10 minute read
From Data to Action: Mapping Milkweed for Monarch Recovery
By Sarah Cusser, Ph.D., Terrestrial Invertebrate Conservation Ecologist
Western monarch butterflies (Danaus plexippus) are beloved for their beauty, extraordinary migration feats, ecological importance, and cultural significance. Despite their special place in our hearts, it’s not easy being a monarch out in the world. Western monarchs have dropped in population size by 99% in the last 20 years. While researchers don’t know exactly why this has happened, we do think that the monarch’s dependent relationship on milkweed (Asclepias spp.) may be the key to figuring it out.
In 2023, ecologists at Santa Barbara Botanic Garden partnered with the Los Padres National Forest and the California Department of Fish and Wildlife to check in on four of the species of milkweed that grow in the Santa Barbara area. We have two years to figure out where milkweeds are growing in the forest and how they are doing. But with one year of sampling behind us, and a lot of unexpected road closures, we found ourselves needing to pivot.
Got Milkweed?
To understand the journey of our research plan, first you need to understand some basics about the monarchs’ lifecycle as it relates to milkweed. Monarch butterflies will only lay their eggs on milkweed. Milkweed leaves contain compounds that, while poisonous to most other animals, are tasty to developing monarchs and make the caterpillars (and eventually the adults) unpalatable to predators. How’s that for a superpower? And it’s all thanks to milkweed — the singular host for this iconic butterfly, which makes it possible for them to avoid getting eaten!
The Monarch Migration Cycle
The migration of the western monarch is truly epic. In the fall, monarchs arrive in groves along the California coast where they cluster together to conserve energy and camp out for the coldest part of the year. Come spring, the monarchs mate and then set out in search of early emerging milkweed species, where they lay their eggs. Once the year’s first generation has metamorphized in these edible nurseries, the butterflies begin their migration northward. Though we don’t know exactly how or why, this is a generational migration. That means, by the end of the summer, the great-great-great-grandchildren of the butterflies born here in Santa Barbara will eventually make it to Oregon, Washington, and Canada. Come fall, adult monarchs, fat with northern nectar, will gear up to make the epic flight back to groves along the California coast, including Ellwood Mesa here in Santa Barbara.
Early-Season Milkweed Is a Must, But Is There Enough?
The first part of the migratory cycle can be a doozie. When western monarch moms leave the protection of the coastal groves in February and March, they need to find early-season milkweed plants with enough leaves to support their young. This can be difficult so early in the year. Most milkweed species don’t emerge, let alone get large enough to support hungry caterpillars, until much later in the summer. Only a few species of milkweed start growing early enough to be helpful to western monarch moms in the early spring. These include California milkweed (Asclepias californica), woollypod milkweed (A. eriocarpa), desert milkweed (A. erosa), and woolly milkweed (A. vestita). If there aren’t enough milkweeds to serve early spring monarch moms, it has a cascading impact on the exponential growth of monarch populations over the course of a year. The size of the first clutch of early spring butterflies largely determines monarch breeding success for the entire year. Put another way, if this one step in the journey goes poorly, the whole year’s a bust. So, where are these critical, elusive native milkweeds and how well are they serving butterflies? This is where the Garden comes in.
The Garden’s location, directly between coastal overwintering sites and the first generation’s milkweed habitat, is an ideal spot to investigate how the butterflies are making this first tremulous step in their long migratory journey. Like all conservation efforts, we need basic information about how the plants are doing before we can help.
Hunting for Milkweed Populations
To figure out where these species of milkweed are, ecologists here at the Garden first gathered as much information as we could from the milkweed scouts of yesteryear. Using publicly available online data from Western Monarch Milkweed Mapper, Calflora, and iNaturalist (along with personal communication with friends and collaborators), we amassed nearly 1,000 potential milkweed populations to visit. During summer 2023, with historic points in hand, we surveyed 286 miles (460 kilometers) of trails and roads in search of milkweed populations. That’s roughly the width of Iceland!
Not All Research Goes According To Plan
Due to extreme weather events during winter 2022, many of the forest roads throughout the Los Padres National Forest were closed. Combined with the limitations of our mere human bodies, we were only able to make it to about one-third of the historic points on our list. While we did manage to locate 167 milkweed populations, recording detailed information about their location, habitat type, slope, aspect, and other environmental variables, this was just a drop in the bucket of what we wanted to accomplish during our two-year project.
We needed a better-informed plan. We needed the data we collected in 2023 to help us narrow our focus on the more-than-600 remaining historic points. We needed to focus on only the most promising and accessible populations. We needed species distribution modeling!
In late 2023, we started talking with University of California, Santa Barbara, students in the master of environmental data science degree program at the Bren School of Environmental Science & Management. Thanks to all our walking and driving, we were able to provide the students with information on the environmental variables important to the 167 early-season milkweed populations we’d found so far. With this data, students Sam Muir, Amanda Herbst, Melissa Widas, and Anna Ramji are working currently to produce two types of maps to inform our second year of survey efforts.
First, the group is using species distribution models to figure out the exact habitat types that milkweeds prefer. Specifically, they are investigating slopes, aspects, elevations, and vegetation communities where we would expect to find milkweed plants. With this, the team can look for those types of habitats in parts of the forest we haven’t visited yet. In this way, they are producing a heat map that predicts the likelihood of finding milkweed at any given location, anywhere throughout the national forest. Second, students are considering obstacles like road and trail closures, steep treacherous slopes, and private property boundaries to produce an accessibility map. By combining the species distribution and accessibility maps, the group is creating what we’re calling a “priority index,” which will be essential in literally prioritizing our 2024 surveys. The priority index will be key to completing our research.
Once complete, the Garden will be sharing these models via a user-friendly, interactive web application on its website. This tool will allow website visitors to explore both the species distribution and accessibility maps to find milkweed populations near them!
We believe that all data collected as part of this project will provide significant information on the distribution and quality of early-season breeding habitat for the western monarch butterflies in Los Padres National Forest. Not only will this inform forest management, but it will also help protect early breeding habitat and restoration efforts. The project is a meaningful step toward western monarch population recovery, and we’re excited to see it through for the beautiful, cultural, and ecological significance these creatures provide.
Get Involved To Help Monarchs
If you want to help early-season western monarch (Danaus plexippus) populations, record your milkweed sightings on iNaturalist. That includes California milkweed (Asclepias californica), woollypod milkweed (A. eriocarpa), desert milkweed (A. erosa), and woolly milkweed (A. vestita). Bonus points if you find monarch eggs or larvae snacking on the leaves and stems!
You can also plant native milkweeds and nectar plants in your yard to help western monarchs. While California milkweed and woollypod milkweed can be hard to find in some nurseries, the Garden Nursery tries to keep them in stock. Also, there’s currently a major push to increase the availability of these seeds and plugs. Even though it comes up a little later in the season, narrowleaf milkweed (A. fascicularis) can also be a good plant to consider.
We recommend that you avoid planting the nonnative milkweed (tropical milkweed, A. curassavica), which does not die back during the winter, and unfortunately provides great, year-round habitat for the parasite Ophryocystis elektroscirrha (OE) that can negatively impact monarch larvae.
If you live within 1 mile (1.6 kilometers) of a monarch overwintering site, like Ellwood Mesa or More Mesa, we recommend that you avoid planting milkweed at all. Because milkweeds do not historically occur along the coast, their presence may confuse overwintering monarchs and cause the monarchs to breed during our California winter and settle here permanently, instead of maintaining their migratory life cycle. The transition from migratory to resident butterflies has been seen in Florida and the Caribbean, as well as parts of Australia, where monarchs have been introduced. The risk in having a large winter breeding population is that the resident, winter breeding monarchs typically become a breeding ground for parasites and disease.
If you live close to an overwintering site, consider introducing nectar plants for adult butterflies. Good choices for adult monarchs looking for a snack include many of California’s native plants: native buckthorn (Ceanothus spp.), coast sunflower (Encelia spp.), buckwheat (Eriogonum spp.), California poppies (Eschscholzia spp.), goldenbrush (Hazardia spp.), California horkelia (Horkelia spp.), goldenweed (Isocoma spp.), goldfields (Lasthenia spp.), and vervain (Verbena spp.), among others.
