The Unexpected Wealth of Serpentine Habitats

Most avid birders might be more interested in the incredible diversity of avian species which visit and inhabit the Bay Area than in the rocks beneath. But the two are part of the same story: geologic diversity typically underlies ecologically diverse areas, and the bedrock beneath our beautiful lush areas governs unseen, determining crucial life conditions like how water moves through an area, how the composition of soils can support or restrict plant growth in an area and the elevation and slope at which vegetation grows. The uniqueness of the geologic setting beneath Marin County makes for some pretty interesting conditions for life, which supports some very rare endemic species of plants and animals. 

Serpentine is very erodible and as it weathers, it becomes a thin, rocky soil layer, rich only in nickel, chromium, and cobalt, heavy metals which choke plant growth and inhibit organic matter build up in the soil. Primary plant nutrients such as potassium, magnesium, and calcium, which support basic life functions like nutrient uptake and photosynthesis, are scant. The soil’s thin composition hampers water retention, a quality which makes it difficult for plants to survive through California’s famously dry summers.    

The nutrient poor substrate of serpentine is an unlikely haven for rare endemic species of grasses and wildflowers. But many invasive species, which have adapted to thrive in as many different environments as possible, find the heavy, dry, metallic serpentine soils intolerable. And so these rare environments are conducive only to native plant growth; a number of endemic California wildflowers and plants have evolved to withstand the toxicity of this native soil, and grow here undisturbed by the crawling growth of invasives which threaten much of the state’s iconic biodiversity. Although only one percent of California’s soils are serpentine, they support a staggering ten percent of endemic California flora! A 2009 study found that there are over two hundred serpentine endemic plant taxa. Many serpentine endemic plants have necessarily developed unique physical, chemical, biological, and temporal adaptations to withstand the growing conditions of the soil. Some have special mechanisms to block the accumulation of heavy metals like nickel, and others can extract calcium more efficiently than other species. Most plants that grow on serpentine soils appear stunted, as they direct most of their energy into dense root systems beneath the soil. One study found that many species have involved different adaptations to serpentine soils independently, over time.

A few different natural environments can form atop these soils, including grasslands, barrens, chaparral, and even wetlands. All of these niches are quite rare across North America, but California has the most serpentine soils in North America, and all of these niches can be found throughout the state.

Serpentine barrens, the rarest of all serpentine communities, are both extremely harsh and extremely conducive to rare endemic species growth. These barrens are defined as rocky, open areas with no woody growth and sparse herbaceous cover. Mount Tamalpais is a great place to go to observe the endemic plant communities which have settled on serpentine barrens! Here you can peer at thriving populations of Mount Tamalpais Bristly Jewelflower, Tiburon Buckwheat, Marin Navarretia, and Tamalpais Jewelflower, as well as other rare endemic species that occur on different soil substrates.

There are other spots to go in Marin County to see these rare serpentine endemics. You can seek the seemingly otherworldly Tiburon Mariposa Lily, which grows on a single sloped serpentine outcropping on Ring Mountain, and nowhere else in the world. This lily is found in close proximity to other regional endemics including serpentine reedgrasses, serpentine wheatgrass, Tiburon buckwheat, Tiburon paintbrush, and Marin dwarf flax.

There are also a number of invertebrates which occur only on serpentine soils in the Marin area. One such species is the Tiburon microblind harvestman (closely related to the more common Daddy Long Legs), a tiny spooky looking spider-like creature which sightlessly navigates the subterranean ranges of serpentine grasslands, but only at night and only when moisture is high. Another serpentine dweller is the Muir’s hairstreak, an elusive coin- sized butterfly which lays its eggs only on two species of serpentine endemic Cyprus. The Bay Checkerspot butterfly, endemic to the San Francisco area, also breeds in natural communities on serpentine soils. 

So how do serpentine soils relate to birds? Again, geologic diversity is generally conducive to all ecological levels of diversity, because complex, varied landscapes support more species with different habitat needs. And the rare flora that grow on serpentine soils play an unusual role in their ecosystem, offering habitat to endemics which have coevolved with the serpentine dwelling plants. Serpentine grasslands are a particularly important habitat to many native Californian birds, such as the Golden Eagle, Eastern Bluebird, Horned Lark and Grasshopper Sparrow. 

Additionally, while there are less endemic birds than endemic plants in the state, more species of birds breed in California than any other region in the US, and therefore sufficient habitat is crucial for supporting the broad array of stopover birds. The variety of serpentine environments in the Bay area, as well as the state- including serpentine barrens, serpentine grasslands, and serpentine chaparral- are used by avian travelers of the great Pacific Flyway, which extends from Alaska down to Patagonia. 

Understanding the history and presence of Serpentine in this area enriches my appreciation of our beautiful flora and fauna that grows over it. This spring as you’re enjoying the chirping songbirds and the blankets of orange and yellow wildflowers, emblems of the beatific spring beauty of the bay, keep your eye out for the blue green hues of California’s state rock, and see if you can identify any of the rare endemics which stubbornly reside in its rocky soils.

References

• Brady, K. U., Kruckeberg, A. R., & Bradshaw Jr., H. D. (2005). Evolutionary ecology of plant adaptation to serpentine soils. Annual Review of Ecology, Evolution, and Systematics, 36(1), 243–266. https://doi.org/10.1146/annurev.ecolsys.35.021103.105730
• Calgeographic. (2023, January 3). Serpentinite habitats in California. Cal Geographic. https://calgeographic.com/2023/06/18/biogeography-serpentinite-habitats/
• Elam, D. R. (1998). Draft Recovery Plan for Serpentine Soil Species of the San Francisco Bay Area. US Fish and Wildlife Service, Pacific Region.
• Holloran, P. (n.d.). Serpentine Grasslands and Maritime Chaparral. FOUNDSF. https://www.foundsf.org/Serpentine_Grasslands_and_Maritime_Chaparral

• Safford, H. & Viers, J. & Harrison, Sandy. (2009). Serpentine endemism in the California flora: A database of serpentine affinity. Madroño. 52. 222-257. 10.3120/0024-9637(2005)52[222:SEITCF]2.0.CO;2.

• Sloan, D. (2006). Geology of the San Francisco Bay Region (Vol. 79). Univ of California Press.

• Tukiainen, H., Toivanen, M., & Maliniemi, T. (2022). Geodiversity and Biodiversity. Geological Society, London, Special Publications, 530(1), 31–47. https://doi.org/10.1144/sp530-2022-107
• U.S. Fish and Wildlife Service. 1998. Recovery Plan for Serpentine Soil Species of the San Francisco Bay Area. Portland, Oregon. 330+ pp