Although the actual counts taken this summer aren’t available yet, summer population censuses have routinely been mysteriously high compared to the low population sizes documented in the winter. Last winter’s Mexican population was 27 percent lower than the previous year.
The population data difference results from an increase in the number of volunteers counting and the number of sites monitored. They saw more butterflies because more people were looking in more areas. This past winter’s California population is only a fraction of the 1.2 million recorded in the late 1990;s and is 39 percent lower than the longterm average population size.
In 2017, California overwintering monarchs were slammed with bad storms at the beginning of spring migration. Professor James told me this affected migration and summer range numbers but to some extent low summer mortality helped.
I would say that the western population took a hit in late winter with the Feb/March storms in CA affecting overwintering colonies just as they were about to break up. I think many colonies were dispersed by the storms and it seems likely that much mortality occurred making the pioneer breeding population for 2017 much smaller than usual.
This poor start translated into lower numbers migrating into the PNW in May and June. The low numbers continued in some areas all summer but in other places summer breeding populations were very successful (low apparent immature stage mortality) and numbers have built up again. However, prolonged heatwaves in some areas have depressed populations... so its a mixed bag! but ultimately I'm expecting similar overwintering numbers in CA to last year.
We have evidence of problems at the beginning of spring’s northward migration from both California and Mexico winter colonies that should be noticed as reduced summer population sizes. But generally it isn’t. Why do numbers from summer counts appear large but overwintering numbers are historically low? Summer counts across the area of the U.S. and Canada used by the eastern subpopulation don’t mirror the winter declines in Mexico. Nor do anecdotal observations suggest fewer monarchs during summer in the western subpopulation over the years.
These discrepancies between summer and winter counts have been attributed to reduced summer breeding and survival due to loss of milkweed habitat in the summer ranges. Extensive agricultural use of glyphosate (Roundup) on crops with engineered-tolerance to the herbicide also kills milkweed and nectar plants, destroying monarch habitat.
But one scientist considered that perhaps the problem wasn’t a reduction in milkweed (the larval host plant) but something that happens during southward migration. Cornell University Professor Anurag Agrawal called the glyphosate-induced milkweed loss causation false “dogma” based on his statistical analyses. He said the problem “appears to take place after they take flight in the fall,” and blames monarch declines on conditions during migration south: sparse nectar sources in the fall, inclement weather, and habitat fragmentation.
This renegade stance stirred up controversy as Agrawal’s position regarding the importance of milkweed populations is an outlier. The accuracy of his analyses depends (in part) on the validity of the high summer counts of monarchs.
Professor John Pleasants along with world-renowned monarch scientists Karen Oberhauser and Lincoln Brower further investigated what might be the cause of these discrepancies — both the population size differences and the “it’s not due to milkweed” contention. They published their results in July: Interpreting surveys to estimate the size of the monarch butterfly population: Pitfalls and prospects.
This eastern subpopulation study focused on the north-central portion of the summer range because chemical analysis of monarch wings indicated this is where the majority of butterflies migrating to Mexico originates.
Their analyses looked at the number of milkweed in agricultural and non-agricultural areas and the change in the proportion of milkweeds in these habitats in the Midwest over the last two decades. Study methodology controlled for variables (e.g. “relative utilization of the milkweeds in those two types of habitats”) and data holes (e.g., “we only have milkweed density information for Iowa”).
After crunching all the numbers, the authors concluded that the summer population data Agrawal used was misleading and that summer numbers are not representative. They also found evidence against the hypothesis of increased mortality during fall migration.
Turns out the pertinent question isn’t “where do the monarchs go?” The mysterious difference is due to where people are looking for summer monarchs relative to where the monarchs are hanging out. More people are looking in parks and public areas and more monarchs are using these areas because the agricultural lands no longer offer enough suitable habitat.
The seeming discrepancy between higher numbers in the summer and drastic declines in the winter is due to two factors. One is what is repeatedly found to be the cause: less milkweed and nectar plant habitat in agricultural lands limiting the monarch populations. And the second is what we have done in response to the milkweed problem: increase milkweed and nectar plants in parks, other public lands, and have more people look for monarchs there. (The data supporting these findings is presented in Tables 1-3 of the study.)
The sad truth is both summer and winter population numbers are decreasing. Concentrating census taking in the areas where people intentionally offer monarch habitat gives the illusion of a robust summer population.
The significant positive relationship between overwintering hectares and three measures of milkweed availability, namely, the number of milkweed stems, milkweed resource, and annual egg production . . . provides direct evidence that the declining availability of milkweed resources is the driver of the decline in the size of the overwintering population . . . it would be surprising if the loss of 46% of the milkweeds on the landscape . . . did not have an effect.
On a more positive note, this study also shows that monarchs will shift to new suitable habitat when their former habitat is lost. Development, agriculture, and climate change are diminishing monarch habitat, but people are replacing it. The study’s results validate the importance of projects to increase milkweed and nectar plants, such as plantings along roads, among large-scale solar and wind turbine installations, hedgerows in cropland, and in parks and other public lands.
When we create new quality habitat, the monarchs move in. There’s no mystery about that.