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Changes in climate drive recent monarch butterfly dynamics


Declines in the abundance and diversity of insects pose a substantial threat to terrestrial ecosystems worldwide. Yet, identifying the causes of these declines has proved difficult, even for well-studied species like monarch butterflies, whose eastern North American population has decreased markedly over the last three decades. Three hypotheses have been proposed to explain the changes observed in the eastern monarch population: loss of milkweed host plants from increased herbicide use, mortality during autumn migration and/or early-winter resettlement and changes in breeding-season climate. Here, we use a hierarchical modelling approach, combining data from >18,000 systematic surveys to evaluate support for each of these hypotheses over a 25-yr period. Between 2004 and 2018, breeding-season weather was nearly seven times more important than other factors in explaining variation in summer population size, which was positively associated with the size of the subsequent overwintering population. Although data limitations prevent definitive evaluation of the factors governing population size between 1994 and 2003 (the period of the steepest monarch decline coinciding with a widespread increase in herbicide use), breeding-season weather was similarly identified as an important driver of monarch population size. If observed changes in spring and summer climate continue, portions of the current breeding range may become inhospitable for monarchs. Our results highlight the increasingly important contribution of a changing climate to insect declines.

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Fig. 1: Overwintering monarch population size in Mexico and summer glyphosate use in the Midwestern United States.
Fig. 2: Relative importance of seasonal factors influencing the size of the eastern North American monarch population (2004–2018).
Fig. 3: Spring weather (1994–2018) and estimated effects on summer monarch population size (2004–2018).
Fig. 4: Summer weather (1994–2018) and estimated effects on summer monarch population size (2004–2018).
Fig. 5: Relationships among monarch population sizes in summer, early winter and late winter between 2004–2018.

Data availability

Monarch data from the overwintering grounds and covariate data are available on Zenodo ( Monarch data from the summer breeding grounds are proprietary and were obtained from the North American Butterfly Association (, the Iowa Butterfly Survey Network (, the Illinois Butterfly Monitoring Network (, the Michigan Butterfly Network ( and the Ohio Lepidopterists ( These data may be available upon reasonable request to L.R. and with permission from the aforementioned programmes.

Code availability

Code needed to run analyses (R scripts and Stan model files) is available on Zenodo ( and Github (


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We thank the many volunteers who contributed to data collection. S. Altizer shared data and insights on the effects of disease and N. Haddad provided comments on the manuscript. This work was supported by NSF grant nos. EF-1702635 (EFZ), DBI-1954406 (EFZ) and EF-1702179 (LR).

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Authors and Affiliations



E.R.Z., L.R., K.S.O. and E.F.Z. conceived of the research. L.R., N.N., M.I.R., E.R.-S. and K.S.O. contributed data. E.R.Z, S.P.S., M.T.F. and E.F.Z. constructed the model. E.R.Z. ran analyses. E.R.Z. and E.F.Z. wrote the first drafts of the paper. All authors contributed to the interpretation of results and edits to the paper.

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Correspondence to Erin R. Zylstra.

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The authors declare no competing interests.

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Peer review information Nature Ecology & Evolution thanks Diana Bowler and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Locations of monarch butterfly surveys on summer breeding grounds between 1994–2018.

Locations of surveys conducted between 14 Jun–15 Aug by the North American Butterfly Association (NABA; blue) and state-specific butterfly monitoring networks (BMNs; red) in a, 1994–2003 and b, 2004–2018. Counties (U.S.) and census districts (Canada) that are included in our delineation of the summer breeding range for the 1994–2003 reduced annual-cycle model and the 2004–2018 full annual-cycle model are outlined in grey.

Extended Data Fig. 2 Model-based index of monarch butterfly population size during peak summer, 1994–2018.

Model-based predictions (posterior medians with 95% credible intervals [CI]) of the expected number of adult monarchs observed per hour on an average NABA survey conducted between 19 Jul–15 Aug, 1994–2018, with linear trend (grey line) and 95% CI (shaded area; slope = –0.15 adults/hr/yr, 95% CI: –0.30, 0.01). Vertical dashed line denotes the break between our 1994–2003 and 2004–2018 analyses.

Extended Data Fig. 3 Changes in summer climate on monarch summer breeding grounds.

Percent change between 1994–2003 and 2004–20018 in a, average temperatures (GDD from 3 May–15 Aug) and b, cumulative precipitation (mm, Apr–Aug) for each U.S. county included in our delineation of the monarch summer breeding range. Temporal trends over a recent 15-year period (2004–2018) in c, GDD (°C/yr), and d, cumulative precipitation (mm/yr). Positive values indicate increases or positive trends in weather variables; negative values indicate decreases or negative trends. Canadian counties were excluded from panels a and b because data limitations prevented us from including these regions in our 1994–2003 model of monarch population dynamics.

Extended Data Fig. 4 Residuals from the winter submodel describing variation in the area occupied by monarch butterflies.

Estimated residuals (posterior medians) from the winter submodel describing the area occupied by monarchs in each of the overwintering supercolonies, when monarchs were present in early winter, 2004–2018. Solid grey line and shaded area represent a linear trend with 95% credible interval (slope= –0.003, 95% CI = –0.014, 0.008).

Extended Data Fig. 5 Effects of summer weather on monarch population size, 1994–2003.

a, Estimated marginal effects (median and 95% credible intervals) of GDD (deviation from county 10-year average) on expected monarch counts during peak summer (expected mean count of adult monarchs per search hour, 19 Jul–25 Jul), for typical cool, average, and warm counties (avgGDDc = 711, 898, and 1033 °C, respectively) within the summer breeding range, 1994–2003. b, Estimated marginal effects of precipitation (deviation from county 10-year average) on expected monarch counts during peak summer, for typical dry, average, and wet counties (avgPCPc = 422, 525, and 578 mm, respectively), 1994–2003.

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Zylstra, E.R., Ries, L., Neupane, N. et al. Changes in climate drive recent monarch butterfly dynamics. Nat Ecol Evol 5, 1441–1452 (2021).

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