- Monarch butterflies make an annual migration to warmer temperatures, with some populations traveling as far as 3,000 miles.
- Santa Cruz is one of the monarch overwintering sites in California.
- Monarchs have faced declines over the past 20 years, and 2018 marked the lowest count in 5 years for the California population.
- Factors such as loss of flowers, degradation of stopover sites along their migration, and the loss of overwintering habitat in Mexico contribute to their decline.
It was late December when Dan and I packed up our camera gear and set out for Santa Cruz to see the monarchs. Ever the worry-wart, I anxiously clasped and unclasped my hands the entire one-hour drive over. “What if we missed them?”, I asked, eyes wide with potential disappointment as we wound down Highway 17. It was our first time hearing about the incredible journey of this butterfly, and I was avid about witnessing it for myself despite it being towards the end of their migration.
Racing the sunset, we arrived in the Lighthouse Field parking lot and made our way down a field path leading to a small stand of eucalypts. Rounding the a corner on the trail, we were met with a small cluster of trees dripping in orange. Tiny flutters gave life to the bright masses, which were unmistakably made of hundreds of butterflies.
The Journey South
Most people, myself included, will react in disbelief upon hearing that a delicate orange butterfly migrates thousands of miles to escape cold temperatures. “But migration is for the birds!” they’ll think, when trying to picture an insect achieving such a feat. Sure enough, monarch butterflies are the only known butterfly species to migrate in two directions (north and south), and they happen to overwinter in Santa Cruz, just outside of the Bay Area.
This incredible journey is made by one specific generation of butterflies, termed the “migratory generation”. Cooling temperatures, shorter days, and the aging of milkweed, the primary source of food for monarch caterpillars, signal to the butterflies that it’s time to begin the journey south to escape cold temperatures.
There are two populations of migrating monarchs: one east of the Rocky Mountains, and one west. The eastern population, with individuals from places as far as southern Canada, make their journey down to Mexico where they congregate in the oyamel fir forests. For this population of butterflies, the migration can be as long as 3,000 miles. The western population makes their way over to California’s coast, overwintering in locations from Santa Cruz all the way down to San Diego.
Relying on navigational cues such as the Earth’s magnetic field and the position of the sun, the butterflies make their long flight. Upon reaching their destinations, the monarchs congregate together and form large clusters on the branches of trees. This allows the insects to retain enough warmth to survive the winter.
Trouble in Paradise
The Western Monarch Thanksgiving Count is a citizen-science project (a scientific study based on data collected by the public) that stands as the longest-running and most comprehensive count in for the monarchs overwintering in California. Citizen-scientists, biologists, and other professionals join forces for a 3-week long effort to count butterflies at multiple sites.
But lately, the numbers have been grim. The western population of monarch butterflies has been facing declines over the past 20 years, and 2018 marked the lowest count in 5 years. According to one source, 1.2 million butterflies were recorded on a survey two decades ago. The graph above shows the decreasing numbers, and in 2018 a survey using almost the same number of locations recorded just 300,000 butterflies.
A Complex Decline
Many sources have suggested that the loss of the monarch caterpillar’s only food source, milkweed, has been the major contributor of the monarch decline. However, a recent study on long-term monarch population trends conducted by a Cornell University academic suggest otherwise.
Anurag Agrawal, from Cornell’s Department of Ecology and Evolutionary Biology, wanted to take a closer look at all of the monarch population data. Pulling together multiple data sets, he worked with the intent to identify where the biggest hits to the population occurred along the monarch’s migratory route.
Surprisingly, he found that populations trends appear to be doing the best at points where they are relying on milkweed, meaning that milkweed’s decline may not be the only cause of the monarch population decline. Instead, the problem is happening at points during their migration and in parts of their overwintering sites in Mexico. The degradation of sites that they use to stopover along their migration, the lack of sites with nectar-rich flowers to fuel their migratory journey, and the loss of their overwintering forests in Mexico appear to be the main contributors to population loss.
That’s not to say that planting native milkweeds to help the butterflies will be a vain effort – as providing additional resources to the butterflies can never hurt. However, these efforts can be greatly amplified by other efforts as well. Planting habitats for monarchs to feed in your own backyard, only buying FSC-certified wood (Forest Stewardship Council certification, certifying that the wood has come from a responsibly-managed forest and supply chain), and volunteering for monarch counts are all other ways that we can help.
Public education, in particular, can also work to help the butterflies – for that allows us to approach the complex problem on a unified front. With more people aware, we can work together to save the monarchs before they vanish from our coast.