Regional climate on the breeding grounds predicts variation in the natal origin of monarch butterflies overwintering in Mexico over 38 years

Abstract

Addressing population declines of migratory insects requires linking populations across different portions of the annual cycle and understanding the effects of variation in weather and climate on productivity, recruitment, and patterns of long-distance movement. We used stable H and C isotopes and geospatial modeling to estimate the natal origin of monarch butterflies (Danaus plexippus) in eastern North America using over 1000 monarchs collected over almost four decades at Mexican overwintering colonies. Multinomial regression was used to ascertain which climate-related factors best-predicted temporal variation in natal origin across six breeding regions. The region producing the largest proportion of overwintering monarchs was the US Midwest (mean annual proportion = 0.38; 95% CI: 0.36-0.41) followed by the north-central (0.17; 0.14-0.18), northeast (0.15; 0.11-0.16), northwest (0.12; 0.12-0.16), southwest (0.11; 0.08-0.12), and southeast (0.08; 0.07-0.11) regions. There was no evidence of directional shifts in the relative contributions of different natal regions over time, which suggests these regions are comprising the same relative proportion of the overwintering population in recent years as in the mid-1970s. Instead, interannual variation in the proportion of monarchs from each region covaried with climate, as measured by the Southern Oscillation Index and regional-specific daily maximum temperature and precipitation, which together likely dictate larval development rates and food plant condition. Our results provide the first robust long-term analysis of predictors of the natal origins of monarchs overwintering in Mexico. Conservation efforts on the breeding grounds focused on the Midwest region will likely have the greatest benefit to eastern North American migratory monarchs, but the population will likely remain sensitive to regional and stochastic weather patterns.

Within-wing isotopic (δ2H, δ13C, δ15N) variation of monarch butterflies: implications for studies of migratory origins and diet

“…The remaining Table 2. Mean and SD (‰) of expected monarch wing stable isotope values used by Flockhart et al (2017) to assign individual monarchs to regions of natal origin. This illustrates the problem of any systematic differences among laboratories amounting to as much as a 5‰ difference in δ 2 H and a 0.5‰ difference in δ 13 C that can arise from lack of solvent rinsing and differential use of black vs orange pigmented regions.…”

“…Open Access more and more, retrospective studies based on meta analyses rely on comparability of isotopic measurements among laboratories to infer changes in migration, origins or sites of productivity of monarchs in North America (Flockhart et al 2017). Moreover, to our knowledge, few researchers routinely homogenize wing material and instead rely on small subsections cut from wings for stable isotope analyses (but see Yang et al 2016).…”

“…Care was taken to remove only wing membrane and to avoid wing veins in the sample. Thus, our sample was designed to test for isotopic measurement the effects of natal origin on parasite loads (Altizer et al 2015), the role of wing coloration in flight distance (Hanley et al 2013) and general conservation concerns related to where most individuals are being produced (Flockhart et al 2017). Such tracking studies have used continental patterns, or isoscapes, of both precipitation δ 2 H and milkweed δ 13 C. Despite the demonstrated value of stable isotope measurements of butterfly wings to investigate migration, to our knowledge, there has been no previous investigation into the isotopic variance among different regions of wing material (primarily chitin) despite the fact that monarch wings, as with those of many lepidopterans, are highly variable in pigmentation.…”

“…Funding was provided by an operating grant to KAH. We thank T. Flockhart for providing summary material used in assignment of the Flockhart et al (2017) paper. Two anonymous reviewers and A. Davis made valuable comments on an earlier draft of this paper.…”

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

“…The Eastern population summer breeding range is bordered on the west by the eastern portions of Kansas, Nebraska, South Dakota and North Dakota and extends through the Midwest to the Northeast and north into lower Canada. The Midwest portion of the summer breeding range has been identified using stable isotopes as the major contributor to the overwintering pop [2,3]. Pleasants and Oberhauser [4] and Flockhart et al [5] made the case that the decline was due to a loss of milkweeds, the sole host plant of monarch butterflies, in Midwest corn and soybean fields due to the use of glyphosate herbicide (the milkweed limitation hypothesis).…”