Profile of Sandra M. Díaz

Fluctuations in biodiversity are not just collateral damages in the face of global environmental change, according to Sandra Díaz, a professor of community and ecosystems ecology at Córdoba National University in Argentina and a senior principal researcher of the Argentine National Research Council. Díaz explores how the chemical and physical traits of plants—such as size, texture, and nutrient content of leaves, wood density, palatability to herbivores, and canopy architecture—influence a plant’s response to the environment and play an active role in the planet’s constant evolution in response to environmental change.

Sandra M. Díaz.

Díaz has uncovered the important variables driving ecosystem functions and services, such as fodder and timber production, habitat for wildlife, and aesthetic enjoyment. Although biodiversity is often estimated by the number of species present, Díaz revealed that the function and benefits of an ecosystem are more accurately predicted by the combination of organisms and the specific traits they bring to the table.

Her recent studies bridge the gap between ecology and social sciences. In her Inaugural Article (1), Díaz and colleagues from the ecological and social sciences offer a conceptual framework for linking different aspects of functional trait diversity with the needs of different social groups. “Almost everyone now agrees that biodiversity is important to people and that different people want different things; the challenge is therefore how to go about this in practice. We wanted to combine methodological guidelines, on-the-ground field experience, as well as new concepts,” she says. The guidelines and concepts Díaz introduced have earned her a seat among the world’s top environmental researchers as a member of the National Academy of Sciences of Argentina, the Academy of Sciences of the Developing World, and a foreign associate of the National Academy of Sciences. Her 2007 article in PNAS won the 2008 Cozzarelli Prize and the Sustainability Science Award from the Ecological Society of America (2009).

Backyard Biodiversity

Sandra Díaz was born in Bell Ville, a small city in central Argentina. As long as she can remember, she has been drawn to biology. Her parents loved plants and animals, and nurtured a large garden. It was common at the time to bring wild animals from the fields and forests into the home. “I know it sounds horrible today, but at that time, most animal fans did that,” explains Díaz. When her father went fishing, he occasionally brought home a baby owl or baby hare, “things that he really liked very much and he thought we could keep in the house, in the garden, for me.”

As an undergraduate, Díaz studied biology at the Córdoba National University. Her parents provided support, although it was not yet clear how her biology degree would translate into a successful career. When she began her graduate work, she already knew she wanted to be an ecologist, and she remained at the Córdoba National University. In the Argentinean scientific circles at the time, she says, community ecology was considered an alternative area of biology.

“In Argentina, this area of science was considered a little bit soft; if you like outdoors, and you are not very precise, then field ecology was for you. But I really felt attracted to the interconnected view of things that was essential for ecology, and the chance to dig out mechanistic explanations.” For her PhD thesis, Díaz teamed up with Marcelo Cabido and Alicia Acosta, skilled and enthusiastic ecologists who were barely older than her. “Those were not easy times in Argentina, including science, so the whole team was pretty much self-taught.” Cabido and Acosta were establishing their own reputations as scientists, but they welcomed Díaz into their fledgling team. “When I started with all these crazy ideas about plant traits, they said, ‘Go ahead. We support you.’”

The group was a good match. Other members had a strong foundation in plant taxonomy and vegetation studies, but Díaz brought a fresh perspective. She was interested in plant functional traits, and her initial research focused on how different forms of land use and climates affected the structure of plant communities. In central Argentina, for example, some types of land use did not affect the range of species but dramatically altered the plants' predominant traits. “You had pretty much the same list of species, but their proportions, and the overall structure and function of the community, were completely different” (2, 3).

It became obvious to Díaz that differing traits hidden behind highly similar species could potentially impact everything from soil microorganisms to animals to water retention, and ultimately the people who make a living from them. “A functional trait approach was clearly needed if we were to connect plants with land use and with ecosystem processes.” In her PhD and subsequent work, Díaz developed the conceptual basis and practical protocols enabling other researchers to apply the functional trait approach. “Looking at the tremendous amount of attention that functional traits receive today,” says Díaz, “it amazes me that, in the early nineties, only a minority [of scientists] thought they were important.”

Learning How to Experiment

Díaz completed her PhD research in 1989 and began looking for postdoctoral opportunities abroad, expecting to continue her focus on general plant traits. “I was interested in trying to find generalities. I have never been very good at small details. I have always liked trying to extract general patterns from things.” Around that time, she came across a book by Philip Grime, a famous British ecologist and then Director of the Unit of Comparative Plant Ecology at the University of Sheffield and the Buxton Climate Change Impacts Laboratory. “I was immediately intrigued by the way he connected different styles of being a plant with environmental factors, and with ecosystem processes, in such a mechanistic and yet simple way. So I wrote Grime a letter asking to work with him.”

When Díaz arrived at Sheffield in 1991, she began investigating how plant communities respond to CO₂ concentrations that have steadily increased since the beginning of the industrial age. “I was not particularly interested in CO₂ in the beginning, but I loved the prospect of learning experimental techniques to study community ecology,” Díaz says. The results that emerged from their study were unexpected, and the findings received much recognition. Other researchers suggested that, although higher CO₂ concentration stimulated the growth of individual plants cultivated in the laboratory, the same would not be possible in ecosystems in which soils are poor in mineral nutrients. However, Díaz and colleagues showed experimentally that mineral nutrient constraints also occur on fertile soils. Fast-growing, weedy plants responded negatively to high CO₂ levels, even when extra fertilizer was added to the soil. But in the same conditions, the microorganisms thrived. This all suggested a feedback mechanism by which, when CO₂ was elevated, soil microorganisms outcompeted the fast-growing plants for nitrogen. By contrast, slow-growing plants such as heather have positive associations with soil fungi and are stimulated. The most exciting aspect, according to Díaz, was the interplay of soil organisms and plants with contrasting resource-use strategies because they yielded ecosystem-level results that would have been very difficult to anticipate from single-plant, leaf-level physiology (4).

Her time at Sheffield had a lasting impact on her career. “It was a vibrant research group, very productive and with an excellent team spirit. I learned from Phil Grime how to experiment with communities. He could do groundbreaking experiments even on a shoestring budget. I also learned to go for bold, very fundamental questions, to delve deeply into the processes of ecology.”

Choosing a Path

After returning to Argentina in 1993, technical and financial constraints hindered Díaz from continuing her experimental work on CO₂. Instead she shifted gears and returned to her long-term study of plant traits, this time studying traits in the context of global environmental change. She wanted to determine how functional trait diversity responds to climate and land-use change while actively influencing ecosystem properties.

“From a very philosophical point of view, I agree that every living form on Earth has the right to exist,” says Díaz. However, she says, the number of species present in a system is often not a good indicator of how the system functions, how it reacts to environmental changes, or what benefits it can offer to people. Researchers can more easily gauge the ecosystem’s basic characteristics and capabilities by determining factors like the average height of the plants, the nitrogen content of the leaves, what kind of species are dominant locally, and what combinations of plant traits are present in the system. “I started to come up with this idea of how to deal with what I call ‘functional trait diversity,’” explains Díaz (5).

Eventually she began to manipulate the functional diversity on plots of land by dividing a patch of vegetation into smaller plots, some lacking fast-growing herbaceous plants and others lacking slower-growing woody plants or the most abundant plant species. The plots, which simulate various scenarios of climate- or land use-driven changes, allow Díaz and her team to explore how ecosystem processes are affected by plant variables (6). These field manipulations are more realistic than controlled laboratory experiments, says Díaz, but, ultimately, scientists will need to work with the real-world environments created by humans. “These are messy, full of uncontrolled factors, not very amenable to traditional statistics, and logistically difficult,” she says. “But this is the arena where accelerated environmental change is happening.”

While participating in the Intergovernmental Panel on Climate Change and the Millennium Ecosystem Assessment, Díaz was struck by the massive scale of environmental change facing humans, and the inextricable links between climate change and socioeconomic issues. The challenges are forcing dramatic changes in the way ecologists work; the research questions can no longer be fully tackled by a single discipline, and the massive scale of the questions require large networks of scientists. On her part, Díaz helped lead an international initiative to compile a global database of tens of thousands of plants, a task that she says would have been impossible without an interdisciplinary team effort. “The launching article has 135 authors!” she says. “There are plenty of challenges that arise, but scientists have to rise to the occasion,” she says (7).

In 2004 (8), Díaz and colleagues from four countries showed a fundamental tradeoff in plant design between a set of plant attributes that allow rapid acquisition of resources and another set that permit conservation of resources within well-protected tissues. In simplistic terms, Díaz explains that, underneath the overwhelming diversity of plants on Earth, there are very few basic designs or, as Díaz put it, “basic ways of being a plant.” These designs have to do with whether plants grow quickly or slowly; whether they have flimsy, nutritious, short-lived leaves, or dense, tough, highly defended leaves; whether they are big or small; what kind of wood they have; and how they reproduce. There are basic combinations of traits that always go together, Díaz says. The dominant combination in a particular location and climate has a considerable impact on ecosystem function, she found. Her findings held true across major plant lineages and floras in very different climates, land-use regimens, and biogeographical histories. Such consistent patterns of plant specialization had been predicted in previous studies, but Díaz confirmed the phenomenon for a vast set of plants and traits.

Linking Plants and People

The next step was to unravel how certain plant characteristics make an ecosystem more or less productive, resilient, palatable, or nutrient-rich, and how these effects are linked to the concept of functional diversity. In a 2007 Cozzarelli Prize-winning paper in PNAS (9), Díaz et al. laid out the framework for connecting different aspects of functional diversity—including traits of the dominant species, variety of traits contained in the community, and the presence of particular species or trait characteristics—with specific ecosystem processes and services to people. Before those findings were publicized, Díaz recalls that people generally agreed that biodiversity was important for human well-being, but in a vague and general way. “I think what made this paper noteworthy is that we provided a mechanistic framework to link specific ecosystem properties and the benefits that people perceive from them with specific components of plant functional trait diversity.”

In her Inaugural Article (1), Díaz et al. take the methodology established in 2007 a step further by applying it to real-world ecosystems that are shaped by different groups in complex societies. For example, a swath of forest can provide carbon sequestration and therefore contribute to climate stabilization. It provides habitat for animals; it can yield timber or fodder, aesthetic enjoyment, or food. However, usually it cannot produce all these at the same time. Díaz wanted to know who cared about each service and exactly what plant traits were connected to those benefits or disadvantages.

She assembled an interdisciplinary team with social scientists and ecologists. The social scientists were tasked with figuring out what each social group wanted from a particular ecosystem and why. A subsistence farmer, a commercial farmer, an eco-tourist, and the global community will all value different aspects of a piece of land. For example, a rancher wants fodder for his cows. The social scientists ask the farmer which aspects, in his opinion, are linked to good fodder—is it a particular species or a specific trait or the mixture of plants with different characteristics? In parallel, ecologists measure all these functional diversity components and also the relevant ecosystem process, in this case, fodder productivity and nutritional quality.

“So, through this fairly detailed work, one starts to understand the links all the way from specific aspects of the functional composition of the plant community to specific needs and preferences of different groups of people; what we are basically proposing is the unpacking of the general idea that biodiversity is good for people—we are unpacking it both ecologically and socially.” How the living fabric of the planet modulates ecological processes, and how people not only benefit from them but also actively shape this fabric, ecologically and evolutionarily, is something that deeply interests Díaz. “This requires a genuine interdisciplinary mindset, and therefore it is not easy, but at the same it is a hugely stimulating challenge, both practically and intellectually.”