Profile of Susan P. Harrison

Susan P. Harrison, a professor of environmental science and policy at the University of California, Davis (UC Davis), has made seminal contributions to basic, theoretical, and conservation ecology. Her research, which includes studies concerning the spatial dynamics of plant and animal communities, large-scale diversity patterns, natural climatic variability, and anthropogenic climate change, continues to advance our understanding of the forces that shape natural communities. Elected to the National Academy of Sciences (NAS) in 2018, Harrison reports in her Inaugural Article (1) some of the first consistent patterns of plant diversity changes with water supply at multiple spatial scales. The findings could help to inform plant community conservation strategies, particularly in regions with water-limited climates.

Fig. 1.

Susan. P. Harrison. Image courtesy of Michael J. McRae (photographer).

A “Convert” to Ecology

Harrison grew up in Sonoma, California. Her father, a physician, and social worker mother were both active in civic affairs and shared a love of the outdoors. They took Harrison and her five siblings on many camping and hiking excursions. Harrison says, “They deeply valued the natural environment, scholarship, and public service, and I like to think all of those elements found their way into my choice of profession.”

Harrison majored in zoology at UC Davis, intending to attend medical school. Before earning a bachelor’s degree in 1983, Harrison was hired by ecologist Richard Karban at the university as a summer field assistant. “Some of this work took place in stunning natural settings, like the coastal bluffs at Bodega Bay, California,” she says. Her interest in ecology grew. “By the end of the summer, I was a convert and cancelled my plans to start medical school.”

Early Mentors

Karban became Harrison’s advisor when she pursued a Master’s degree in ecology at UC Davis. She says, “He had a formative influence on how I think about questions and approaches in ecology, how to design experiments, and also how to think about ethics and personal values in science.” Her other primary mentors at UC Davis were James Quinn, now emeritus director of the university’s Information Center for the Environment, and theoretical ecologist Alan Hastings.

After earning a Master’s degree in 1986, Harrison visited Stanford University as a prospective doctoral student. There she met biologist Paul Ehrlich. She expressed interest in plant–insect interactions, habitat fragmentation, and the ecology of serpentine soils. This prompted Ehrlich and his laboratory manager Dennis Murphy to unroll a map of southern Santa Clara County, a region known for serpentine soil outcroppings that support populations of the endemic and threatened Bay checkerspot butterfly (Euphydryas editha bayensis). Harrison says, “I instantly recognized that this model system was for me.” She was admitted into Stanford’s biology doctoral program, and began fieldwork at the site under Ehrlich’s guidance.

Large-Scale Field Test of Metapopulation Theory

A study led by Harrison and coauthored by Ehrlich and Murphy demonstrated that the butterfly persists as metapopulation: A set of populations that are interdependent over ecological time (2). The researchers applied their fieldwork data to metapopulation theory, marking a large-scale test of the popular theory. The results aligned with the theory in that the butterfly goes locally extinct and recolonizes small habitat patches, but the study countered the theory by finding that the species mainly persists due to the existence of one large permanent population.

A subsequent review article, which is still the most cited among Harrison’s body of work, further assessed metapopulation theory (3). She concluded that metapopulations tend to exist within three qualitatively different situations: Species that depend upon one or more extinction-resistant populations, species with high dispersal between patches of habitat, and nonequilibrium metapopulations in which local extinction occurs in the course of a species’ overall regional decline.

Revision of Spatial Pattern Formation Theory

The articles on metapopulation received wide attention, contributing to the Bay checkerspot butterfly becoming a paradigm for conservation. Harrison is grateful to Ehrlich for his support. She says, “The opportunity to work in his [laboratory] was the lucky break that played the biggest role in getting my career off the ground.” She stayed at Stanford for a postdoctoral stint at the Morrison Institute for Population and Resource Studies in 1989 before obtaining a second postdoctoral position at the Center for Population Biology at Imperial College at Silwood Park, England in 1990.

In 1991 Harrison accepted a position as an assistant professor at UC Davis’ Division of Environmental Studies. By 2019, she had advanced to the position of distinguished professor. At the university, she continues to devote significant time to research. In 1997, for example, Harrison and ecologist John Maron, now at the University of Montana, tested another popular mathematical hypothesis, spatial pattern formation theory (4). It holds that populations may form patchy distributions within continuous habitats through strong predator–prey or host–parasite interactions combined with limited dispersal.

The study, which marked a large-scale field test of the theory, focused on a system involving the Western tussock moth (Orgyia vetusta) and associated parasitic insects. In agreement with the theory, they found that the parasites prevented the spatial spread of their host victim. Harrison later revised the theory after she, Hastings, and ecologist Kevin McCann combined data from the parasite–moth system with abstract mathematical and parameter-rich simulation models (5). The theory now incorporates the counterintuitive prediction, corresponding to their field observations, that the prey species are likely to exhibit the highest population density at the edge of an outbreak.

Uncovering Large-Scale Diversity Patterns

Harrison has mentored 20 graduate students and 10 postdoctoral fellows. With graduate student Brian Anacker, she analyzed field data and large-scale floristic data for plant communities across California (6). They found that the amount of phylogenetic diversity within plant communities is best explained, not by the characteristics of the local environment, but by where the community is located in a larger geographic, climatic, and historical context.

At the first Presidential Debate of the American Society of Naturalists, Harrison argued in 2015 that species diversity is controlled from the “top down” by historical contingencies rather than from the “bottom up” by limited resources. She says, “It was pure intellectual fun in which clearly neither side was totally right or wrong.” Her position, summarized in an article, emphasizes that ecology has moved beyond notions of ecosystem equilibrium at a carrying capacity and toward a richer view of communities as highly dynamic in space and time (7). While known for her research on plant and insect systems, Harrison has also coauthored several articles concerning birds, terrestrial mammals, and marine ecosystems with her husband, UC Davis biological sciences professor emeritus Howard Cornell.

Evidence for Thermophilization

Over the past decade, Harrison’s research has increasingly focused on how plant communities respond to natural climatic variation and anthropogenic climate change. In 2010, she and colleagues resurveyed 185 vegetation plots in Oregon’s Siskiyou Mountains. The plots were originally sampled from 1949 to 1951 by ecologist Robert Whittaker. Since Whittaker’s work, the climate has become warmer and drier at low elevations. The researchers found that, through a process known as thermophilization, plant communities that were not located in the highest elevations, where there is a regular yearly snowpack, shifted toward a greater resemblance to communities in warm climates (8).

Harrison and her team transplanted low-elevation herbs to higher and cooler environments in the Siskiyou Mountains and found that the plants survived in their new locations but not in their original ones, where they died from summer drought stress (9). The researchers determined that in the cooler environments, the transplanted herbs benefited from vegetative cover, which kept the ground slightly warm at night.

Reasons for High Climate-Resistance

Results from numerous studies conducted by Harrison’s laboratory and others have shown that plants growing in low-fertility soils seem to be less affected by climate change than plants in fertile soils. Harrison and Anu Eskelinen, who is now at the German Center for Integrative Biodiversity Research, conducted a 5-year experimental study in a semiarid annual grassland system with highly heterogeneous soil nutrient content (10).

The study uncovered two reasons for the higher climate resistance observed in low-fertility soil plants. The first is that nutrient scarcity constrains plants’ abilities to respond to variation in the water supply. The second is that plants in low-nutrient environments have particular functional traits that make them more resistant to both nutrient and water stress.

Drought and Deluge

Using long-term data and an experimental water manipulation on 100 plots, Harrison and her team examined the resilience of a heterogenous annual grassland community in California to a prolonged series of dry winters, from 2000 to 2014, followed by a near-record wet winter, from 2016 to 2017 (11). They found that plant community diversity declined and did not rebound, even after the extremely wet winter.

The water manipulation experiment showed that the likely mechanism underlying the decline is seedling mortality during dry winters. Harrison explains that many grasses and wildflowers produce seedbanks, which may lie dormant until germination-promoting conditions occur. Seedling mortality diminishes seedbanks and thus also lowers the recovery potential of a plant community. The resulting diversity losses over time could foreshadow extinctions, especially in regions that are becoming increasingly dry.

How Plant Diversity Responds to Climate

With funding from the National Science Foundation, Harrison—a fellow of the California Academy of Sciences (2004), the Ecological Society of America (2013), and the John Muir Institute of the Environment (2018)—is currently working on a synthesis of three large-scale datasets. Her Inaugural Article, which presents an extensive analyses of plant diversity in California over space and time, is one product of this effort (1). The researchers determined that plant taxonomic, functional, and phylogenetic diversity declined during prolonged dry climate periods, leading to the prediction that the most vulnerable species at any location within the state will be those with the most mesic, or moderate, climatic affinities.

Harrison has initiated a project to test whether plant community diversity in a changing climate is constrained by the limited abilities of plants to disperse from dry to formerly wet climates. Harrison and her team are transplanting annual grassland species from southern California to her focal field site, the McLaughlin Reserve, in the northern part of the state. Beginning in 1997, she helped to establish the site as one of six reserves managed by UC Davis within the University of California’s natural reserve system. Harrison says, “Biological field stations and their associated lands are a critical resource for studying our changing natural environment. They have totally made possible my career and many others’. The science establishment needs to be continually reminded of their value.”