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Summer populations are quite correlated with winter populations

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Summer populations are quite correlated with winter populations

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Saunder et al., winter abundances of monarchs were predicted by variation in the summer breeding populations as well as floral nectar availability and forest patch sizes.

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Saunder et al., winter abundances of monarchs were predicted by variation in the summer breeding populations as well as floral nectar availability and forest patch sizes.

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1.4 billion stems of Milkweed need to be restored in the midwest in order to return the Monarch numbers to a mean of 6 hectares(10-50 million Monarchs) of overwintering habitat

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a tagged butterfly from the Midwest is about 4 times more likely to be recovered than a tagged butterfly from the Northeast

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This paper supports the Milkweed Limitation hypothesis, stating that the milkweed lost due to increased herbicide in agricultural fields and land use by humans is the limiting factor for Monarch population growth.

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The correlation between migration success and NDVI suggests that nectar availability in this region affects migration success of all butterflies, no matter their point of origin

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Table 2 Migration Mortality Hypothesis.PNG

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“In 1986: “Herbicides are a major threat to monarchs, because the butterflies need weeds and wild- flowers to survive.” In 1990 on deforestation in Mexico: “But until it is clear that cutting has stopped, there is danger of a catastrophe that’s going to spell the end of monarch butterflies in eastern North America.” In 2002 on monarch mortality caused by a winter storm in Mexico: “It was really macabre, I’ve been going down there for 25 years, and I’ve never seen anything like it.” In 2006: “The biggest threat to the migration is the steady attrition of forests because of illegal logging.” In 2011 on use of herbicides in agricultural fields: “It kills everything. It’s like absolute Armageddon for biodiversity over a huge area.” And in 2017 on climate change impacts on monarch butterflies: “It’s hard to know what’s going to happen, but I don’t think it will be good.”

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These declines are thought to be a combination of several factors coming together.

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Migration success has not decreased over time

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100 Year drought in Texas

Monarch and other butterflies are experiencing floral nectar limitation.

Small scale farms being abandoned

The use of pesticides (neonicotinoids) and herbicides.

Ophryocystis elektroscirrha.

The introduction of non native milkweed.

An increased levels of disease in the population.

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The conservation effort that would have the greatest effect on Monarch numbers would be to increase milkweed populations

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Overall, there has been a general decline in the populations. As years have passed, the butterflies/hectare have decreased significantly and proportionally to some of the major causes listed below. The circled section on Figure 1 shows the time period of data that was explored in this paper.

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Boyle et al.’s key conclusion is that the monarch decline began over 60 y ago and was concordant with the abandonment of small-scale farms (which served as a reservoir for milkweeds). This result is concordant with the long-term decline of monarchs in California supported by observational data remarkably collected by a single researcher over a 45-y period (13).

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Overwintering population numbers are correlated with late summer population size

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Migration Success was not signifigantly correlated with overwintering population size.

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Evaluating the Migration Mortality Hypothesis Using Monarch Tagging Data

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Evaluating the Migration Mortality Hypothesis Using Monarch Tagging Data

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Advances in understanding the long-term population decline of monarch butterflies

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Advances in understanding the long-term population decline of monarch butterflies