In the patchy landscapes of California, Susan Harrison uncovered nature's fragile interconnectedness. Her lifelong inquiry into its mysteries now reveals the profound impact of a changing climate.
Susan P. Harrison once traced the fate of a single butterfly across the rugged serpentine soils of California. Today, she tracks the future of entire plant communities as they respond to a warming world. As a Distinguished Professor at the University of California, Davis, and a member of the prestigious National Academy of Sciences, Harrison has spent over three decades deciphering the intricate forces that shape and sustain life on our planet 1 6 .
Her journey from studying insect metapopulations to sounding the alarm on climate-driven diversity loss represents a career dedicated to understanding ecology at every scale. This is the story of how a scientist, her questions, and her beloved California landscapes have together illuminated critical truths about the natural world and its uncertain future.
Susan Harrison's path to ecology was not a straight line. Growing up in Sonoma, California, as one of six children in a family that valued the outdoors, she was immersed in nature from the start. Her father, a physician, and her social worker mother shared a love for scholarship and public service, often taking the family on camping and hiking trips 6 .
Initially a zoology major at UC Davis with plans for medical school, Harrison's trajectory shifted dramatically when she worked as a summer field assistant for ecologist Richard Karban 6 . "By the end of the summer," Harrison recalls, "I was a convert and cancelled my plans to start medical school" 6 . This conversion led her to pursue a Master's degree in ecology at UC Davis under Karban's mentorship, followed by a PhD in biology at Stanford University 1 6 .
"By the end of the summer, I was a convert and cancelled my plans to start medical school."
At Stanford, Harrison found the perfect convergence of her interests in plant-insect interactions, habitat fragmentation, and serpentine soil ecology. When her prospective advisor, renowned biologist Paul Ehrlich, and his lab manager Dennis Murphy unrolled a map of serpentine soil outcroppings in Santa Clara County, Harrison instantly knew she had found her study system 6 7 . This moment would launch her into a career that would fundamentally reshape how we understand ecological communities.
Harrison's doctoral research on the Bay checkerspot butterfly produced what would become a landmark study in ecology.
Harrison and her colleagues employed a multi-faceted approach to understand the butterfly's population dynamics:
Harrison's research required simple but meticulous methods to track butterfly populations:
| Finding | Significance |
|---|---|
| Butterfly distribution followed a "mainland-island" pattern | Challenged classic metapopulation theory that assumed identical patches |
| A few large, stable populations acted as sources | Highlighted the disproportionate importance of certain habitats |
| Distance from source populations predicted colonization | Demonstrated the critical role of dispersal limitation |
| Small populations frequently went extinct | Confirmed vulnerability of small, isolated populations |
The study's most significant contribution was demonstrating that the butterfly's persistence depended not on the classic "balance" of extinctions and colonizations, but rather on a few large, resistant populations that served as sources for smaller, more vulnerable satellite populations 1 7 . This "mainland-island" model deviated from established theory and had profound implications for conservation.
Harrison's checkerspot research "made me, almost instantly, an expert on metapopulation dynamics," she would later reflect 7 .
The paper led to an invitation to the first-ever conference on metapopulation dynamics in Finland and a highly-cited review paper that remains her most referenced work 7 .
As Harrison's career progressed, her research focus expanded while maintaining its foundational interest in how species persist across landscapes. After joining UC Davis as a faculty member in 1991, she continued to explore spatial ecology but increasingly turned her attention to plant communities 1 6 .
Primary Research Focus: Metapopulation dynamics
Key Study Systems: Bay checkerspot butterfly
Primary Research Focus: Spatial ecology and diversity patterns
Key Study Systems: Serpentine plant communities
Primary Research Focus: Climate change impacts on biodiversity
Key Study Systems: California grasslands, Oregon forests
In 1997, Harrison helped establish the McLaughlin Natural Reserve, part of the University of California's Natural Reserve System, which would become a focal point for her research 2 6 . "There's nothing like it and it's an amazing resource," Harrison says of the reserve system. "Not only is there this beautiful piece of land that protects some special part of the natural heritage, but over time it develops this kind of knowledge base" 2 .
"There's nothing like it and it's an amazing resource... over time it develops this kind of knowledge base."
Harrison's work began to reveal that diversity at the local level strongly reflects larger-scale regional influences, with climate patterns overriding local conditions in determining which species thrive 3 5 . This "top-down" perspective challenged conventional ecological thinking that emphasized local resource competition as the primary driver of diversity 6 .
Over the past decade, Harrison's research has delivered some of the most compelling evidence of how climate change is already reshaping California's ecosystems.
In one pivotal 15-year study at the McLaughlin Reserve, Harrison analyzed how plant species richness responded to a series of dry winters from 2000-2014, followed by an extremely wet winter in 2016-2017 2 6 . The research design incorporated:
The findings were stark: grassland diversity declined during the drought years and failed to rebound even after the exceptionally wet winter 6 . The research identified the mechanism behind this pattern: seedling mortality during dry winters gradually depleted the underground seed banks, diminishing the recovery potential of plant communities 1 6 .
Harrison also helped identify the phenomenon of "thermophilization" — the process by which plant communities shift toward species preferring warmer conditions — through resurveys of plots in Oregon's Siskiyou Mountains originally sampled by Robert Whittaker in the 1950s 6 . Her transplantation experiments further demonstrated that low-elevation herbs could survive at higher, cooler elevations but succumbed to drought stress in their original locations 6 .
Beyond her research contributions, Harrison has been a steadfast advocate for the protection of California's natural landscapes as living laboratories. She describes the UC Natural Reserve System as the product of a "grassroots movement of environmental scientists" who saw their research sites "getting developed and paved over" 2 .
Her commitment to future scientists is equally profound. Having mentored 20 graduate students and 10 postdoctoral fellows 6 , Harrison values her role in guiding the next generation of ecologists. Her former assistants have gone on to distinguished careers of their own, including Karen Holl, now a professor of restoration ecology at UC Santa Cruz 7 .
Harrison's work has earned her numerous honors, including fellowship in the California Academy of Sciences (2004) and the Ecological Society of America (2013), culminating in her election to the National Academy of Sciences in 2018 1 .
The NAS citation recognized her as "a leader in the study of ecological diversity at different spatial and temporal scales," whose work is "of fundamental importance for understanding the impact of global change on ecological communities" 5 .
UC Davis
Elected 2018
20 graduate students, 10 postdocs
Co-founded 1997
Today, Harrison continues her synthesis of decades of data, working to predict how California's rich plant diversity will respond to ongoing climate change 6 . Her findings suggest that the most vulnerable species across the state are those with "mesic" or moderate water requirements, while stress-tolerant species may persist 6 .
Plants with "mesic" or moderate water requirements are most at risk as climate patterns shift.
Plants in low-fertility soils appear less affected by climate change due to inherent stress tolerance.
She has also identified that plants growing in low-fertility soils appear less affected by climate change than those in fertile soils, both because nutrient scarcity constrains their response to water variation and because they possess traits that confer resistance to multiple stresses 6 .
"It never made all that much sense to go study someplace a thousand miles away when there's all this cool, incredible natural diversity right here in lovely Northern California" 2 .
Looking ahead, Harrison is concerned about "potentially a lot of change in forests" due to increased temperatures, water scarcity, and fires 2 . Yet she remains committed to the places that have shaped her career, expressing a sentiment that has guided her work from the beginning.
From the scattered patches of serpentine soil that sustain a rare butterfly to the sweeping grasslands responding to a changing climate, Susan Harrison's work reminds us that understanding the delicate balance of nature begins with careful attention to the landscapes we call home.