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Course:EOSC311/2025/Milkweed Habitats for Monarch Butterflies in North America

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Monarch butterflies rely on milkweed to survive, however due to increasing temperatures from climate change and other growth factors, milkweed is becoming toxic as a food source for the butterflies, further threatening an already limited species.

Monarch Butterflies

Monarch butterfly

Monarch butterflies, recognized for their distinctive bright orange and black wings, are a beautiful pollinating species at risk of extinction. Their annual migration is stunning to watch, with thousands of people travelling to see it firsthand. Each year, they migrate thousands of kilometers across North America, traveling between summer breeding grounds in Canada and the United States and overwintering sites in central Mexico.[1] This migration is incredibly unique and complex, with travel to up 4,000 kilometers completed over multiple generations.[2][3] Each stage of their journey depends on environmental cues, temperature changes, and the availability of critical resources, such as milkweed. Unfortunately, monarch populations have been steadily declining in recent years, especially in the West, driven by a reduction in milkweed availability from a combination of habitat loss and changing climate conditions.[4] The destruction of habitat has resulted in a drastic decrease in the amount of available milkweed for monarchs, a plant absolutely crucial to their survival. In addition, climate change has resulted in shifts in seasons, as well as extreme weather events such as drought, wildfires, impacting monarch life cycles and migration[5]. Monarchs rely exclusively on milkweed (Asclepias), which they use as host plants for reproduction, laying eggs, and as a food source[6], and without milkweed, there are no monarchs. In addition to habitat loss, researchers have more recently identified that changing climate conditions are chemically altering the plant itself. Rising global temperatures, altered seasonal patterns, and increasing carbon dioxide levels are all influencing the growth, chemistry, and toxicity of milkweed. These environmental changes can lead to elevated toxic levels of cardenolides or a decline in nutritional value, potentially affecting monarch caterpillar development and survival[3]. Even when milkweed is present, it may no longer be a safe food source for the monarchs who depend on it, adding additional challenges to monarch conservation efforts.

North American Monarch Migration Map

Milkweed

Milkweed (Asclepias) is diverse, with over 70 species native to North America, each adapted to different climates and ecosystems.[7] While there is variation among species, most milkweeds have similar growth requirements. Milkweeds tend to prefer well-drained soils, and thrive in areas like roadsides, prairies, and fields. Although some species tolerate light shade, most milkweeds require at least 6–8 hours of direct sunlight daily to grow enough to support monarch development[8]. As monarch caterpillars feed singularly on milkweed leaves, the quality and abundance of this plant are crucial to their survival. In terms of monarch breeding, the most important regions across North America are the Midwest and Great Plains where milkweed is abundant during the spring and summer laying seasons. Western monarchs rely on milkweed in California and surrounding areas, and other populations breed along the Gulf Coast and southern Atlantic regions.[9]

Soil Composition

Milkweed distribution across North America is closely tied to regional geological history. Long-term Earth processes, from retreating glaciers to uplifting mountain ranges to forming volcanic bedrock, have created different soil mineral compositions, drainage capacities, and climates, contributing to plant and insect biodiversity, including monarchs.[10] Soil forms through erosion and weathering of existing rock material, and the resulting mineral composition determines the physical and chemical environment in which plants grow. Soils derived from limestone, shale, or sedimentary rock tend to be higher in calcium and carbonates, supporting deep roots and stable moisture retention. In contrast, igneous and metamorphic rocks such as granite or serpentine weather into soils that are often low in nutrients, and high in magnesium, iron, and trace metals like nickel and chromium.[11] These minerals can stress plants, often resulting in elevated defense responses such as increased cardenolide production in milkweed.[12]

Swamp Milkweed Asclepias incarnata Flowers

These geological variations help explain not only where milkweed grows but also how it functions at the cellular level. Differences in mineral composition affect plant growth, defense chemistry, and water retention, while regional location greatly impacts water availability[8]. Shallow soils over bedrock or well-drained sandy substrates may dry out quickly, stressing milkweed during heat waves or droughts. Alternately, floodplain or glacial lake bed soils rich with silt and organic matter retain moisture, offering more stable growth conditions. Serpentine soils from ultramafic rock are low in nutrients like calcium and nitrogen, but high in heavy metals like nickel and chromium, potentially stressing and stunting plant growth. Glacial till and sedimentary basins in the Midwest and North East America contain loamy, nutrient-rich soils with nitrogen, calcium, and organic matter, ideal for milkweed growth. Alternately, in California and the West, volcanic and serpentine landscapes create well-drained soil rich in magnesium and iron, supporting drought tolerant milkweed.[13] These different soil types determine which milkweed species can grow in a region, as well as how they respond to environmental stress, influencing their nutritional quality and chemical defenses and by extension monarch populations. Rather than looking only at milkweed itself, researchers can benefit from examining regional geological differences and soil compositions when determining which milkweed species should be planted in conservation and restoration efforts. Identifying the parent rock, mineral composition, and drainage characteristics of a site can help predict whether a given milkweed species will do well, and how its chemistry may affect monarch survival.

Table 1. Comparisons of Different North American Milkweed Species
Milkweed Species Location Cardenolides Soil Composition Common Elements Climate threats Uses & Significance
Common Milkweed

(A. syriaca)[14]

Eastern & Central North America Moderate to High. Beneficial to monarch defense Loamy clay rich, glacial till/sedimentary basins Nitrogen, potassium, organic carbon Drought & heat stress (increases toxicity), elevated CO2 (lowers leaf nitrogen) Primary host for egg laying & caterpillar development, strong defense from cardenolides, grows well in disturbed fields and roadsides
Showy Milkweed

(A. speciosa)[15]

Western U.S. (CA, OR, WA, ID) Low to Moderate. Lower cardenolide content, usable Sand/gravel soils, well drained glacial valleys Silica, calcium, trace iron Drought & heat stress, wildfires, habitat fragmentation Western host plant, lower toxicity, used in restoration, good with dry open land
Swamp Milkweed

(A. incarnata)[16]

Eastern wetlands, Great Lakes, Southern Canada Low. Not ideal for chemical defense Clay/silt soils, floodplains/glacial lake beds Nitrogen, iron, organic matter Wetland drying, extreme flooding High caterpillar survival, less cardenolide defense, prefers moist habitats (such as along rivers)
Butterfly Milkweed

(A. tuberosa)[17]

Eastern & Central U.S. uplands Very low. Not ideal for larval development Sandy/rocky uplands/prairies Calcium carbonate-rich (from limestone), high magnesium elevated CO2 (lower nutritional quality), dominant milkweed strain Not a major larvae host plant, essential source for adults, does well in poor soils and sunny grasslands
California Milkweed

(A. californica)[18]

Southern & Central California Low to Moderate (poorly studied). Moderate defense, not well-researched Dry hillsides, volcanic, chaparral, serpentine Magnesium, iron, nickel, low calcium Drought, wildfire, soil erosion & habitat loss, urbinization Western host, prefers rocky environments, less studied (toxicity & habitat)
Green Antelopehorn

(A. viridis)[19]

South-Central U.S. (TX, OK, KS) High. Rich in cardenolides Clay/loam, prairies/open meadows Calcium carbonate, nitrogen, potassium Spring drought, sensitive to grazing, prairie loss Early season host, high cardenolide defense levels, aligns with northward migration

Climate Impacts

In order for monarchs to progress through their life cycle, they have to be able to access milkweed in the right stages at the right times. In North America, monarchs begin migrating north in early spring from overwintering sites in central Mexico and coastal California.[2] To successfully reproduce, they need young freshly growing milkweed along their migration routes at the right time. However, climate change is shifting blooming patterns, potentially resulting in a difference in when monarchs arrive in a region and when milkweed is at the right developmental stage for egg laying and feeding. Warmer spring temperatures are leading to earlier growth of milkweed in some regions, but monarchs may not adapt their migration patterns in time. This can result in monarchs arriving too late to take advantage of the growth or too early when there isn’t any milkweed around yet.[4] Both scenarios can reduce reproductive success and caterpillar survival. Additionally, some milkweed species respond differently to climate cues. For example, Asclepias speciosa in the western U.S. may bloom later in drought years, while A. syriaca in the Midwest can sprout early under warmer conditions. These interspecies differences further complicate monarch conservation and restoration efforts across the continent.

Cardenolides

Cardenolides are chemical compounds found in several plant species, including milkweeds (Asclepias). These compounds are primarily composed of carbon, hydrogen, and oxygen, and their production in plants is influenced by nutrients in the soil, such as nitrogen, magnesium, phosphorus, and trace metals, as well as by environmental conditions.[20] Cardenolides are a chemical defense that helps deter insects and animals from eating the plant by making the plant bitter, toxic, or even lethal in high enough doses. While this deters most herbivores, some specialist insects, such as monarch caterpillars have evolved to tolerate and even use cardenolides. As they feed on milkweed, monarchs store the consumed toxins in their body, making themselves toxic and bitter to nearby predators like birds.[21] Understanding these background relationships between soil chemistry, environmental factors, and milkweed toxicity is important for assessing threats to monarch populations.

Soil Composition

While they don’t directly contain elements like nitrogen or calcium, cardenolide production in milkweed depends on the plant’s access to nutrients in the soil.[22] Nitrogen is essential for growth and defense. When nitrogen is high, milkweed focuses more on growing leaves and less on cardenolides, increasing its nutritional value and weakening its defenses. Magnesium helps the plant produce energy and maintain function, especially under stress. Phosphorus plays an important part in energy transfer in plants, helping fuel the production of cardenolides and other chemical defenses, especially when the plant is stressed. Trace metals such as nickel and iron can trigger stress responses in plants, sometimes leading to an increase in defensive toxins like cardenolides.[10][23]

Climate Factors

In addition to soil mineral composition, climate factors also influence milkweed chemistry. Drought results in a reduced amount of water and an increased cardenolide concentration in its leaves, likely part of a stress response.[24] While this may help protect the plant, too high levels of toxicity can harm monarch larvae. Heat stress from prolonged high temperatures can affect cardenolide composition and distribution in milkweed tissue, potentially making milkweed less edible and more toxic to monarchs, even if the plants survive high heat[8]. Elevated CO2 levels in the atmosphere can lead to faster plant growth, often at the cost of reduced nutrition due to reduced nitrogen and protein content in milkweed leaves, meaning that even if milkweed is growing faster or looks healthier, it may actually provide less nutrition to caterpillars[23]. Some studies show that milkweed under high CO2 levels produced fewer or weaker cardenolides (CITE), while others suggest it depends on the species (CITE).

Impact on Monarchs

Monarchs use cardenolides stored in their tissues as a chemical defense strategy. As monarch caterpillars feed on milkweed leaves, they absorb and store cardenolides in their tissues as a chemical defense strategy, making both the caterpillars and adult butterflies toxic.[25] Over time, monarchs have evolved to withstand cardenolides levels that would be lethal to most other insects. However, excessive levels can still be harmful.[26] When milkweed plants produce extremely high concentrations of cardenolides, especially under stressors like drought or heat, the toxins can negatively impact feeding, growth rates, and development success rates. In addition, soils with low nitrogen content may result in lower nutritional quality, forcing caterpillars to consume more and exposing them to higher toxicity. Soils low in nitrogen lead to milkweed leaves with lower protein and nutrient content, meaning that caterpillars need to consume more to meet their dietary requirements.[24] In an attempt to compensate, they may consume more leaf tissue. Eating more means they also ingest more cardenolides, increasing their exposure to toxins. The combination of poor nutrition and elevated toxins can result in less growth, smaller adult size, and greater energy costs during important events such as metamorphosis and migration.[27]

Applications

Monarch butterflies were first designated a species of concern by COSEWIC in 1997, and later added to Canada’s endangered species list in 2016 due to increased population declines driven by habitat loss and climate change[5]. A major issue is the combined effect of less milkweed and increasingly toxic milkweed, which puts caterpillars at risk even when host plants are present. As climate change alters seasonal patterns and raises stress levels in plants, milkweed may produce excessive cardenolides, reducing survival and slowing development of monarchs. Conservation efforts have focused mainly on planting more milkweed, but not all species are equal in terms of value or adaptability. Recent studies show that some species, like Asclepias incarnata, perform better under specific temperature regimes and may not be interchangeable in a warming climate, making consideration for both location and species vital.[28] In order to be more successful as the climate keeps changing, conservation needs to move beyond quantity and examine quality to efficiently and effectively choose the right species for the right location. Geology and soil chemistry directly influence how milkweed grows as well as its chemistry. This requires understanding not just what species monarchs can use, but how different milkweeds respond to their environments. Matching milkweed species to their soil types, such as A. viridis to prairie clay-loam or A. incarnata to silt, can ensure healthier and less toxic food sources that will still provide the defensive compounds monarchs rely on.[19][16] Since monarchs are often assumed to be host specialists but habitat generalists, a one-size-fits-all approach to planting may overlook components such as regional differences in soil, temperature, or moisture.  As conservation efforts continue to advocate for planting more milkweed, it is more and more important to consider not just how much milkweed is available, but which species are planted, where they are located, and how they respond to changing environmental conditions. Understanding the geological and mineral composition of soils is vital to studying plant chemistry, and by extension the animals that interact with them. By understanding and utilising that relationship, researchers can move towards more targeted and effective strategies and make more informed decisions for conservation and restoration efforts.

Connection Statement

In EOSC, we learn how Earth's geological processes, such as weathering, erosion, and plate tectonics, create the landscapes and soils that surround us. This topic connects directly to my bio-environmental studies in Global Resource Systems by demonstrating how Earth’s physical systems influence biological diversity and conservation efforts. This project shows how geology directly impacts where milkweed survives and thrives, as well as how they influence biochemical monarch development. It shows that conservation efforts can’t just be about planting more milkweed, it requires understanding the geology of a region, the minerals in the soil, how water interacts with it, and whether the resulting environment can support the balance that monarchs require. Conservation based in Earth science allows for more effective strategies. This demonstrates the integrated relationship that geology and ecology share and how it can be used to address real world issues in conservation, agriculture, and climate adaptation. Geological history shapes the conditions where monarchs feed, develop, and survive, reminding us that successful habitat restoration depends not only on the plants we choose but also the ground they grow in.

References

  1. ↑ "Monarch Butterfly". National Wildlife Foundation.
  2. ↑ 2.0 2.1 "Migration and Overwintering". U.S. Forest Service.
  3. ↑ 3.0 3.1 Matthew J. Faldyn, Mark D. Hunter, Bret D. Elderd (04 April 2018). "Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies". Check date values in: |date= (help)CS1 maint: uses authors parameter (link)
  4. ↑ 4.0 4.1 Wayne E. Thogmartin, Ruscena Wiederholt, Karen Oberhauser, Ryan G. Drum, Jay E. Diffendorfer, Sonia Altizer, Orley R. Taylor, John Pleasants, Darius Semmens, Brice Semmens, Richard Erickson, Kaitlin Libby, Laura Lopez-Hoffman (September 2017). "Monarch butterfly population decline in North America: identifying the threatening processes".CS1 maint: multiple names: authors list (link)
  5. ↑ 5.0 5.1 "Monarch (Danaus plexippus): COSEWIC assessment and status report 2016". Government of Canada.
  6. ↑ "Everything You Need to Know to Help Monarch Butterflies". National Wildlife Foundation.
  7. ↑ Antonio Giovino , Carmine Guarino , Annalisa Marchese , Rosaria Sciarillo , Gianniantonio Domina , Marco Tolone , Isabel Mateu-Andrés , Bouchaib Khadari , Calogero Schillaci , Miguel Guara-Requena , Sergio Saia (10 January 2023). "Genetic variability of Chamaerops humilis (Arecaceae) throughout its native range highlights two species movement pathways from its area of origin".CS1 maint: uses authors parameter (link)
  8. ↑ 8.0 8.1 8.2 Anurag A. Agrawal, Mark Fishbein (21 July 2008). "Phylogenetic escalation and decline of plant defense strategies". National Institute of Health - National Library of Medicine.CS1 maint: uses authors parameter (link)
  9. ↑ D. T. Tyler Flockhart, Leonard I. Wassenaar, Tara G. Martin, Keith A. Hobson, Michael B. Wunder, D. Ryan Norris (7 August 2017). "Tracking multi-generational colonization of the breeding grounds by monarch butterflies in eastern North America".CS1 maint: uses authors parameter (link)
  10. ↑ 10.0 10.1 Richard D. Bardgett, Wim H. van der Putten. "Belowground biodiversity and ecosystem functioning". Nature.CS1 maint: uses authors parameter (link)
  11. ↑ Steven Earle, Karla Panchuk (2019). Physical Geology. BCcampus.
  12. ↑ Sergio Rasmann, Anurag A. Agrawal (03 March 2011). "Latitudinal patterns in plant defense: evolution of cardenolides, their toxicity and induction following herbivory". Check date values in: |date= (help)
  13. ↑ "Soil Survey Manual". U.S. Department of Agriculture - Natural Resources Conservation Service. March 2017.
  14. ↑ "COMMON MILKWEED Asclepias syriaca L." (PDF). U.S. Department of Agriculture - Natural Resources Conservation.
  15. ↑ "SHOWY MILKWEED Asclepias speciosa Torr" (PDF). U.S. Department of Agriculture - Natural Resources Conservation.
  16. ↑ 16.0 16.1 "SWAMP MILKWEED Asclepias incarnata L." (PDF). U.S. Department of Agriculture - Natural Resources Conservation.
  17. ↑ "BUTTERFLY MILKWEED Asclepias tuberosa L." (PDF). U.S. Department of Agriculture - Natural Resources Conservation.
  18. ↑ "NARROW-LEAVED MILKWEED Asclepias fascicularis Decne" (PDF). U.S. Department of Agriculture - Natural Resources Conservation.
  19. ↑ 19.0 19.1 "GREEN MILKWEED Asclepias viridis Walt" (PDF). U.S. Department of Agriculture - Natural Resources Conservation.
  20. ↑ Anurag A. Agrawal, Georg Petschenka, Robin A. Bingham, Marjorie G. Weber, Sergio Rasmann (31 January 2012). "Toxic cardenolides: chemical ecology and coevolution of specialized plant–herbivore interactions". New Phytologist Foundation.CS1 maint: multiple names: authors list (link)
  21. ↑ Keith S. Brown Jr. & Ronaldo B. Francini. "Evolutionary strategies of chemical defense in aposematic butterflies: Cyanogenesis in Asteraceae-feeding American Acraeinae". Springer Nature Link.
  22. ↑ David S. DeLaMater, John J. Couture, Joshua R. Puzey, Harmony J. Dalgleish. "Range-wide variations in common milkweed traits and their effect on monarch larvae". Wiley Library.CS1 maint: uses authors parameter (link)
  23. ↑ 23.0 23.1 Thomas E. Dilts, Madeline O. Steele, Joseph D. Engler, Emma M. Pelton, Sarina J. Jepsen, Stephanie J. McKnight, Ashley R. Taylor, Candace E. Fallon, Scott H. Black, Elizabeth E. Cruz, Daniel R. Craver, Matthew L. Forister (2019). "Host Plants and Climate Structure Habitat Associations of the Western Monarch Butterfly". Frontiers.CS1 maint: uses authors parameter (link)
  24. ↑ 24.0 24.1 John J. Couture, Shawn P. Serbin, Philip A.Townsend (April 2015). "Elevated temperature and periodic water stress alter growth and quality of common milkweed (Asclepias syriaca) and monarch (Danaus plexippus) larval performance". Research Gate.CS1 maint: uses authors parameter (link)
  25. ↑ Patricia L. Jones, Georg Petschenka, Lara Flacht, Anurag A. Agrawal (22 Feb 2019). "Cardenolide Intake, Sequestration, and Excretion by the Monarch Butterfly along Gradients of Plant Toxicity and Larval Ontogeny". National Institute for Health - National Library of Medicine.CS1 maint: uses authors parameter (link)
  26. ↑ Mackenzie Hoogshagen, Amy P. Hastings, Joselyne Chavez, Marissa Duckett, Rayshaun Pettit, Andrew P Pahnke, Anurag A. Agrawal, Jacobus C. de Roode. "Mixtures of Milkweed Cardenolides Protect Monarch Butterflies against Parasites". National Institute for Health - National library of Medicine.CS1 maint: uses authors parameter (link)
  27. ↑ Leiling Tao, Kevin M. Hoang, Mark D. Hunter, Jacobus C. de Roode. "Fitness costs of animal medication: antiparasitic plant chemicals reduce fitness of monarch butterfly hosts". National Health Institute - National Library of Medicine.CS1 maint: uses authors parameter (link)
  28. ↑ Nathan P. Lemoine. "Climate Change May Alter Breeding Ground Distributions of Eastern Migratory Monarchs (Danaus plexippus) via Range Expansion of Asclepias Host Plants". National Institute for Health - National library for Medicine.


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