title: 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.
authors.affiliation: Department of Integrative Biology, University of Guelph, Guelph, ON, N1G 2W1, Canada.
authors.forename: D T Tyler
authors.identifier: 0000-0002-5832-8610
authors.initials: DTT
authors.lastname: Flockhart
authors.affiliation: Department of Biology, Sweet Briar College, Sweet Briar, VA, 24595, USA.
authors.forename: Lincoln P
authors.initials: LP
authors.lastname: Brower
authors.affiliation: Centro de Investigaciones en Geografía Ambiental, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro No. 8701, C.P. 58190, Morelia, Michoacán, Mexico.
authors.forename: M Isabel
authors.initials: MI
authors.lastname: Ramirez
authors.affiliation: Environment Canada, Saskatoon, Saskatchewan, S7N 3H5, Canada.|Department of Biology, University of Western Ontario, London, Ontario, N6A 5B7, Canada.
authors.forename: Keith A
authors.initials: KA
authors.lastname: Hobson
authors.affiliation: International Atomic Energy Agency, Department of Nuclear Sciences and Applications, A-1400, Vienna, Austria.
authors.forename: Leonard I
authors.initials: LI
authors.lastname: Wassenaar
authors.affiliation: Odum School of Ecology, University of Georgia, Athens, GA, 30602, USA.
authors.forename: Sonia
authors.initials: S
authors.lastname: Altizer
authors.affiliation: Department of Integrative Biology, University of Guelph, Guelph, ON, N1G 2W1, Canada.
authors.forename: D Ryan
authors.initials: DR
authors.lastname: Norris
mesh_terms: D025041:Animal Migration; D000818:Animals; D002080:Butterflies; D002980:Climate; D008800:Mexico; D012098:Reproduction
keywords: 
  Asclepias
  ; 
  Danaus plexippus
  ; carbon; hydrogen; migratory connectivity; provenance; seasonal migration; stable isotopes