The International Union for Conservation of Nature placed the monarch butterfly on its Red List of Threatened Species as endangered in July, but despite decades of declining numbers, the delicate insect’s population rebounded in 2021.
Invertebrates aren’t often the first animals to garner people’s sympathy when their populations dwindle, but conservation awareness of pollinators is gaining mainstream popularity. In Colorado, the summer destination of two monarch populations’ migration routes, people may be latching onto the idea of growing native plants over grass lawns, avoiding pesticides and supporting native birds and invertebrates.
The monarch butterfly, or Danaus plexippus, is a charismatic poster child of sorts for pollinating invertebrates, said Amanda Kuenzi, community science director at Mountain Studies Institute and chairwoman for the southwest chapter of the Colorado Native Plants Society.
“They’re big and beautiful and recognizable. But what it really comes down to is that we are in a pollinator crisis globally,” she said.
The monarch butterfly population has been in steep decline for decades, falling as much as 90% in the last 30 to 40 years, she said. The IUCN in Switzerland attributed its endangered designation of the monarch butterfly to habitat destruction and climate change.
The monarch is not currently listed as an endangered species according to the United States Endangered Species Act – meaning it doesn’t qualify it for federal protection or funding – but it is scheduled to be reassessed in 2024 by the U.S. Fish and Wildlife Service.
In 2020, the Fish and Wildlife Service announced that “listing the monarch butterfly under the Endangered Species Act is warranted but precluded,” according to the Butterfly Pavilion in Denver.
Monarchs that overwinter in California and migrate to Colorado for the summer increased from a population of just 2,000 in 2020 to 200,000 in 2021, according to data from the Colorado Butterfly Monitoring Network organized by the Butterfly Pavilion.
Shiran Hershcovich, lepidopterist at the Butterfly Pavilion, said it is too early to say whether the monarch population has caught a break, and although the jump in population size recorded last year is exciting, its cause remains a puzzle to scientists.
“Booms and busts are not uncommon in the invertebrate world,” she said. “Many insect species will have big kinds of cycles in their populations. But not as dramatic as what we have seen with the monarch last year.”
The rise in population size hasn’t returned the monarch butterfly to its former numbers, but Hershcovich considers the development a hopeful sign.
“We are very interested in what the data will show this year. In Colorado, we reported a 180% increase in monarch sightings. Which is consistent with reports from across the country, too,” she said.
Scientists have several leading theories as to how the monarchs might be making a comeback.
For starters, environmental pressures such as pollution on native ecosystems and environments could have been reduced by slowed human activity during the COVID-19 pandemic.
Another theory is that Colorado’s fire season started late this year and the state received a healthy amount of moisture, at least by milkweed standards. Milkweed is a perennial flowering plant that monarch larvae, or caterpillars, almost exclusively feed on, Hershcovich said. And if it’s burned to ash, it can’t be eaten by butterfly larvae.
The smoke caused by wildfires can also threaten monarch butterflies.
“Butterflies are really sensitive to environmental cues. So fire seasons can also be really difficult for butterfly populations,” she said. “Butterflies also sense the world differently than we do. So they can pick up a lot of chemical cues from the environment that us humans are not as sensitive to.”
Heavy smoke can upset or confuse a butterfly’s senses, so scientists theorize wildfires could lead to drastic effects on a species’ population.
Milkweed is also a riparian species, meaning it requires a lot of moisture to grow and thrive, Hershcovich said.
“Caterpillars are often very, very picky and will only eat one very specific plant or one very specific group of plants, like is the case of monarchs and milkweed,” she said.
If milkweed isn’t present in a monarch’s environment, the butterfly won’t bother to lay eggs because the next generation won’t be successful.
But Hershcovich said humans are also doing their part to help the monarch population.
“Pollinators in general are kind of having a buzz at the moment,” she said. “They’ve been in the spotlight a lot more. There’s been a lot more advocacy for pollinator health. So there’s been a lot more people involved than ever in invertebrate conservation and monarch conservation.”
People have taken to their own backyards to support monarchs and other pollinators by planting milkweed and other native flowering plants, she said.
But, she warned, it’s too early to tell for sure how Colorado’s monarch population will fare in the future.
Kuenzi said she has spotted more milkweeds “than ever” in the Durango area than in past years.
“I feel like people are getting the message,” she said. “I don’t know if it’s intentionally planted or who is doing it or what, but I have noticed that there’s milkweed plants popping up along the river trail.”
She said local milkweed would still fare better with a wetter habitat. But anywhere there is runoff, such along the sides of a river or a road where water will flow downhill, makes good habitat for the plant.
Still, Kuenzi warned that people shouldn’t adopt the misconception that milkweed is the only plant that the monarch species needs. After their larval stage, they require a diverse plethora of wildflowers to take nectar from.
She said the increased attention for monarch conservation is good for all pollinators – a rising tide raises all ships.
But Hershcovich cautioned that although prospects for monarch butterflies appear optimistic, there isn’t enough evidence to say definitively that they are truly recovering from years of population decline.
“So is this a trend? Will we continue to see the populations go up? Was this a one-off event? Will we see a decrease? It’s a little too early to tell pattern-wise what is going on,” she said.
She said Colorado is a unique place for monarchs because it is the epicenter of two monarch “communities” with separate migration routes.
“The monarch populations are divided into two groups: The eastern monarch and the western monarch,” she said. “The eastern monarch is the largest of the two. It spends its winters down in Mexico. So it makes that long journey south to spend the cold months there.
“The western monarch will spend its winters on the California coast. Colorado is actually the place where those two monarch populations are divided,” she said. “So any monarchs – and this is more of a general rule – but most monarchs east of the Rockies belong to the eastern population of monarchs and will migrate south for the winter. And then monarchs west of the Rockies belong to the western monarch population and will spend their winters in California.”