The Curious Case of the Disappearing Butterflies
Why are monarch butterflies disappearing? They're part of a decades-long vanishing act that has scientists worried.
[Close-ups of monarch, wider views of monarchs flying, monarch clusters]
JESSICA WARE (Curator, Division of Invertebrate Zoology) [off-camera]: How do you keep tabs on a butterfly?
Monarch butterflies perform one of the most amazing feats in the natural world: some crossing a continent, traveling 3,000 miles from Canada to Mexico in a multi-generational migration and settling in the same special places year after year after year… even though these individual butterflies have never been there before.
Unlike most butterflies, monarchs fly high—riding thermals over 3,000 feet above the ground. And during migration, they can speed along at 5-8 miles an hour, maybe a little faster than most of us jog. So, it can be really hard to follow a single butterfly over their long trip.
And it wasn’t until the 1970s that American and Canadian scientists began to understand the full scope of this journey. To them, it was a mystery where monarchs went during the winter. But for thousands of years, people in Mexico, and likely in coastal California had known where the monarchs were spending the colder months.
Now, there’s a new kind of monarch disappearance mystery: Why are their numbers plummeting?
In the Western U.S., monarch numbers have dropped from as many as an estimated 10 million butterflies in the 1980s to less than 2,000 in 2020.
To find out what’s happening, researchers are harnessing the power of new technology and community scientists to track monarchs in California.
[Ware talks in Butterfly Vivarium]
DIRECTOR [off-camera]: Some people like me might have read The Very Hungry Caterpillar at some time in their life, but what’s the scientific distinction that makes a butterfly a butterfly?
WARE: So butterflies are in the order Lepidoptera. We thought for a long time butterflies and moths had a clear distinction. But then a lot of genetic information seemed to suggest that instead maybe there isn’t really like a clear scientific distinction between the two groups. We thought that maybe butterflies flew in the day, but it turns out there's day flying moths.
WARE: Lepidoptera tend to have scales on their wings. They’re all holometabolous, which means they have complete metamorphosis. There are other insects that have incomplete metamorphosis, like grasshoppers. Like, a baby grasshopper kind of just looks like what an adult grasshopper looks like.
[Video of butterfly life stages]
WARE: But for holometabolous insects, they have an egg that hatches to a caterpillar. The caterpillar goes through a series of instars, a series of molts to a larger and larger size.
[Ware to camera, indicating pupae]
WARE: Then they go into a pupal stage which is what these are, and in the pupal stage, there is a complete kind of internal and external rearrangement of their body, and then they emerge as an adult.
Because they’re turning into monarch soup inside that pupal case, it makes it particularly difficult to track an individual between life stages. You might be able to catch a slower moving caterpillar, but a tag wouldn’t transfer over to a winged adult’s body.
DIRECTOR (off-camera): And even when you can track one adult, you’d still only get part of the migration story, right?
WARE: For monarchs there's multiple generations that go varying distances along the journey from Canada down to Mexico.
WARE [off-camera]: The round-trip takes 3-5 generations to complete. Most of those will only live about 2-6 weeks, mating and laying eggs during the summer months. But one “super” generation is born in late summer, early fall.
DIRECTOR (off-camera): Ah, so it’s that super generation that does the longest leg of migration?
[Rippling cluster of monarchs, dozens of monarchs flying against a blue sky]
WARE [off-camera]: Yeah, and if you get a chance to see them gathered at an overwintering site, it’s truly stunning. From October through March, monarchs aggregate in particular spots. Eastern monarchs make their way to Mexico, and most western monarchs wing their way to spots on the California coast. We already know where many of these sites are based on decades of observational data.
[Ashley Fisher to camera]
ASHLEY FISHER (Monarch Overwintering Specialist, Xerces Society): I think it’s really special that they overwinter basically really in your face on the coast of California, with thousands of them hanging out together.
[Fisher and conservation biologist Isis Howard at Pismo Beach Monarch Butterfly Grove]
WARE [off-camera]: Conservation biologists Ashley Fisher and Isis Howard are researchers with the Xerces Society, a science-based nonprofit focused on conserving invertebrate species. They’re tracking monarchs during their stay in California to understand changes in populations over time, and what factors might be causing the big declines in their numbers.
[Isis Howard to camera]
ISIS HOWARD (Western Monarch Community Science, Xerces Society): What we see here are monarch butterflies that have traveled from Utah, Montana, Northern California, Oregon, Washington…
ASHLEY: Because of the decrease in daylight and temperature, it triggers something in them that makes them want to travel south, basically.
[Footage of monarch butterflies]
WARE [off-camera]: But not only do these genetic triggers tell monarchs that it’s time to get a move on, things start to change in the butterflies’ bodies. They’ll start to do things like store more fat, and slow down their aging. This allows them to live for 8 to 9 months—enough time to fly long distances and survive as adults through the winter.
[B-roll of monarchs]
FISHER: We are really interested and basically everyone wants to know where the monarchs are going during their migration. So, the best way we can do that is to track individuals. So, to track individuals, we're using a technique called radio telemetry. And it's actually been around for a really long time to track a lot bigger animals.
Historically, even the lighter tags have been, you know, 0.15 grams. And for a butterfly, even that is a lot because monarch butterflies, even though they're one of the larger species in North America, they only weigh less than a gram.
[ECU of tag in Fisher’s hand]
FISHER: These, on the other hand, are incredibly light. It's just a solar panel that is connected to the tail, which is basically its little antenna.
[Images of butterfly tagging]
FISHER: If we can better understand how individuals travel and they spend their time in overwintering habitat on the coast, we can better understand what kind of habitat they need to complete their overwintering cycle.
[B-roll of monarchs at Pismo Beach]
WARE [off-camera]: Monarchs look for pretty specific amenities during their winter vacation: dappled sunlight, high humidity, access to fresh water, and no freezing temps or strong winds.
[Howard + B-roll of butterfly clusters]
HOWARD: When monarch butterflies aggregate at the overwintering site, they often form what we call a cluster. When the ambient temperature starts to reach or exceed 55 degrees Fahrenheit, monarchs are then able to utilize their wing muscles and start flying.
They basically start rippling. And you start to see the flashes of orange and black. And then eventually one butterfly will make the courageous decision to- to leap out into flight, and then so many will end up following them.
[B-roll of monarch cluster]
WARE [off-camera]: While it seems like clustering might be about staying warm, it could also protect them from storms and from overheating. That’s important both so they don’t lose their fat stores too quickly, and to maintain their suspended aging.
FISHER: One of the factors we think is really important for determining where monarchs like to cluster in a grove is wind. We want to figure out what wind speed is a threshold that starts breaking up monarch butterfly clusters. So, there’s a graduate student from Cal Poly named Kyle Nessen, and he put up sensors all around our grove.
[Kyle Nessen working with equipment]
KYLE NESSEN (Graduate Student, Cal Poly): I'm going to check the batteries right now and just make sure everything is good to go. And then once I'm done, it will record the average wind speed and the max speed every minute for the next couple of months.
Right now I'm collecting actual wind measurements throughout the grove at different points. And then the second half of this will be coming here with a LiDAR drone and we’ll fly this whole area with really high resolution 3D data and recreate the forest structure in the computer.
[Nessen working + wide shots of grove + butterfly clusters]
WARE [off-camera]: That data could tell researchers things like where planting extra trees might create a better spot for monarchs to hang.
[Signage of monarch count numbers + animated GFX]
INT. BUTTERFLY VIVARIUM
WARE [off-camera]: In 2020, when the western monarch population dropped to 2,000, that seems like too much.
[Fisher and Howard on bench + B-roll of monarchs]
FISHER: They were close to what we would call an extinction vortex, which is where there's so few numbers that there's no way that they could mathematically recover. Luckily, we had a really big bounce back and really good years. If you compare it to the population numbers we saw in the eighties, we are still in a long term decline. The one good year does not necessarily make up for-
HOWARD: Dozens of-
FISHER: Decades. Yeah many decades of not good years.
[B-roll of monarchs on milkweed]
WARE [off-camera]: With a vulnerable population, protecting habitat can be key. Monarch caterpillars have evolved to dine almost exclusively on a plant called milkweed. Milkweed contains toxins called cardenolides that monarch caterpillars can sequester in their bodies and retain as adults, a defense against would-be predators.
They need these plants to survive. But due to habitat loss, milkweed breeding areas are in increasingly short supply. That, plus pesticide use, and climate change are all big threats facing these butterflies.
[Fisher + B-roll of development around monarchs]
FISHER: It's so great that we have monarchs in our communities, but it's also pretty unfortunate that where they choose to overwinter is on the coast of California pretty prime real estate.
HOWARD: The plain fact is that if we lose the overwintering sites along the California coast, we're going to lose the monarch migration as well.
[Ware to camera]
WARE: We know more about monarch populations perhaps than any other type of insect that's out there. And yet we still don't really have enough information to know the magnitude of what this kind of massive drops and increases and drops mean in terms of long term survival for monarchs. I think people should be worried.
But there are amazing efforts underway to keep tabs on monarch numbers. Isis works with volunteers for the Western Monarch Count, a community science-driven initiative that’s been collecting data since 1997.
EXT. PISMO BEACH MONARCH GROVE
[Fisher + Pismo/butterfly b-roll]
FISHER: We're putting a lot of brainpower and a lot of money into conserving habitat for the monarch butterflies to try to save their population. I think we care because it's just such an incredible phenomenon that a lot of people feel a lot of emotional connection to.
[Howard + pollinator b-roll]
HOWARD: The declines in monarch butterflies, especially on the West Coast, really represent and reflect the declines of other beneficial native insects and pollinators. So, our efforts to provide habitat and protect habitat for monarch butterflies is really benefiting more than just these butterflies.
Monarchs have touched most of our lives in one way or another. So, they really represent this symbol and icon that can not only bring people together over pollinator conservation, but biodiversity conservation at large.
INT. BUTTERFLY VIVARIUM
[Ware to camera]
WARE: As you might imagine, for people who live in the area where monarchs arrived in tens of thousands of individuals, there was a significant event that happened every year.
[Día de Muertos + butterfly b-roll]
WARE [off-camera]: The monarchs’ migration to the southwestern United States and Mexico coincides with Día de Muertos in early November. To some people, the butterflies represent the souls of deceased loved ones who come back to visit. Preserving a species can help us preserve cultural heritage, too.
So what can you do to help monarchs? Well, you can plant species of milkweed that are native to your area. You can plant nectaring flowers for adult butterflies to enjoy. You can reduce pesticide use and find a community science project in your area. You don’t need radio tags or high tech scanners to make a difference!
Before you go, if you’re a fan of underappreciated animals (and if you’ve made it this far that seems likely) we wanted to tell you about Sharks Unknown with Jasmin Graham. It’s another awesome animal series right here on Terra, which follows a diverse team of shark researchers as they study these amazing, yet often misunderstood creatures. There’s a link in the description, we think you’ll love it. Thanks!