Commentary: Evaluating the Migration Mortality Hypothesis Using Monarch Tagging Data

Fordyce, James A.; Nice, Chris C.; Forister, Matthew L.

doi:10.3389/fevo.2020.604914

Cited by 4 publications

(4 citation statements)

References 6 publications

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“…Considering the general lack of widespread breeding season declines found here, our evidence suggests, alongside the ongoing declines at winter colonies, that monarchs must be experiencing increasingly higher levels of mortality during their fall migration. Contrasting evidence of no change in the number of tagged monarchs returning to Mexico in the fall suggested otherwise (Taylor et al, 2020), but that finding remains contested due to difficulties in accounting for changing tagging effort through time (Fordyce et al, 2020). In support of our assessment, a recent study of the monarch parasite, Ophryocystis elektroscirrha , has shown that nation‐wide prevalence has increased in the last 15 years, and that this increase is leading to considerable migratory losses and corresponding reductions in winter colony sizes (Majewska et al, 2021).…”

Section: Discussionmentioning

confidence: 99%

Opposing global change drivers counterbalance trends in breeding North American monarch butterflies

Crossley

Meehan

Moran

et al. 2022

Global Change Biology

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Many insects are in clear decline, with monarch butterflies (Danaus plexippus) drawing particular attention as a flagship species. It is well documented that, among migratory populations, numbers of overwintering monarchs have been falling across several decades, but trends among breeding monarchs are less clear. Here, we compile >135,000 monarch observations between 1993 and 2018 from the North American Butterfly Association's annual butterfly count to examine spatiotemporal patterns and potential drivers of adult monarch relative abundance trends across the entire breeding range in eastern and western North America. While the data revealed declines at some sites, particularly the US Northeast and parts of the Midwest, numbers in other areas, notably the US Southeast and Northwest, were unchanged or increasing, yielding a slightly positive overall trend across the species range. Negative impacts of agricultural glyphosate use appeared to be counterbalanced by positive effects of annual temperature, particularly in the US Midwest. Overall, our results suggest that population growth in summer is compensating for losses during the winter and that changing environmental variables have offsetting effects on mortality and/or reproduction. We suggest that density-dependent reproductive compensation when lower numbers arrive each spring is currently able to maintain relatively stable breeding monarch numbers. However, we caution against complacency since accelerating climate change may bring growing threats. In addition, increases of summer monarchs in some regions, especially in California and in the south, may reflect replacement of migratory with resident populations. Nonetheless, it is perhaps reassuring that ubiquitous downward trends in summer monarch abundance are not evident.

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Section: Discussionmentioning

confidence: 99%

Opposing global change drivers counterbalance trends in breeding North American monarch butterflies

Crossley

Meehan

Moran

et al. 2022

Global Change Biology

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“…In total, this evidence suggests, alongside the ongoing declines at winter colonies, that monarchs must be experiencing increasingly higher levels of mortality during their fall migration. Contrasting evidence of no change in the number of tagged monarchs returning to Mexico in the fall suggests otherwise (Taylor et al 2020), but that finding remains contested due to difficulties in accounting for changing tagging effort through time (Fordyce et al 2020). Our results argue that following the winter period, monarchs experience high population growth, perhaps facilitated by reduced intraspecific competition among larvae and generally favorable environmental conditions during breeding.…”

Section: Discussionmentioning

confidence: 50%

No broad decline of breeding monarch butterflies in North America: implications for conservation efforts

Davis

Crossley

Moran

et al. 2021

Preprint

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Many insects are in clear decline, with monarch butterflies (Danaus plexippus) drawing particular attention as a flagship species. Falling numbers of overwintering monarchs are well documented, but there has been debate regarding population trends of summer breeding populations. Here, we compile a series of long-term monarch monitoring datasets, some which are analyzed here for the first time, that reveal highly variable responses across the migratory geographic range, but no broad net decline in numbers of breeding monarchs. We also did not find evidence that sampling biased towards natural sites was masking declines at disturbed sites. Overall, our results suggest a robust resiliency in summer populations that thus far has allowed recovery from losses during the winter. Thus, monarchs may not require as much breeding habitat restoration as once thought, and focus should be on conserving the fall and spring migration.

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“…The “migration survival hypothesis” (MSH) suggests survival during the fall southward migration may be declining, because breeding populations estimated from summer surveys in the US have remained stable, yet overwintering numbers have declined 12 , 26 . Taylor et al 27 , based on monarch tagging data, argued migration success has not declined, but concerns exist about this conclusion 28 . Others have argued the lack of observed summer trends in the breeding region stems primarily from a sampling design that does not account for loss of monarchs in agricultural lands 24 , 29 – 31 .…”

Section: Introductionmentioning

confidence: 99%

Changes in landscape and climate in Mexico and Texas reveal small effects on migratory habitat of monarch butterflies (Danaus plexippus)

Diffendorfer,

Botello,

Drummond

et al. 2024

Sci Rep

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The decline of the iconic monarch butterfly (Danaus plexippus) in North America has motivated research on the impacts of land use and land cover (LULC) change and climate variability on monarch habitat and population dynamics. We investigated spring and fall trends in LULC, milkweed and nectar resources over a 20-year period, and ~ 30 years of climate variables in Mexico and Texas, U.S. This region supports spring breeding, and spring and fall migration during the annual life cycle of the monarch. We estimated a − 2.9% decline in milkweed in Texas, but little to no change in Mexico. Fall and spring nectar resources declined < 1% in both study extents. Vegetation greenness increased in the fall and spring in Mexico while the other climate variables did not change in both Mexico and Texas. Monarch habitat in Mexico and Texas appears relatively more intact than in the midwestern, agricultural landscapes of the U.S. Given the relatively modest observed changes in nectar and milkweed, the relatively stable climate conditions, and increased vegetation greenness in Mexico, it seems unlikely that habitat loss (quantity or quality) in Mexico and Texas has caused large declines in population size or survival during migration.

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Commentary: Evaluating the Migration Mortality Hypothesis Using Monarch Tagging Data

Cited by 4 publications

References 6 publications

Opposing global change drivers counterbalance trends in breeding North American monarch butterflies

Opposing global change drivers counterbalance trends in breeding North American monarch butterflies

No broad decline of breeding monarch butterflies in North America: implications for conservation efforts

Changes in landscape and climate in Mexico and Texas reveal small effects on migratory habitat of monarch butterflies (Danaus plexippus)

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