by DK
See also: The distribution of species over space and time • Climax community • Open community theory • Biogeography
IN CONTEXT
KEY FIGURE
Eugene Odum (1913–2002)
BEFORE
1905 In Research Methods in Ecology, American botanist Frederic Clements writes about plant communities and how they change over time.
1935 Arthur Tansley, a British botanist, proposes the term “ecosystem” to describe a community of plants, animals, soil minerals, water, and air.
AFTER
1954 Eugene and Howard Odum’s study of the coral Eniwetok Atoll in the Pacific Ocean applies the principles of holistic ecology.
1974 British environmentalist James Lovelock and American biologist Lynn Margulis first publish their Gaia hypothesis. It states that Earth is a self-regulating system that maintains the conditions necessary for life on our planet.
The American ecologist Eugene Odum was not the first scientist to write about ecology, but in the 1950s he proposed that it deserved to be a discipline in its own right. Until then, ecology was viewed as a relatively insignificant subdivision of the biological sciences—the poor relation of biology, zoology, and botany. However, Odum believed passionately that studying plant and animal species in isolation could never lead to a full knowledge of the living world. He argued that it was more important to study the places and roles that the species held in their community, rather than simply finding out more about what they were. Odum’s new approach to the subject—first set out in his 1953 book The Fundamentals of Ecology—revolutionized the purpose and influence of ecological research.
Salt marshes, such as these on the coast near Porthmadog, North Wales, form their own ecosystem, with the seawater and its nutrients providing a unique habitat for wildlife.
The “new ecology”
The holistic view of Earth involves studying the systems of organisms as a whole. As Odum explained, one organism, or any one group of organisms, cannot be understood without studying the ecosystem in which it lives. The holistic approach examines all the roles played by each member of an ecosystem, and how that system interacts with others. Climate, geology, water and mineral input, and human activity all affect—and are affected by—a multitude of living communities.
Odum was writing in the 1950s and ’60s, when there was a growing awareness of the environmental destruction wrought by humanity. The role of people was a crucial part of “systems ecology,” as he called his idea. Odum wanted humans to be sympathetic allies with the natural world—collaborators rather than manipulators—and his views of an all-embracing ecology did much to inspire the first Earth Day, which was celebrated in 1970.
The holistic concept of Odum’s “new ecology” deals with Earth as a whole, bringing together physics, chemistry, botany, zoology, geology, and meteorology. The fundamental assumptions of ecology are that the ecosystem is the basic unit of nature, that biological diversity increases the ability of ecosystems to survive, and that the whole is greater than the sum of its parts. Systems in the natural world—whether they are groups of cells in an animal’s body, the whole animal, or the ecosystem in which the animal lives—are able to self-regulate to provide stability.
“…ecology has been badly presented and has been broken into too many antagonistic subdivisions.”
Eugene Odum
Integrated investigation
A holistic study of a lake ecosystem would involve looking at all the inputs into the lake and its margins as well as all the outputs, including energy, water, minerals, and nutrients. It would also consider any human inputs. The study would examine the roles played by both producer organisms, such as plants and algae, and consumers such as herbivores and carnivores. The holistic approach also examines changes over time, in which developments that benefit some organisms in the short-term might lead to a lack of diversity in the future. For example, although trout thrive in warmish, alkaline waters, if those waters become too warm or acidic due to ecological change, the fish can no longer breed.
Odum’s holistic approach leaves a legacy of a far more detailed appreciation of what is happening in an ecosystem than a series of individual species studies.
Earth Day
The first Earth Day on April 22, 1970, saw crowds such as this one in Philadelphia, Pennsylvania, gather across the US to protest against pollution and the use of pesticides.
After witnessing a horrific oil spill in Santa Barbara, California, in 1969, US Senator Gaylord Nelson decided to focus on growing worries about pollution during a national “teach-in” on the environment. He could not have envisaged the size of the movement he would inspire. On April 22, 1970, 20 million Americans took part in the first Earth Day, with rallies, marches, and lectures taking place nationwide. Such was the effect of the protests that later that year the Clean Air, Clean Water, and Endangered Species Acts became law, and the Environmental Protection Agency was established in the US that December. Earth Day became a global phenomenon, with 200 million people participating in 141 countries in 1990—and built momentum for the 1992 UN Earth Summit in Rio de Janeiro. Earth Day celebrations are held every April, with a different theme each time. In 2018, the focus was on ending global pollution by plastics.
See also: The ecosystem • Macroecology • The peaceful coexistence of humankind and nature • The Green Movement
IN CONTEXT
KEY FIGURE
Alfred Wegener (1880–1930)
BEFORE
1596 Abraham Ortelius, a Dutch scholar, is one of several geographers who observe that the two sides of the Atlantic Ocean seem to “fit” each other.
AFTER
1929 British geologist Arthur Holmes proposes that convection in Earth’s mantle drives continental drift.
1943 George Gaylord Simpson dismisses fossil evidence for continental drift and argues for “stable continents.”
1962 American geologist Harry Hess explains how the seafloor spreads, by molten magma rising from below.
2015 A group of Australian scientists propose that periods of rapid evolution in the oceans were triggered by collisions between tectonic plates.
The surface of Earth is constantly moving, very slowly, and has been doing so for more than three billion years. The lithosphere (Earth’s crust and upper mantle) is divided into seven large sections and many smaller ones, called tectonic plates. Where plates meet, the type of movement determines the nature of the boundary. Where plates push against each other, new mountains are created. If plates pull apart, new crust forms on the ocean floor.
The first inkling that the continents may not have always been in their current positions came in the late 16th century. European explorers sailing to the Americas saw from their newly created maps that the coastlines on each side of the Atlantic Ocean mirrored each other. Later, geologists found strong structural and geological similarities between the Caledonian-era mountains of Northern Europe and the Appalachian Mountains of North America.
Lookalike fossils
There are various examples of fossil finds straddling different continents that can only be explained by continental movement—since the animals or plants concerned would have been unable to cross the ocean divide. These include Cynognathus crateronotus, a mammal-like reptile that lived over 200 million years ago in southern Africa and eastern South America. Glossopteris, a genus of woody trees, grew in South America, South Africa, Australia, India, and Antarctica, but nowhere else, around 300 million years ago.
To German geophysicist Alfred Wegener, such fossil patterns indicated that these continents had once been joined together. In 1915, he published his theory that all the continents were once a single land mass, “Pangaea,” which has since broken up and drifted apart.
Wegener’s theory was not well received at first. In 1943, George Gaylord Simpson, one of the most influential paleontologists in the US, criticized the theory. He argued that the fossil record could be explained by static continents linked and unlinked by
periodic flooding.
This fossilized head of the extinct reptile Cynognathus crateronotus was found in southern Africa. The same fossils occur in South America: evidence that the two continents were once one.
Evidence and evolution
Despite early doubts, evidence for the plate tectonics theory grew. A series of discoveries established that the seafloor was spreading and that new oceanic crust was constantly being created. We now understand that the movement of the tectonic plates is driven by convection currents carrying heat from deep inside the planet to the surface.
Once Wegener’s theory was accepted, the fossil evidence made much better sense. Continental drift has had a profound influence on how species have evolved. For example, if a continent splits apart, the two separated populations of a species can start to evolve in completely different directions. On the other hand, if two continents collide, or a bridge of land forms between them, different species begin to mix and compete, and some may become extinct as a result.
Tectonic plates can move in three different ways, forming different types of boundary. When plates diverge, new oceanic crust is formed. When they converge, new mountains form. When plates slide past each other, the rift is known as a transform fault.
“The forces which displace continents are the same as those which produce great fold-mountain ranges.”
Alfred Wegener
Marsupials in America and Australia
Marsupials are strongly identified with Australia, yet they evolved in America and are still also found there.
Marsupials are nonplacental mammals whose young complete their gestation feeding from their mother’s teats, typically in a pouch on the belly. Now found only in the Americas (mainly South and Central) and Australia, they are thought to have evolved in North America 100 million years ago. They spread to South America and diversified into many different species.
Several groups later moved into what is now Antarctica and on into southern Australia. It is thought that they traveled via a belt of vegetation straddling the three areas, which were once all part of the southern landmass called Gondwana.
By 55 million years ago, the continents had separated, and marsupial species began to evolve differently. The only known Antarctic marsupial fossils, found on Seymour Island in 40-million-year-old rocks, resemble South American marsupials of the same period, but not those of Australia.
See also: Island biogeography • The distribution of species over space and time • Macroecology • Metapopulations • Biogeography
IN CONTEXT
KEY FIGURE
James Lovelock (1919–)
BEFORE
1935 British botanist Arthur Tansley uses “ecosystem” to describe an interdependent community of biological and nonbiological components.
1953 In Fundamentals of Ecology, American ecologist Eugene Odum describes Earth as a collection of interlocking ecosystems.
AFTER
1985 In the US, the first conference on the Gaia hypothesis is held, entitled, “Is the Earth a Living Organism?”
2004 James Lovelock voices his support for nuclear power over renewable energy.
In 1979, British scientist James Lovelock’s book Gaia: A New Look at Life on Earth presented his Gaia hypothesis to a general readership. In essence, Lovelock claimed that Earth is a single, self-regulating system, in which living and nonliving elements combine to promote life. The book quickly became a bestseller, and caught the imagination of the growing Green movement, offering a fresh approach to environmentalism.
What Lovelock proposed was not without precedent. In the 1920s, Vladimir Vernadsky, a Russian scientist, had developed the idea of the biosphere, the zone of Earth that holds all living organisms, and suggested that it could be seen as a single entity in which organic and inorganic elements interact. The British botanist Arthur Tansley then took this idea further in the 1930s, with his concept of an “ecosystem” that regulates itself into a state of equilibrium.
Tansley’s theory was at the heart of Lovelock’s hypothesis: that all living organisms and their environment form one complex super-ecosystem that regulates and balances conditions to sustain life on Earth. The idea first occurred to Lovelock in the late 1960s, but it was after discussing it with US microbiologist Lynn Margulis that it began to take shape. Together, they presented the hypothesis in a paper in 1974, giving it a name suggested by the writer William Golding—Gaia, after the ancient Greek Earth goddess. Lovelock and Margulis portrayed Earth as a living entity, composed of the biosphere, living organisms; the pedosphere, the surface layer of the Earth; the hydrosphere, the bodies of water on the Earth’s surface; and the atmosphere, the gases surrounding the Earth. These spheres and their complex interactions maintain Earth in “homeostasis.” This concept is borrowed from physiology, which describes the stable internal conditions, such as temperature and chemical composition, that allow organisms to function optimally. They are controlled by self-regulating mechanisms that react to change in those conditions. Lovelock’s use of the word homeostasis reinforced the implication that Earth, or Gaia, is a living entity.
A stone relief shows Gaia, the Greek goddess of Earth. The nonscientific name chosen by Lovelock for his hypothesis initially hindered its acceptance by many scientists.
“Evolution is a tightly coupled dance, with life and the material environment as partners. From the dance emerges the entity Gaia.”
James Lovelock
Keeping the balance
The hint of mysticism in the Gaia principle chimed with the “New Age” thinking of the time. This helped popularize the idea, but it also led to a negative reception from the scientific establishment. However, behind the Earth “goddess” metaphor was a serious science-based hypothesis that the interactions of living organisms and their physical surroundings—including the cycles of oxygen, carbon, nitrogen, and sulfur—form a dynamic system that stabilizes the environment.
According to Lovelock, Gaia is controlled by the action of “feedback loops,” which are the checks and balances that compensate for disturbances in the system, bringing it back into equilibrium. To function well, life on Earth depends on a particular balance of variables such as water, temperature, oxygen, acidity, and salinity in its environment. When these are constant, Earth is in a stable state of homeostasis, but if the balance is disturbed, the planet encourages the organisms that will restore the equilibrium, while being hostile to those that reinforce the disturbance. The organic components of the Earth system do not simply react to changes in their environment, but control and regulate them.
These feedback mechanisms operate in a complex global network of interconnected natural cycles, to maintain the optimum conditions for the organisms within them. They can resist change, but only to a certain extent. A big enough disturbance can push the system to a “tipping point,” where, with the balance of its components altered, it is likely to settle into a very different state of equilibrium. Such a tipping point, argued Lovelock, occurred about 2.5 billion years ago, at the end of the Archean Eon, when oxygen first appeared on Earth. At this time, Earth was a hot, acidic place in which methane-producing bacteria were the only life that thrived. Bacteria capable of photosynthesis then evolved, which created an atmosphere that was conducive to more complex forms of life. Eventually, the equilibrium conditions that exist on Earth today were established.
In the Gaia hypothesis, Earth, the only known planet to support life, is itself a “superorganism,” where the sea, land, and atmosphere work together to maintain the right living conditions.
JAMES LOVELOCK
Inspired by writers such as Jules Verne and H.G. Wells, James Lovelock, born in 1919, was fascinated by science and invention from an early age. He graduated in chemistry from Manchester University in 1941. Lovelock was a conscientious objector during World War II and worked for the National Institute for Medical Research in London. In 1948, he received his Ph.D. in medicine, and then spent time in the US on a Rockefeller fellowship. After retu
rning to Britain in 1955, he turned his attention to inventions, notably the electron-capture detector (ECD), which detects trace atoms in a gas sample. In the 1960s and 1970s, he held visiting professorships in Houston, Texas, and Reading, England, during which time he developed the Gaia hypothesis. In 2003, Lovelock was made a Companion of Honour by Queen Elizabeth II.
Key works
1988 The Ages of Gaia
1991 Gaia: The Practical Science of Planetary Medicine
2009 The Vanishing Face of Gaia: A Final Warning
“If there were a nuclear war, and humanity were wiped out, Earth would breathe a sigh of relief.”
James Lovelock