its long-standing decline and 12 embarked on a trajectory of increasing abundance. We examined this possibility through 13 changepoint analyses, first assessing whether a change in trajectory existed and whether that 14 change was sufficient to alter our estimated risk for the population. We found evidence of a 15 change in trajectory in 2014, but insufficient statistical support for a significantly increasing 16 population since that time (β = 0.285, 95% CI = -0.127, 0.697). If the population estimate for 17 winter 2019 is ≥4.0 ha, we will then be able to credibly assert the population has been increasing 18 since 2014. However, given estimated levels of time series variability, presumed habitat capacity 19 and no recent change in status or trend, there was a 13.5% probability of observing a population 20 estimate as large or larger than was reported for winter 2018. Despite insufficient evidence for an 21 increasing population, near-term risk of quasi-extinction by 2023 has declined (mean risk 22 23 highlight the incredible difficulty in drawing robust conclusions from annual changes in 24 abundance over a short time series, especially for an insect that commonly exhibits considerable 25 year-to-year variation. Thus, we urge caution when drawing conclusions regarding species status 26 and trends for any species for which limited data are available. 27 28 Shapiro 32 Populations vary over time in their abundance, and this variability can impart uncertainty to the 33 status and trend of a species. As population dynamics approach extinction, dynamics become 34 more variable (Holmes and Fagan 2006), which means short-term highs might become higher, 35 even while abundance is declining on average. In addition to the stochastic variation in 36 abundance imposed by the environment, uncertainty in species status and trend arises from 37 population sizes most often being estimated rather than counted; trends being inferred from 38 limited duration time series; and latent characteristics of a population, such as its relation to 39 carrying capacity or quasi-extinction thresholds, generally being inferred properties rather than 40 an observable quantity. Thus, given these various sources of uncertainty, it is difficult enough to 41 determine the trajectory for a population, let alone any change that may occur in that trajectory, 42 especially one that may occur near the terminus of a time series based on limited data.43 3 Estimates of the population size of the eastern North American migratory population of 44 monarch butterflies (Danaus plexippus, hereafter monarchs) in their overwintering locations in 45 high-elevation oyamel fir (Abies religiosa) forests of central Mexico suggest a long-term decline 46 in abundance. Using a model allowing separation of observation-induced error from n...

Change‐point analysis of lambda‐cyhalothrin efficacy against soybean aphid (Aphis glycines Matsumura): identifying practical resistance from field efficacy trials

“…The continued evaluation of these insecticides at these locations may refine these estimates as trajectories of change near the terminus of a time series may be difficult to determine. 36 Resistance to pesticides is an ever-increasing challenge to global agriculture with nearly a 1000 species of pests, including approximately 600 arthropod species, resistant to one or more pesticides. 20 Resistance to a pesticide can be directly documented by demonstrating a reduction in susceptibility over time within a population.…”

“…At UMORE, the estimated slope of percent control after the change point was approximately five times that of SWROC, although not statistically different because of the wide confidence intervals. The continued evaluation of these insecticides at these locations may refine these estimates as trajectories of change near the terminus of a time series may be difficult to determine 36 …”

“…Within these unique publications, 16 were related to entomology, and 21 involved the use of pesticides. Within our literature search, examples were found of change point and similar statistical methods used in entomology when modeling population dynamics in monarch butterflies (Danaus plexippus), 36 pupation success and behavior of western flower thrips (Frankliniella occidentalis), 37 and the residual efficacy of pesticide-impregnated ear tags for cattle. 38 Also, similar methods have been used when evaluating the dissipation of pesticides used as seed treatment in soils, 39 examining the behavior and environmental fate of glyphosate in water and sediments, 40 and for determining the effects of defoliation on yield in field beans.…”

Grappling with uncertainty in ecological projections: a case study using the migratory monarch butterfly

“…Public concern for the monarch escalated after declines were identified in the eastern population (Brower et al 2012). The population remains well below its previous maximum-observed size (Thogmartin et al 2020) and has recently been declared deserving of protection under the Endangered Species Act, although it was not listed so that other species could be prioritized for management (USFWS 2020). This means that continual monitoring and reassessment of the monarch will remain a critical need both to understand its dynamics but also to provide the US Fish and Wildlife Service the information they require for their yearly deliberations.…”

Using ecosystem services to identify inequitable outcomes in migratory species conservation

“…The eastern North American migratory monarch population has "undergone a precipitous decline over the last two decades" (Semmens et al, 2018), and recent indications of a population resurgence are not reliable (Thogmartin et al, 2020). Wilcox et al (2019) identified five broad threats to monarchs, with changing climate conditions and the loss of overwintering and breeding habitat likely the greatest (see also Saunders et al, 2018 andFlores-Martínez et al, 2019).…”