Life on Earth:
Why Biodiversity Varies

By Carolyn Krause


Editor's note: In the early 1990s biological diversity—the variety and distribution of life on the earth—became a hot political issue. The chief reason: concern about extinctions of animal and plant species as a result of human activities such as the felling of tropical rain forests to provide land for agricultural production. Because the number of different plant species is highest in rain forests, some experts fear that tropical deforestation will aggravate the problem of species extinction, add more plants and animals to the endangered species list, and destroy species that may have not-yet-discovered medicinal properties, such as the ability to cure cancer. In addition, environmentalists seek to preserve forests because they take up atmospheric carbon dioxide, curbing global warming.

In 1994 the Cambridge University Press Ecology Series published a book Biological Diversity: The Coexistence of Species on Changing Landscapes by Michael Huston, a biologist in ORNL's Environmental Sciences Division (ESD). It has been described by one reviewer as "a bold attempt to find a common link, a grand unifying theory, underpinning many of the major patterns in the way in which organisms assemble themselves into communities on the face of the planet." The book, which earned Huston the honor of Author of the Year and Publications Award at the 1996 Lockheed Martin Energy Systems Awards Night, has been well received by the scientific community—more than 4000 copies have been sold, and it is being translated into Spanish and Portuguese. The following article tells the story of how Huston came up with his theory and the response to his work.

I was trying to answer one of the major unanswered questions in ecology—why are there more species in some regions than in others?

Michael Huston examines the core of a young tulip poplar on the ORNL reservation. The core shows evidence of slower tree growth during the 1990s than in previous decades, possibly because of drought.

In 1976, when Michael A. Huston was a graduate student in biological sciences at the University of Michigan, he wrote a research paper that almost got him kicked out of graduate school. The paper proposed a hypothesis that challenged the widely accepted theory about coexistence of species of plants and animals in various environments, known as the principle of competitive exclusion.

"I was trying to answer one of the major unanswered questions in ecology—why are there more species in some regions than in others?" Huston says. "I proposed that species diversity was regulated by nonequilibrium processes rather than generally accepted equilibrium processes. Even more controversial was my suggestion that the productivity of tropical rain forests is low, since most ecologists think that tropical rain forests are the most productive ecosystems in the world because they have the highest number of species." The paper marked the beginning of Huston's scholarly attempt to refute two commonly held beliefs: that plants in tropical ecosystems grow at an unusually fast rate and that tropical rain forests can be converted to profitable agriculture and productive forestry.

Species diversity is the result of a dynamic equilibrium between competitive displacement and disturbance.

A Gulf Fritillary, a type of butterfly found only in the tropics, visits a common roadside weed in Everglades National Park. Huston's most recent studies in the tropics have been conducted in this park.

Ignoring the recommendations of his graduate adviser, who told him not to waste his time working on species diversity because all of the top ecologists in the country were already concentrating on it, Huston developed his ideas and submitted his paper to one of the major journals in ecological and evolutionary theory. He also submitted the paper to the faculty evaluation committee as part of the preliminary examination process for his doctoral degree. The faculty committee found the paper totally unacceptable, and in early 1977 Huston was on the verge of being ushered out of the graduate program with a "terminal" master's degree when news arrived that the unacceptable paper had been accepted by the journal.

However, even before his paper was published, Huston's theory was attacked in a March 1978 Science magazine article by a prominent ecologist, who concluded (somewhat prematurely, Huston adds) that "among neither tropical rain forest trees nor corals is there a consistent correlation of diversity and growth rates, as predicted by the (Huston) hypothesis." When the paper, "A General Hypothesis of Species Diversity," finally appeared in January 1979 (The American Naturalist, Vol. 113, pp. 81–101), it attracted the attention of ecologists all over the world. "Because more than 1000 reprint requests came in the first two years," Huston says, "the university Biology Department took over the task of making copies and mailing them out." Needless to say, Huston was allowed to continue his graduate studies at the University of Michigan.

The writing of that paper was not the only time Huston would encounter resistance to his ideas. The publication of the paper, however, marked one of several times that he gained the attention of ecologists and exerted a potentially significant influence on the field of ecology.

Coexistence of Species

In 1859 Charles Darwin explained how species arose in his book The Origin of Species by Means of Natural Selection. In recent decades, a question of intense interest to ecologists has been, how do existing species survive and coexist in spite of the many processes that cause extinction?

"The basic premise of most ecologists," Huston says, "is that the coexistence of species is based on the balance of nature. The idea is that no two species can live on the same resource, so if species coexist, they must be using different resources. For example, if you had two kinds of woodpecker eating the same type of insect inhabiting the same type of tree, one species would outcompete the other species, which would vanish from this forest. But you can have two different species of woodpeckers in a forest if the two species use different trees or eat different insect species."

A strangler fig competes for light and will eventually kill this cypress tree in a South Florida swamp forest.

The composition of a forest is largely determined by competition. Trees compete with each other for energy from sunlight, soil nutrients, and water. The leaves take in light and the roots take in water and soil nutrients. The tallest trees soak up most of the sunlight. Only the smaller trees that can tolerate shade, such as sugar maples, will survive in such a forest. Eventually, the forest will reach a state of equilibrium in which only a particular combination of species of trees, ferns, other plants, insects, birds, and mammals coexists. This picture of nature is called the competition equilibrium model.



Where most ecologists see stability in nature,
Huston sees instability.

An American egret stalks its prey in a flooded "prairie" surrounded by "tree islands" in Big Cypress National Preserve in Florida.

Where most ecologists see stability in nature, Huston sees instability. As a graduate student, Huston found that models based on the assumption of competitive equilibrium were not particularly useful. "An unappreciated aspect of the principle of competitive exclusion," he says, "is that it may take a long time to reach the equilibrium state where weaker competitors become extinct." Period disruptions—death-causing disturbances such as drought, disease, insect attacks, asteroid impacts, fires, extreme cold, hurricanes, windstorms, lightning strikes, floods, bulldozers, and excessive predation—interrupt the process of competitive exclusion and prevent it from reaching its equilibrium end point. Even in the absence of disturbances, the rate at which the process of competitive exclusion approaches equilibrium can vary greatly in response to such environmental conditions as temperature, soil fertility, precipitation, and solar radiation. When the rate of competitive exclusion is slow, fluctuations in environmental conditions can shift the competitive balance between species, allowing formerly inferior competitors to become dominant.

According to Huston, the species diversity found at any location is the result of a dynamic equilibrium between competitive displacement and disturbance. Consider the forest at Walker Branch Watershed, an ORNL research area for which Huston was project leader for five years. There, tulip poplars are a potentially dominant tree species under moist conditions. But in the past eight years, based on measurements of tree growth rings, their rate of growth has slowed dramatically. The probable reason is the series of unusually dry years, particularly 1988, 1993, and 1995. In contrast, the white oaks in the watershed are growing at nearly the same rate as before the droughts, because they are more drought tolerant than tulip poplars. A little drought has changed the forest dynamic, reversing the relative growth rates of two abundant species and slowing the process of competitive exclusion.

Diversity vs Productivity

So, how did Huston come up with the dynamic equilibrium model? And, what does the dynamic equilibrium model have to do with biological diversity in the tropics? His diverse life experiences seem to have influenced his views on species diversity.

"I grew up in Iowa, where the soil is unbelievably rich and fertile," he says. "I saw field after field of lush crops of corn, soybeans, and hay used for animal feed. I learned that the land had once been prairie and grassland, but human exploitation of the landscape had driven away wildlife such as elk, bison, wolves, and cougars, and some species such as the passenger pigeon have become extinct. Before I graduated from Grinnell College, I attended Deep Springs College near Death Valley, where I experienced a desert landscape that is unaffected by humans and sparsely populated with remarkably adapted plants and animals.

"In my first year as a graduate student at the University of Michigan, I traveled to Costa Rica to study tropical ecosystems. The tropics have played such a critical role in the development of evolutionary and ecological theory that every student there hopes to make some new discovery, such as a new species of beetle or fungus. I went to Costa Rica full of questions about species diversity. During the wettest July ever recorded at the La Selva field station in the lowland rain forest, I wandered through treefall gaps, traced the roots of lianas across the forest floor, and watched the river flood the forest. I couldn't avoid noticing how different the tropical soils were from the soils of Iowa, in the red color, high clay content, lack of any organic layer, and apparent infertility. Yet there were some impressively large trees, and an overwhelming diversity of plants of all sizes. I didn't know anything about soil science, but I felt that the differences between the vegetation of Iowa and Costa Rica had something to do with the soils.

"Unable to fit what I was seeing with what I had learned, I was struck with an idea that, at the time, contradicted the foundations of ecological theory. Rather than an equilibrium balance of finely coevolved competitors reaching maximum diversity at the highest levels of productivity, I hypothesized that the high diversity I saw around me resulted from the opposite situation. Equilibrium had been disrupted by various types of disturbances, such as windstorms that blew over large trees. Low fertility of the soils seemed to ensure such a slow return to competitive equilibrium that many species were able to coexist in a state of nonequilibrium. I saw high diversity but low productivity."

In other words, plants in the tropics—numerous species of trees, vines, orchids, cacti, ferns, begonias, and epiphytes—compete for light, water, and soil nutrients, just like plants everywhere. However, because the soils are poor and high temperatures increase energy losses through respiration, the plants don't grow very fast from day to day, although they often grow year-round. As a result of this slow growth, superior competitors are not able to dominate very quickly, periodic dry years may alter competitive hierarchies, and many species can coexist in a nonequilibrium state. So the number of different species is highest in the tropical forests in spite of, or in Huston's view, because of the infertile soils.

In contrast, during the much shorter growing season in the temperate zone, plant growth on a day-by-day basis is actually much higher than in most of the tropics because of higher soil fertility and more favorable temperature conditions. However, the short growing season results in a lower annual total plant production than in the tropics, contributing to the conventional wisdom of ecologists that plant productivity is highest in the tropics. It is this "common wisdom" that Huston has tried to refute, but not without a lot of resistance.

Coral Reef Diversity

The second time Huston ran into a brick wall was in 1978 when he sought to describe patterns of diversity in coral reefs using data he had gathered while scuba diving.

"I have been repeatedly surprised by the resistance to ideas and data that contradict the common wisdom," he says. "I had some difficulty publishing data on coral species diversity that I collected as part of a field course in Jamaica during my second year in graduate school. Based on my hypothesis about species diversity, I predicted that the diversity of corals growing together on the reef surface should be higher in deeper water, where there was less light, than near the surface where coral should grow rapidly. During six weeks of daily diving on the reefs at Discovery Bay, I collected enough data to demonstrate that coral diversity did indeed increase from the surface to 20 meters in depth, after which it began to decrease.

"Reviewers of the paper I submitted on coral diversity patterns rejected the paper with the criticism that the data were obviously in error because it was well known that coral diversity decreased continuously with depth. A few years later, another paper came out demonstrating that the same pattern I had found also occurred in the Indian Ocean. So in 1985, seven years after I had completed my field work, I compiled and published a review paper on coral diversity documenting that diversity increased with depth on coral reefs around the world."

For his doctoral thesis, Huston sought to identify specific biological mechanisms that produced the patterns of diversity he had observed. At the La Selva field station in Costa Rica, he set up experimental tree plantations in which he monitored the growth and mortality of more than 1000 individual plants of four forest species—some in plots for mixed species and others in plots of just one species. The experimental treatments consisted of two levels of light (full sunlight and 30% of full sunlight, created under a giant tent of shade cloth) and two levels of soil fertility (fertilized and unfertilized).

"My goal was to understand forest succession in terms of the interaction of individual tree physiology with the environmental conditions under which each individual grew," Huston says. "The trees grew rapidly, and I based my thesis on the first two years of data, although I continued to monitor the plots periodically for over 10 years. I saw clearly that both the rate of succession and species diversity are strongly affected by soil fertility." Both rain forest trees and reef corals show the same pattern of diversity in relation to growth rates. The slower the growth is, the higher the diversity.

Individual-Based Population Models

Computer models of forest succession can predict changes in upcoming generations of forests that result from competitive exclusion and disturbances. Since the 1960s, ORNL scientists had been studying forests using systems ecology models. This approach ignored individual trees and treated the forest as a mass of plant tissue. Then, in the mid-1970s, ORNL's Hank Shugart and his students began working with computer models that simulated forest succession using the interactions of individual trees with each other and their environment. Huston was interested in working with Shugart on these models when he came to ORNL.

"In physics, the fundamental units are subatomic particles," Huston says. "In ecology, the fundamental units are individual organisms. The survival or extinction of individuals in each species is increasingly seen as important to ecological as well as evolutionary processes."

In 1986, after Shugart had departed the Laboratory, Huston worked with ESD's Mac Post and Don DeAngelis on individual-based models that incorporate ideas of nonequilibrium. DeAngelis was studying size bimodality in fish—why some fish populations have only large and small individuals and no individuals of an in-between size. Because Huston was interested in size bimodality in plant populations, he began interacting with DeAngelis.

Huston and Post used individual-based models to predict forest succession. They looked at oaks and maples as individual trees that respond in specific ways to the availability or lack of sunlight, water, and soil nutrients. They didn't tie a yellow ribbon around a young oak tree and watch it grow in a forest; instead they represented individuals of each species as equations in a mathematical model. They were seeing the whole forest through the individual trees; they were simulating the interactions of individual trees with each other and the environment to better understand how the forest would look. They were identifying ecological processes that affected the evolution of forests.

"Competition in forests even influences biochemical processes," Huston says. "Suppose you have a forest of mostly oaks and pines. The pine trees decline in growth and number because of age, drought, and perhaps an attack of pine beetles. The oak trees continue to thrive. As a result, the forest litter will consist of more decomposing oak leaves and fewer pine needles. The chemical composition of the soil will change, causing a shift in nutrients and the types of species that will be found in the future forest."

In 1988 Huston, Post, and DeAngelis wrote an article published in BioScience that linked individual and population processes with ecological and evolutionary patterns. The paper described ORNL's models of forest succession and fish population structure, which simulated the growth and mortality of individual organisms and ultimately added them together to predict the composition of a forest community or a fish population. The paper made a big splash in the field of ecology and received the 1990 award of distinction (first place) in the category of Scholarly/Professional Articles in the International Technical Publications Competition of the Society for Technical Communication.

We introduced a major new approach to ecological modeling, which has the potential to integrate ecological processes across organizational levels and spatial scales.

"Our paper articulated the rationale for and potential applications of what we called 'individual-based models,' " Huston says. "We introduced a major new approach to ecological modeling, which has the potential to integrate ecological processes across organizational levels and spatial scales. It was quickly embraced by researchers who could see that the models related directly to the individual organisms they were studying. Perhaps the most significant contribution of our paper was the demonstration that phenomena at all levels of organization, from the individual organism to the entire ecosystem, could be understood within the single conceptual framework of organisms interacting with each other and their environment."

The individual-based modeling approach was illustrated by a variety of specific examples in a book from a symposium that the ORNL ecologists organized with their University of Tennessee (UT) collaborators Lou Gross and Tom Hallam. The approach is now widely used in a variety of theoretical and practical applications. Tropical rain forest management; accumulation of environmental contaminants in fish and daphnia; power plant impacts on fisheries; and the survival of the deer, panthers, and wading birds of the Everglades are among the many current issues being addressed by individual-based models. (See "Saving the Florida Everglades".)

In collaboration with Tom Smith, an ORNL postdoctoral researcher and a graduate of the UT Ecology Program, Huston developed a model that describes the interactions of physiological adaptations of different plant species with variations in light and water availability. This approach produced a unified theory of plant community dynamics that predicts how shade tolerant different species would be and how fast they would grow under different conditions of water availability.

Book on Biodiversity

In 1986, Huston received a request from Cambridge University Press to write a book on species diversity. "The request," he says, "provided me the opportunity to pull all of my work together and integrate it in a way that was simply not possible in a series of publications. While reorganizing my work to reflect my current understanding, I was pleasantly surprised to come up with a few new insights."

The goal of the research was and still is to determine the distribution of soil and water resources across the landscape and interactions between them and living organisms.

Huston stands among the plastic troughs used to investigate the effects on forests of both increased and decreased precipitation that might result from climate change.

While writing Biological Diversity: The Coexistence of Species on Changing Landscapes, Huston served as leader of ORNL's Walker Branch Watershed Project, which was started in 1967. During his tenure, the Walker Branch Watershed Project supported a wide range of research in hydrology, geomorphology, soil biogeochemistry, stream nutrient cycling, and forest ecophysiology, and it continued long-term monitoring of precipitation (which could be affected by climate change), stream flow and chemistry, and forest dynamics. The goal of the research was and still is to determine the distribution of soil and water resources across the landscape and interactions between them and living organisms. The project also provided the opportunity for Huston to apply many biodiversity concepts developed in his book.

The book was published in 1994. It received many positive reviews and a few negative ones. Huston says that younger scientists tend to be supportive of his work and older scientists tend to criticize it. "The main criticism of my theory has been that it has not been adequately tested," he says. "The challenge for ecologists is to provide quantitative evidence to convincingly reject my ideas, or they will continue to gain emphasis."

In response to Huston's argument that high plant diversity is found on soils too poor for high agricultural productivity, one critic wrote that it is possible to have high productivity and high diversity on the same land. "Yes, it is possible with frequent disturbance to have high plant diversity and high plant productivity," Huston says. "In my book, I noted that prairie grass and herb communities in the highly fertile midwestern United States have high diversity. Why? Because of frequent fires that eliminated woody vegetation. Also, I pointed out that diversity of animals can be high in productive areas like savannas, where plant diversity is low but the growth rate of grasses is high."

Huston said that he has seen no significant criticisms of his book that would refute his theory. "Most of the new studies that I have seen since my book came out," he notes, "actually support my hypotheses more strongly than I would have expected, given the high variability of natural processes."

Biodiversity and Land Use

A New Award

Michael Huston (along with ORNL's Betty Mansfield and Tuan Vo-Dinh) recently received the Department of Energy's BER 50 Exceptional Services Award. Huston was cited for "developing innovative concepts of the general patterns of biodiversity and how environmental change and human influences affect biodiversity."

How can the world make the best use of its natural resources? How should land be used to ensure that (1) the world's population will be adequately fed and clothed and (2) biological diversity will be maintained, avoiding additional extinction and endangerment of plant and animal species and conserving living organisms that may be the source of tomorrow's life-saving medicines? Huston's research in biological diversity suggests rational answers to these questions. These answers are summarized in the final chapter of Huston's book and in his article "Biological Diversity, Soils, and Economics" in the December 10, 1993, issue of Science magazine (for which he received ESD's Distinguished Scientific Achievement Award in 1994). In the Science article he wrote that plant species diversity "should be higher on unproductive, poor soils than on fertile, productive soils where plants are taller and total plant mass is higher. This pattern of highest plant diversity on poor soils and low plant diversity on the best soils is found throughout the world under a wide variety of conditions."

So, what are the implications of this phenomenon for land use and economic development? Is it possible to feed the world and preserve natural resources without causing major conflicts in land use?

"The only defensible reason to destroy natural habitat," Huston says, "is to improve the human condition. Land is justifiably needed for agriculture, forestry, housing, new factories, and expanding cities. But, in some cases, converting natural habitat to agriculture can destroy biological diversity without producing any real agricultural gains.

"Taking the rational view, we should not farm all over," he continues. "We should confine our farming to soils that are fertile and highly productive and use the most efficient farming techniques. Land that has a high diversity of plant species tends to have poor soil and often is not suitable for agricultural production. Some of our federally protected lands—national parks, national forests, and wildlife refuges—have been set aside because they are not useful for farming; but they have high diversity, so we are inadvertently protecting high diversity. Some parts of the Great Smoky Mountains National Park, for example, have a high diversity of plant species but little value for agriculture."

Huston encountered more resistance when his ideas on biodiversity and economics were first published in Science magazine. "I was surprised by the strong negative reaction from some parts of the conservation community," he says. "There are some who feel that human needs must be sacrificed to preserve nature. Aside from the moral or philosophical issues of human rights versus the rights of other organisms, I see little prospect that any individual will sacrifice his own or his family's well-being to save an endangered species or even an entire ecosystem. I believe biodiversity and natural resources can be preserved and the basic needs of current and future human populations can be fulfilled at the same time. I am optimistic that a better understanding of the functioning of ecosystems in relation to the maintenance of biodiversity and harvestable productivity will provide the key to a sustainable future for all life on the earth."

Dirt Is Destiny

Global mapping shows that the number of plant and animal species increases from either pole toward the equator, peaking in wet lowland tropical rain forests. Along this same latitudinal gradient, per capita gross national product (GNP, a standard indicator of economic condition) shows the opposite pattern, being lowest near the equator and increasing toward the higher latitudes. Huston believes it may be more than just coincidence that both patterns are consistent with his ideas about soils and species diversity.

"The same inverse relationship between species diversity and plant productivity that is seen at the local scale of a hillside also appears at the global scale," Huston points out. "The wealth of nations seems to follow the opposite pattern from plant diversity, being highest in many of the countries with the best soils, such as Japan, the Netherlands, the United States, and western Europe.

Huston notes that throughout human history, the highest levels of economic and cultural development have tended to occur in areas with rich soils, such as the "Fertile Crescent" of the Tigris and Euphrates Rivers; the Nile River Valley; Greece and Italy (before deforestation and erosion destroyed the soils); the Ganges, Yellow, and Yangtze river valleys; central Mexico and the Andes of South America; and, in North America, the Mississippi Valley, and the Pacific Northwest.

This is yet another issue where Huston meets resistance, this time from outside the field of ecology. "These observations make geographers and anthropologists very uncomfortable," Huston says, "because they are a form of 'environmental determinism,' which was misused in support of racist political agendas in the first part of this century. Nonetheless, the constraints imposed by the environment on human activities must not be ignored." Leaving room for the force of human will and ingenuity, Huston observes that even if two farmers work equally hard, the one whose farm happens to be located on fertile soil will make more money and have more time for cultural activities and will probably be able to send his children to a better college than the farmer who has the bad luck of settling on or inheriting less fertile land.

This same "luck of the draw" also applies to the nations of the world, Huston believes. Many less developed countries are located in tropical zones, where the soil is poor. Huston suggests that these countries cannot afford to take the approach of developed nations in temperate zones and arid regions: invest heavily in fertilization and irrigation to achieve high agricultural productivity. Because of low soil fertility in many of the less developed countries, Huston argues that they will never be able to develop a strong agrarian economy but rather must build their economies on mineral exploration, industrial production, tourism, or other activities.

So, who will feed the folk in the tropics? "Of all continents, North America has the highest proportion of fertile soils," Huston says. "Major changes are needed in our agriculture to achieve the highest yields efficiently and sustainably so we can supply food to the populations of less developed tropical nations. Large-scale organic gardening is not the solution because it is less productive than industrial farming. We should farm in a way to minimize erosion and improve the soil."

In the past, Huston points out, many civilizations have destroyed their soil by allowing pastures to be overgrazed or by cultivating hilly land. Erosion occurred, leaving the productivity of the land permanently reduced. He says that two modern methods of agriculture should be used widely to reduce erosion. One method is no-till agriculture, in which little plowing is used and unharvested plant material is returned to the soil. The other is precision farming, in which (1) the field is divided into measured and precisely located parts using Global Positioning System satellites; (2) computerized tractors inject the precise amounts of fertilizer, pesticides, and seeds of the right hybrid species in each part of the field to maximize its productivity; and (3) a computerized harvester takes inventory of the crops it harvests at each field location to help the farm manager determine the changes needed to achieve the highest productivity.

In short, Huston believes that the rational use of land can conserve natural resources and biological diversity while still meeting the human need to be fed and clothed. "Preservation of areas of high plant biodiversity does not require the sacrifice of productive agricultural lands," he says. "There is no inherent conflict between the preservation of biological diversity and the economic improvement of the human condition."

Like an oak trying to grow in a forest dominated by tulip poplars, Huston's ideas will not die from competitive exclusion. His ideas are still alive today, and a growing body of evidence suggests his views on biological diversity may be valid. His theory may still be in the shadow of the current paradigms in ecology, but new evidence obtained by a new generation of ecologists may provide the truth, which though disturbing to some, will certainly change the balance of ecological thought.

How To Order Biological Diversity

Biological Diversity: The Coexistence of Species on Changing Landscapes, Cambridge University Press (1994).

Telephone number: 1-800-872-7423. E-mail address: marketing@cup.org. WWW site: http://www.cup.org

The book has 701 pages, 193 line diagrams, and 24 tables. Its catalog number is 369304 (paperback) and 360935 (hardback).


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