Monarch Butterflies

In colder climes, signs of spring can lift a heavy weight from a tired, frozen spirit. Trees bud, flowers bloom, and migratory species trickle in to announce the approach of summer. In the US, one of those species is a floppy orange gem: the monarch butterfly. These insects winter in amazingly dense clusters in Mexican forests before making a staggeringly long journey (one that spans multiple generations, in fact) to summer homes to the north.

But in recent years, the population of monarchs that stay east of the Rockies has dropped like a rock. Precise population numbers are difficult to come by, but estimates kept by the US Fish and Wildlife Service show about an 80 percent decline over the last decade.

Unfortunately, it appears that humans are responsible. The life cycle of the monarch is tightly linked with the milkweed plant. Females lay almost all of their eggs on these plants, and the larvae happily munch on them when they hatch. Milkweed tends to pop up in areas where the soil has been disturbed, like farm fields.

As with other weeds, farmers have long tried to keep milkweed from growing amidst (and competing with) their crops. But the introduction of genetically modified corn and soybeans that could survive being sprayed by the herbicide glyphosate (better known by its original trade name “Roundup”) suddenly gave farmers a more effective way to clear plants like milkweed.

These “Roundup-ready” crops are now absolutely dominant in the US, with milkweed becoming less and less common on cropland. And less milkweed means fewer monarch butterflies. An Iowa study estimated that milkweed in the state was cut by more than half between 1999 and 2010. Monarchs seemed to utilize milkweed on cropland more so than in other areas, so that actually translated to about an 80 percent drop in the estimated number of hatching monarch eggs. Other factors could be affecting the population, but this looks to be the big one.

With a population decline this stark, the obvious (and gloomy) question is whether it might soon disappear. A group of researchers led by University of California, San Diego’s Brice Semmens worked with the available data to project the risk of “quasi-extinction”—the point of no return for the population east of the Rockies.

Since butterflies are hard to count, the best measure of the population is the area of Mexican forest they blanket in the winter. But the researchers also used counts of monarch eggs in the US Midwest based on monitoring surveys and estimates of the loss of milkweed from farmland. Along with assessments of the uncertainty and variability in those numbers—key to the approach—they built a model of the monarch population. Using that model, they were able to forecast the chances that the population fell below different thresholds within the next 10 or 20 years.

Since the exact population threshold that is too small to recover is unknown, the researchers tried several. While the wintering population in the 1990s covered between 5 and 20 hectares (12 to 50 acres), the most recent number wasn’t much more than a single hectare. The quasi-extinction thresholds in this study ranged from just 0.01 hectares to 0.25 hectares.

The risk of hitting those thresholds within ten years was about 2 to 42 percent. Over 20 years, the risk grew to about 11 to 57 percent—that’s not exactly sunshine and rainbows. As the researchers put it, “Our target population exercise indicates a high level of quasi-extinction risk over relatively short time windows, even when assuming large starting population sizes, which highlights the peril that monarchs currently face. Given the population’s present low numbers, poor reproductive success by monarchs in future breeding seasons due to weather conditions and reduced breeding habitat, followed by catastrophic mortality while overwintering in Mexico, could bring the monarch migration to the brink of extinction.”

As part of a plan for protecting honey bees and other pollinating insects put out last May, the Obama administration set a goal for the monarch population to cover six hectares in the winter by 2020. How can we get there? Encouraging milkweed to grow on non-agricultural land would help. Whenever seeds are spread—on federal land, private land in conservation programs, or roadsides—we can make sure milkweed is in the mix. And transportation departments could avoid spraying herbicides that kill milkweed along those roadsides.

Anything that makes sure monarch butterflies can find a place to eat would help.

Scientific Reports, 2016. DOI: 10.1038/srep23265


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