Ecology is the study of the interaction between organisms and their environments. Ecologists study all facets of life on earth, including genetics, evolution, energy and nutrient cycling, as well as climate change. During the twentieth century, ecology evolved to include all major fields of biology and became a cause célèbre for environmentalists, who co-opted the term in the wake of Rachel Carson's Silent Spring (1962).
Historical Background and Scientific Foundations
During the eighteenth and nineteenth centuries, large collections of nonnative organisms arrived in Europe as naturalists traveled the world and returned with specimens for botanical gardens and zoos. Such global travel and natural history collections greatly shaped the ideas of the two naturalists who laid the foundation for the science of ecology: Alexander von Humboldt (1769–1859) and Charles Darwin (1809–1882).
Between 1799 and 1804 von Humboldt traveled through Central and South America collecting plants and fossils. In 1807 he published his Essay on the Geography of Plants, in which he classified the world's plant communities by their dominant types (e.g., palms, firs, cacti, grasses, and mosses), a study known today as biogeography. Von Humboldt concluded that climate played the dominant role in determining the composition of plants in any geographical region, and that certain types of plants will always be found together due to their mutual dependence. He saw the natural world persisting in a state of harmonious equilibrium. Darwin's theory of evolution, published in 1859, challenged this.
Between 1831 and 1836 Darwin circumnavigated the globe aboard the HMS Beagle. Like earlier naturalists, he was perplexed by the vast array of species he found and questioned the laws that governed their distribution and groupings. What, he wondered, led to this great diversity? And what determined which species inhabited certain locations? The answer, he concluded, is competition. All individuals within a species and all species in general compete for limited resources; those individuals and species best adapted to their environments will leave more offspring and thus will populate a given area. Darwin called this mechanism “natural selection,” and he used it to explain not only the origin of new species but also the distribution and diversity of species.
Ökologie and German Plant Biology
In 1866 the German zoologist Ernst Haeckel (1834–1919) coined the term “ökologie” to define a new branch of physiology, the study of an organism's physical and chemical processes, the relationship between organisms and their environments (both biotic and abiotic), and between mutually dependent organisms. In the late nineteenth century, two German botanists, Oscar Drude (1852–1933) and Andreas Schimper (1856–1901), with their Danish colleague Eugenius Warming (1841–1924), shaped the future of ecology by applying their laboratory training in physiology to studies of biogeography and plant adaptations.
Most influential was Warming's Plantesamfund, published in 1895, which was translated into English as The Oecology of Plants in 1909. Warming analyzed how plants compete for and divide up the limited resources they need to survive, differentiating levels of interdependence between species. He used the term “commensalism” to describe species that coexist by divvying up shared resources, each using a portion that others do not need—squirrels and birds, for example, can live in the same oak tree because the squirrels eat the acorns while the birds use the branches for their nests.
Warming called more intimate interdependences “symbioses”—species that depend on each other for survival. He also analyzed the impact of light, heat, humidity, soil, terrain, and animals on species' abundance and distribution, developing the concept of succession, or change in community composition over time. This became the cornerstone of the dynamic ecology that took root in the United States and Britain in the late nineteenth century.
Ecological Pioneers on the American Frontier
One of the earliest American biologists to adopt the ecological point of view was the entomologist Stephen Alfred Forbes (1844–1930). Forbes worked for the state of Illinois, so his studies were dictated by economic concerns, but he hoped to use the data he collected to draw broader theoretical conclusions. In the 1880s he began to analyze fish populations in Illinois lakes, studying how the competition of natural selection led to equilibrium. In his paper “The Lake as a Microcosm,” Forbes saw the community as a natural unit of ecological study, describing the relationships between predators and prey and how, through competition, each species found its place. Forbes was also concerned about the economic impact of nonnative species. Along with entomologist Leland Howard (1857–1950), Forbes argued for control of introduced species.
In the 1890s two other Midwestern ecologists further refined both ecological theory and methodology, calling their study dynamic ecology, the study of change over time. One was Henry Chandler Cowles (1869–1939), who studied the Indiana Dunes along Lake Michigan. Inspired by Warming's Plantesamfund,
which he taught himself Danish to read, Cowles studied the process of ecological succession, hoping to discover how underlying geological formations shaped plant communities.
Cowles was trained in both geology and botany, and he developed what he called the “physiographic” approach to ecology, linking changes in the vegetative community to changes in the physical environment. Like Darwin and Forbes, he sought to discover the laws that defined the composition of communities and the patterns of change over time. While Cowles saw equilibrium as the ideal end point, he argued that most plant communities persisted in a state of constant change.
Cowles spent his entire career at the University of Chicago, training many ecologists of the next generation, including ecologist and biogeographist Charles C. Adams (1873–1955), botanist and phytogeographer Edgar Transeau (1875–1960), and ecologist William Skinner Cooper (1894–1978).
The second member of this influential duo was Cowles's contemporary, American plant ecologist Frederic E. Clements (1874–1945), who studied the plant communities of Nebraska. Where Cowles was the great teacher of ecology, Clements contributed to the discipline through research and writing, publishing the first American textbook on the subject, Research Methodsin Ecology, in which he strongly emphasized the physiological aspect of ecology. While many earlier naturalists had made analogies between plant communities and individual organisms, Clements took it quite seriously, arguing that the vegetative plant community should be studied like an individual “super organism.”
Drawing on cell theory, Clements proposed that ecologists follow the study model used by laboratory physiologists, explaining the functions of the plant community, the “complex organism,” in terms of the functions of its individual parts. Clements also stressed the use of scientific tools in ecological study, especially the quadrat, a square area of any size marked off in a random location within the vegetative formation. By counting the individuals of each species within a quadrat, ecologists could estimate its relative abundance in the community. Ecologists could also return to the quadrat at regular intervals for new counts.
While many ecologists accepted the organismic analogy as useful in formulating hypotheses, few (if any) went as far as Clements in asserting that the vegetative community is, literally, an organism in its functions and growth patterns. The most famous challenge to this theory came from botanist, taxonomist, and ecologist Henry Allen Gleason (1882–1975) at the New York Botanical Garden, who argued for what he called the “individualistic concept.”
According to Gleason, communities do not really exist; to say anything meaningful about the interactions between organisms and their environments, he believed, the ecologist should study individual organisms. Gleason's counterargument notwithstanding, Clements's metaphor did not disappear. Its legacy continues to haunt ecologists who still find it useful in theorizing about community processes.
From Plants to Animals
Despite the success of Forbes's work, animal ecology developed more slowly than plant ecology. Where plant ecologists could mark off quadrats and count the number of plant species per area, animal ecologists had to contend with species that frequently moved and actively concealed themselves. Nonetheless, a few early practitioners, including Americans Charles C. Adams and ecologist Victor Shelford (1877–1968), made important contributions. Like plant ecologists, they studied the process of succession with communities as their basic units.
A major breakthrough came with British zoologist Charles Elton's (1900–1991) niche concept, which he defined in his 1927 book Animal Ecology as an animal's place in the food chain. The niche and the food chain redirected animal ecology toward the study of food relationships in a community, concepts that have remained fundamental ever since.
Such analysis took animal ecologists beyond the traditional classifications of herbivore and carnivore and allowed them to study animals' functional roles in the community. As Elton put it, “When an ecologist says, ‘There goes a badger,’ he should include in his thoughts some definite idea of the animal's place in the community to which it belongs, just as if he had said, ‘There goes the vicar.’” While Elton's work reflected Forbes's earlier argument about the roles organisms play in the lake's harmonious balance, his work went beyond that of early community ecologists. Through his emphasis on the food chain, Elton defined energy as the major exchange commodity in biological communities.
Ecology Meets Mathematics
In the 1920s a new group of ecologists took a theoretical approach to the study of nature, one that did not require counting real plants and animals. Mathematical ecology began after World War I with statistician Raymond Pearl's (1879–1940) study of changes in human populations. Through his research, Pearl rediscovered the work of the Belgian mathematician Pierre-François Verhulst (1804–1849), who formulated the population growth curve in the early nineteenth century, naming it the “logistic curve.” Pearl fit an equation to this curve, known as the Verhulst-Pearl equation, which represents the change in the rate of population growth over time.
Italian mathematician Vito Volterra (1860–1940) built on the Verhulst-Pearl equation in his model of competition between two species. Volterra's interest in ecological modeling arose in the 1920s in response to a question from his daughter's fiancé, a fisheries biologist, about changes in fish populations in the Adriatic Sea. While much of Volterra's mathematical modeling proved too advanced for ecologists, in the 1930s he developed a very useful equation that modeled predation in a two-species system. Yet, like Pearl, Volterra was not the first to discover his equation. In 1925 the American demographer Alfred J. Lotka (1880–1949) had independently discovered the two-species predation model, which predicts the oscillations of predator and prey populations, with changes in predator populations always lagging behind changes in prey populations. This model, known as the Lotka-Volterra equations, is still widely used by ecologists.
In the 1930s the Russian ecologist Georgii Gause (1910–1986) tested the validity of these mathematical equations. Using laboratory work and field studies he built on an idea suggested by Elton: the competitive exclusion principle, which states that no two species can fill the same niche. Based on this principle, Gause saw, beneath the seemingly stable community structure, continual competition among similar species for limited resources. Gause's work served as an important impetus to the field of evolutionary ecology in the 1960s.
From Organism to Ecosystem
In 1942 American ecologist Raymond Lindeman (1915–1942) synthesized many of the approaches to ecology in “The Trophic-Dynamic Aspect of Ecology.” Conducting his research at Cedar Bog Lake while a graduate student at the University of Minnesota, Lindeman formulated many of his key ideas with English-born American zoologist G. Evelyn Hutchinson (1903–1991), who was pursuing answers to questions raised by Elton's Animal Ecology at Yale University.
Inspired by the work of Russian biogeochemist Vladimir Vernadsky (1863–1945), Hutchinson refined Elton's concept of “energy” movement through communities. In studying energy flow and nutrient cycles, Hutchinson expanded the nonliving world beyond physical factors such as climate and soil to include chemicals such as carbon dioxide, nitrogen, and phosphorous. Instead of simple cause and effect, Hutchinson saw the relationship between the physical and nonphysical worlds as interactive, with energy and nutrients continually cycling between the abiotic and the biotic. Hutchinson hoped to quantify these cycles using mathematical modeling.
He tested his ideas in a long-term study of Linsley Pond in Connecticut. Like Forbes, he believed that by studying a well-defined, relatively simple system he could extrapolate his findings to more complex systems. This theoretical approach was not popular with all ecologists. Limnologist Chancey Juday (1871–1944), for example, argued that lakes are “rank individualists” and had to be studied independently and empirically; they were not reducible to mathematical equations.
Lindeman's famous paper reflected the influence of Hutchinson's approach. Using his Cedar Bog Lake research, he demonstrated how energy transfer and nutrient cycling could be followed through a biological community over time and quantified with mathematical equations. Lindeman further suggested that ecological succession could be studied as the change in energy production over time, using rough quantifications to show how energy production would change as Cedar Bog Lake evolved from a lake, to a bog, and, ultimately, to its climax state as a forest, whence its energy production would remain relatively constant.
While Lindeman's work was inspired by field studies, his limited data did not, in his critics' eyes, justify his theoretical conclusions. Several ecologists, Juday included, thought that the paper should not be published. Hutchinson fought hard for its publication, though, and it ultimately appeared several months after his premature death. Amid its many important theoretical contributions, none had greater impact than the ecosystem.
English ecologist Arthur Tansley (1871–1955) first outlined the ecosystem concept 1935. Countering Clements's “super organism” concept, Tansley suggested
the ecosystem, an isolated unit defined by the problem under investigation. It could be a forest, a pond, or a puddle, an atom or the universe. While Tansley's concept was not widely used in the late 1930s, in the wake of Lindeman's paper it came to define a new and productive branch of ecology.
“The New Ecology”
In the decades after World War II, ecology distinguished itself as a branch of biology with new sets of problems, tools, and sources of funding. Two brothers from North Carolina, Eugene (1913–2002) and Howard Odum (1924–2002), took the lead in this new discipline, which Eugene dubbed the “new ecology.” Its fundamental unit was the ecosystem, which was also the organizing principle of Eugene's 1953 book The Fundamentals of Ecology, (the standard textbook in the field for over two decades). The new ecology, according to the Odums, focused on the study of energy and nutrient flow through ecosystems.
In 1954 the U.S. Atomic Energy Commission (AEC) sent the Odums to study the effects of radiation on the Enewetak atoll, the site of several American nuclear bomb tests from 1948 to 1956. To determine the effect of radiation, the brothers treated the coral ecosystem much like an individual organism, measuring its metabolism by quantifying the energy flowing into and out of it, developing an energy budget for the entire atoll. Eugene and Howard also turned the “economy of nature” into more than just a metaphor, showing how the ecosystem's currency—energy and nutrients—could be measured, and how, in stable systems, the energy budget was balanced with inputs nearly equaling outputs. Their Enewetak study won the Ecological Society of America's Mercer Award in 1956.
Beginning in the 1950s, the AEC also funded ecosystems research at Oak Ridge National Laboratory. In these studies, radioactive isotopes of nutrients, like phosphorus, were added to systems and followed, using radiation detectors, as they moved through the trophic levels of the ecosystem. Ecologists literally watched nutrients cycle through systems, measuring the rates and efficiency of nutrient transfer among organisms.
Modeling ecosystem processes leapt forward at Oak Ridge in the 1950s and 1960s as newly developed computers further allowed ecologists to “experiment” and make theoretical alterations to systems, using the computer to model the repercussions. While these models had limitations, ecologists quickly realized their importance for the future of the discipline.
Long-Term Ecological Studies
Between 1964 and 1974 ecosystem studies and modeling came together as part of the International Biological Program, which studied five biomes—grasslands, coniferous forests, deciduous forests, tundra, and deserts—in an attempt to develop a computer model for each. While these systems proved intractable to such reduction, the program did train a generation of ecologists in ecosystems ecology and employed over 1,800 scientists in biome studies. The program also revealed the important roles these ecosystems play in maintaining a healthy environment, providing evidence of the need for conservation.
Concurrent to these large-scale biome studies, American biologist Gene Likens (1935–) and botanist Herbert Bormann (1922–) began a long-term ecosystem study at the Hubbard Brook Experimental Forest in New Hampshire, where they measured nutrient flow and tested long-held assumptions about ecological succession. These experiments included denuding small areas within the forest and monitoring the regrowth process over decades. The results led them to hypothesize a “shifting mosaic steady state,” where patches within the larger ecosystem existed in different stages of succession. While the system, as a whole, appeared to be balanced, a great deal of change actually took place within it. This challenged an orderly, progressive concept of succession, but maintained that succession leads, ultimately, toward a steady state.
The value of such long-term ecosystem studies was reinforced by the National Science Foundation's funding of the Long Term Ecological Research (LTER) network in 1980. Fittingly, in 1987, Hubbard Brook was designated a LTER site. LTER expanded in 1993 to become the International LTER network (ILTER). By 2005 ILTER included 32 countries and sponsored two urban ecosystem studies in Baltimore, Maryland, and Phoenix, Arizona. These long-term studies are necessary for a scientific understanding of how nature's economy works.
The Diversity of Life
While ecosystems ecology formed the core of the Odums' “new ecology,” it also led to a rift with ecologists who saw populations of organisms as the appropriate level of study. Population ecologists were interested in evolutionary explanations and how organisms evolved to fill their roles. The discrediting of group selection in the 1960s further widened the gap between these two approaches.
Two British ornithologists, David Lack (1910–1973) and Vero Copner Wynne-Edwards (1906–1997), played prominent roles in this debate. Lack studied finch populations in the Galapagos Islands and hypothesized that competitive exclusion led to the diverse types of finches found there. In his 1954 book The Natural Regulation of Population Numbers, Lack explained that natural succession controlled bird population size. Over time, he suggested, birds that produced clutch sizes optimal to the availability of resources would have the most evolutionary success. His explanation was evolutionary.
In the 1960s Wynne-Edwards challenged Lack's hypothesis, arguing that clutch size was a product of group selection. He argued that bird populations controlled reproduction so as not to outstrip their resources. His explanation was functional. In 1966 American entomologist George C. Williams (1926–) attacked Wynne-Edwards's hypothesis in his book Adaptation and Natural Selection, defining individuals as the appropriate level for evolutionary study, and, thus, populations and evolutionary explanations as the basis for evolutionary ecology.
In 1957 American ecologist Robert MacArthur (1930–1972), who had earned his Ph.D. under Hutchinson, spent a year in England working with Lack. Studying the natural laws underlying niches and competitive exclusion, MacArthur developed mathematical models of population ecology and used field data to test them. In 1963 he and Harvard entomologist Edward Osborne Wilson (1929–) published the island biogeography theory, which states that the number of species on an island (its diversity) is a dynamic equilibrium between colonization and extinction.
While controversial, this theory spawned much research. In 1966 and 1967 Wilson and Daniel Simberloff tested the theory by poisoning all of the arthropods on seven small islands in the Florida Keys then monitoring the recolonization. While controversial, their results provided support for the theory and had the effect MacArthur sought: encouraging population ecologists to employ a more theoretical, mathematical, and experimental approach. In the 1980s Wilson became a champion of biodiversity, a concept growing out of his work with MacArthur.
Modern Cultural Connections
While ecologists have generally kept their scientific work separate from the environmental movement, many have used their science to argue for environmental protection. In the 1890s MIT industrial and environmental chemist Ellen Swallow Richards (1842–1911) coined a new field of study, “home ecology” (which later became “home economics”), which she saw as a field involved with improving the living environment for humans both inside and outside the home. Through the study of home ecology, Richards and her female colleagues made important contributions to social reform during the Progressive Era in the United States.
Seventy years later another female scientist brought the word “ecology” into popular usage. In her 1962 book Silent Spring, Rachel Carson examined the detrimental effects of man-made chemicals on ecosystems and individual species, including humans, within vast food chains. In 1968, Stanford biologist Paul Ehrlich (1932–) incorrectly predicted an overpopulation crisis in his book The Population Bomb, asserting that at then-current rates of growth the human population would outstrip its resources, leading to worldwide famines beginning between 1970 and 1985.
Carson and Ehrlich's work led to political action. In 1969 the United States government passed the National Environmental Policy Act, and formed the Environmental Protection Agency in 1970. In 1972 this agency banned the use of DDT, one of the deadly chemicals Carson analyzed in her book.
In 1973 the Ecology Party (which became the Green Party in 1985) formed in England, the first in a movement that soon spread across Europe. The Ecology Party represented a wide array of liberal interests including demilitarization, feminism, and environmentalism. In the 1970s the “deep ecology” movement gained popularity as well. First outlined by Norwegian philosopher Arne Naess (1912–), it aims to remove anthropocentrism from the environmental movement. Naess believes that all life, including human, has equal value and that human needs should not be given greater priority than those of any other species.
At the beginning of the twenty-first century ecology as a discipline reflects the concept of biodiversity. For ecologists working in fields such as conservation biology and restoration ecology, this represents the basis of their study—the protection of life on all levels. For academic ecologists, biodiversity reflects the scope of the discipline, studying genetics, individuals, populations, communities, ecosystems, and landscapes. With the growing awareness of human-caused environmental damage, ecologists also study the effects of global climate change and analyze how humans interact with (and alter) their environment. The two urban studies in the LTER network reflect this new approach toward ecology. Addressing human issues and studying ecological problems on landscape and global scales has proven challenging for ecologists, forcing them to take a more interdisciplinary approach to their studies.
In 1964 Paul Sears called ecology a “subversive science,” arguing that it would ultimately undermine ideas of human progress embodied by the pursuit of scientific knowledge. Today, as the earth's population approaches 7 billion and as organisms are threatened with extinction at an unprecedented rate, it seems that Sears saw the future of ecology clearly. Twenty-first century ecologists are charged with repairing damaged ecosystems and with studying them to see how much change they can tolerate.
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