Scientists have three main theories for the persistent decline in the numbers of monarch butterflies, according to a Forbes article written by Jeff McMahon:
• loss of their food supply to the herbicide glyphosate,
• hazards along their four-generation-long migration from the Midwest and Canada to the mountains of Mexico, or
• effects of climate change.
“We accept that there’s probably some combination of all of these,” said Elise Zipkin, an ecologist from Michigan State University, “and we really want to understand which are having the most effect and what we might be seeing in the future.”
Zipkin’s team collected data covering 30 years of Monarch migrations and found an initial decline that correlated with the planting of Roundup Ready crops in Midwestern fields in the 1990s. The crops were genetically engineered to survive the herbicide Roundup, the brand name for glyphosate. The glyphosate killed the milkweed plants, which monarch caterpillars depend upon as their exclusive food source, between crop rows and at field edges.
“We see the steepest time of the decline in this early part (in the 1990s) when this Roundup Ready use was really ramping up,” Zipkin said in a recent lecture before the National Academy of Sciences, Engineering and Medicine.
But when glyphosate use plateaued early in this century, monarch numbers continued to decline, suggesting another factor had come into play.
“We found strong support for the climate-change hypothesis,” she said, “from about 2004 to almost the present time.”
From 2004 to 2018, monarchs declined as global warming produced more pronounced effects.
Scientists believe warming could be good for monarchs if it warms the cooler parts of their range, such as Minnesota and Michigan. It could be very bad for them if warms the Southern part of their range, such as Texas and Mexico.
Zipkin’s team found conditions worsened for monarchs at both ends of their journey—the southern regions did get warmer during that decade, but the northern regions did not. The northern regions, in fact, got cooler.
“We predict more northern areas are going to be warming quickly, but what actually happened at this time period—the southern part got warmer in the summer and the northern part actually was a little bit cooler on average.”
With climate change expected to worsen, Zipkin and her team set out to discover what will happen next.
“It’s not enough to just understand monarch trajectories. What we really want to do is we want to figure out what we can do to mitigate and reverse these declines.”
So they used their collected data to model monarch populations across various climate scenarios in the future.
Monarchs do best when greenhouse-gas emissions are kept to a minimum, they projected, but even the most optimistic climate outlook includes risk: “What we found is even in the short term under the lowest emissions we have on average about two to three years in the next 20 years that we expect the population to reach a size smaller than it’s ever been before,” she said.
“So that’s something that we want to think about—even though on average it might be okay, there could be years where it drops really low, and that can have some potentially bad consequences.”
Scientists have identified a few areas of Canada and the Midwest—Central Illinois, Indiana and especially Ohio—where conditions for monarchs should improve under the gentlest climate scenario. Conservation efforts should be concentrated in those areas, Zipkin said, to boost monarch populations before they pass through more perilous regions.
Under high-emission climate scenarios conditions worsen for monarchs everywhere.