The national attention on restoring monarch populations through increasing their habitat continues to rise. Recent analyses from the Monarch Conservation Partnership show that to attain a sustainable eastern monarch population, a minimum average population size of 6 hectares in the overwintering grounds must be achieved, and that habitat conservation efforts across the monarch’s range are necessary to reach this goal (Semmens et al. 2016, Oberhauser et al. 2016). Individuals and organizations are taking action to restore habitat. For example, over 60 organizations are now part of the Monarch Joint Venture and states across the U.S. are creating monarch and pollinator conservation plans. With such ambitious goals and limited resources, our habitat restoration efforts must be based on the best available science.
Increased attention on monarch conservation has led to increasing demand for habitat restoration guidance across the landscape. We know monarchs’ basic needs in habitat restoration: milkweed host plants and blooming nectar flowers. However, the specific elements of a “recipe for monarch success” (such as milkweed plants* per acre, diversity of flowering nectar plants, and other factors) vary from one landscape and geographic region to the next. While we do not have all the information needed to design the ideal monarch habitat for all land uses and regions, we cannot afford to wait until further research answers every remaining question. As researchers and conservationists work together to continue exploring effective conservation measures, we must continue forward momentum in habitat conservation activities using the the best available science.
One question that conservationists encounter when designing a restoration project is what native milkweed density and diversity will help monarchs the most. There is limited scientific guidance available on this topic, but some information is available that can help guide these decisions.
Recent research (Kasten et al., 2016) showed that the number of immature monarchs (eggs and larvae) in a given area increased as the number of milkweed plants increased, but only up to a point. This suggests a saturation effect, or a point at which adding more milkweed to a habitat will not necessarily increase the number of monarchs produced. When looking solely at the effect of milkweed density on monarch density, the tipping point found by Kasten et al. was 2,000 milkweed plants per acre, after which more milkweed did not result in more monarchs produced. Data defining the actual or realistic milkweed density on the landscape (and in different land uses) are limited, and current monitoring programs are addressing this data gap. However, the Monarch Conservation Science Partnership has used the best available science and expert opinion to classify an average expectation for milkweed density over a broad range of high quality habitats. This estimate, 200 milkweed plants per acre, was discussed in the context of what could be achieved over a large swath of sites across North America.
Here, we hope to shed some light on how these figures can be used to guide monarch conservation projects, communications, and measurements of success.
The values of 200 or 2000 plants per acre can be used as tools when designing a habitat restoration project. How we use these tools is important; accommodations relevant to the particular project goals or objectives may be needed. The milkweed density estimates presented above are not intended as a “gold standard” of habitat toward which we are collectively striving. While high quality habitat for monarch butterflies should include native milkweed, it is critical to think holistically about the overall quality of the habitat for monarchs and other species, and avoid focusing on a single habitat attribute like milkweed density.
Quality monarch habitat should have a diverse suite of milkweed and blooming nectar plant species, and avoid threats such as pesticide drift. It is also critical to think about seed mix design and short and long term management needs for these sites; balancing the ratio of forbs and grasses while also considering bloom time is critical to designing a successful restoration. While science may provide guidelines on what density of milkweed might encourage high monarch production, it is important to think of every restoration project from an ecosystem perspective, ensuring that it will persist long after the seed mix is planted.
These considerations do not directly answer the question of how many milkweed plants per acre is most appropriate. Milkweed and forb density targets for a high quality restoration project will vary based on environmental conditions and various restoration objectives. However, up to 2000 plants per acre, there is good reason to assume that more milkweed will produce more monarchs. After that, there may be diminishing returns and it may be best to focus on increased flowering plants per acre, species diversity or other habitat qualities.
The Monarch Butterfly Habitat Exchange program led by the Environmental Defense Fund and a wide variety of collaborators has created a Monarch Habitat Quantification Tool (HQT). This tool can assist landowners and managers in determining the current and potential quality of their site for monarch habitat.
Anyone can use the HQT for assessing the quality of their monarch habitat, but it is designed for assessment of land enrolled in the Monarch Butterfly Habitat Exchange, which is being piloted in Missouri, Iowa, Texas, and California. Currently available for the North Central and South Central United States, the HQT will help shift the dialogue around habitat restoration targets to determining how functional the acres are for monarchs, based on a number of habitat attributes and management practices. Learn more about the Monarch Butterfly Habitat Exchange and access the HQT on the Monarch Butterfly Habitat Exchange website.
Not only does the tool provide a viable assessment of monarch habitat, it will also be used to inform future conservation monitoring and management efforts. The assessment protocols of the HQT are compatible with the Monarch Conservation Science Partnership Integrated Monitoring Strategy, which is working to gather monarch and habitat data using a standardized set of protocols. This unified monitoring strategy is essential to our long term conservation planning for monarch butterflies, especially as it relates to the pending species status assessment and Endangered Species Act listing decision.
Moving forward with monarch conservation will require engagement across the U.S. landscape from all sectors, and there will continue to be important scientific discoveries about monarch needs. Evaluating our habitat restoration work based on the diverse components that make up the functionality of habitat for monarchs moves away from the one size fits all mentality in conservation targeting and helps inform future conservation discoveries.
*Note that we use the term plants here, even though some milkweed species, including common milkweed, produce many stems per plant, and it is impossible to determine which stems are connected underground.
Monarch Butterfly Habitat Exchange and Habitat Quantification Tool: edf.org/monarch
Monarch Joint Venture website: http://monarchjointventure.org/
Kasten, K., Stenoien, C., Caldwell, W., and Oberhauser, K. (2016), Can roadside habitat lead monarchs on a route to recovery?. Journal of Insect Conservation 20: 1047 - 1057. doi:10.1007/s10841-016-9938-y
Oberhauser, K., Wiederholt, R., Diffendorfer, J. E., Semmens, D., Ries, L., Thogmartin, W. E., Lopez-hoffman, L. And Semmens, B. (2017), A trans-national monarch butterfly population model and implications for regional conservation priorities. Ecological Entomology, 42: 51- 60. doi:10.1111/een.12351
Semmens, B., Semmens, D., Thogmartin, W., Wiederholt, R., López-Hoffman, L., Diffendorfer, J., Pleasants, J., Oberhauser, K., Taylor, O. (2016), Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus). Scientific Reports, 6: 23265. doi: 10.1038/srep23265.
The Monarch Joint Venture is a national partnership of federal and state agencies, non-governmental organizations, and academic programs working together to conserve the monarch butterfly migration. The content in this article does not necessarily reflect the positions of all Monarch Joint Venture partners. Header photo taken by Minnesota Native Landscapes.