by DK
“Flowering stones” (Lithops) are native to southern Africa, their thick, fleshy leaves well suited to dry, rocky conditions. Related species also occur in similar arid habitats in the US.
Adaptations for extremes
Schimper made a close study of plant physiology—the structures of plants and how they had adapted to varying temperature and moisture conditions. He was particularly interested in plants growing in extreme climatic conditions. Salty environments, for example, require plants to survive high levels of soil and water salinity. Schimper found that vegetation growing on the coastal mangroves of Brazil, on Caribbean and Sri Lankan beaches, and in sulfur-emitting volcanic craters in Java, were similarly tolerant to salt.
Schimper also studied how plants coped in the challenging conditions of arid environments. He found that plants growing in hot, dry places had evolved “varied contrivances for regulating the passage of water.” To illustrate this, he chose a type of vegetation with tough leaves, short internodes (the distances between the leaves along a stem), and leaf orientation parallel or oblique to direct sunlight. This type grew in various parts of the world, where arid conditions meant that water was scarce. The name Schimper gave to these plants—sclerophyll, from the Greek words skleros (“hard”) and phullon (“leaf”)—is still used today.
Epiphytes, plants that grow on the surface of other plants and derive their moisture and nutrients from the air or rain, also fascinated Schimper. He observed epiphytes such as Spanish moss growing in the southern US and the Caribbean islands and similar species in South America, South Asia, and southeast Asia. He found that they were linked by warm temperatures and year-round moisture—traits of what he called a tropical rain forest.
Although the broad geographic divisions devised by Schimper still hold true, there is now a better understanding of how vegetation develops in response to many different stimuli beyond simple climatic differences. For example, measures of potential water evaporation into the atmosphere, water surplus, and water deficit, which can be combined in a moisture index, are more useful determinants of plant distribution than simple temperature and rainfall figures.
“… the time is not far distant when all species of plants and their geographical distribution will be well known.”
Andreas Schimper
Like other epiphytes, Spanish moss lives on other species but draws water and nutrients from the air rather than from its host. It thrives in tropical and subtropical environments.
See also: Evolution by natural selection • Ecophysiology • The ecosystem • The foundations of plant ecology • Biogeography • Biomes
IN CONTEXT
KEY FIGURE
Henry Chandler Cowles (1869–1939)
BEFORE
1825 Adolphe Dureau de la Malle coins the term “succession” when describing new growth in forest cuttings.
1863 Austrian botanist Anton Kerner publishes a study of plant succession in the Danube river basin.
AFTER
1916 Frederic Clements suggests that communities settle into a climax, or stable equilibrium, at the end of a succession period.
1977 Ecologists Joe Connell and Ralph Slatyer argue that succession occurs in diverse ways, highlighting facilitation (preparing the way for later species), tolerance (of lower resources), and inhibition (resisting competitors).
The Indiana Dunes comprise a windswept section of shifting sand along the southern shore of Lake Michigan, US. In 1896, American botanist Henry Chandler Cowles saw these dunes for the first time, and so began his career in the emerging field of ecology. Dunes are among some of the planet’s least stable landforms, and therefore changes to their ecology happen relatively quickly. As Cowles walked among the dunes, he noticed that when certain plants died off, their decomposing matter created favorable conditions for other plants. As these new plants died, even more plants could grow.
Based on his observations, Cowles developed the idea of ecological succession, although groundwork for the concept had been laid by earlier naturalists. In an 1860 address to members of the Middlesex Agricultural Society, Massachusetts, Henry David Thoreau had stated: “Though I do not believe that a plant will spring up where no seed has been, I have great faith in a seed.”
15,000 years ago, there would only have been bare sand around Lake Michigan’s shore. Vegetation developed in a physical gradient, with sand nearest the water and forests farthest back.
Growth of an ecoystem
French geographer Adolphe Dureau de la Malle is regarded as the first person to use the term “succession” with reference to ecology when he witnessed the progression of plant communities after all the trees were removed from a forest. Cowles provided a more formal articulation of his ecological succession theory, in The Ecological Relations of the Vegetation on the Sand Dunes of Lake Michigan, published in 1899. In this seminal paper, he proposed the idea of primary succession—the gradual growth of an ecosystem originally largely devoid of plant life. The stages of primary succession include pioneer plants (often lichens and mosses), followed by grassy plants, small shrubs, and trees.
The process of primary succession begins in barren environments such as bare rock. Hardy species, usually lichens, appear first and then give way to a stable climax community of more complex and diverse life forms over hundreds of years.
Life after disturbance
Secondary succession occurs after a disturbance that destroys plant life, such as a flood or a fire. The plant life reestablishes itself and develops into an ecosystem similar to the one that existed before the disturbance. The stages of secondary succession are similar to those of primary succession, although the ecosystem may start at different points in the process, depending on the level of damage caused by the trigger.
A common example of secondary succession occurs after a wildfire in oak and hickory forests. Nutrients from burned plants and animals provide the right conditions for growth of annual plants. Pioneer grasses soon follow. After several years, due at least in part to the environmental and soil changes resulting from the pioneer species, shrubs and oak, pine, and hickory trees will begin to grow. As the trees grow higher, shading out more of the underbrush, the grasses are replaced by plants able to survive with low sunlight, and, after around 150 years, the forest once more resembles the prefire community.
“I … found indisputable evidence (a) that forests succeeded prairie, and (b) that prairie had succeeded forest.”
Henry Allan Gleason
American ecologist
See also: Field experiments • The ecosystem • Climax community • Open community theory • Biomes • Romanticism, conservation, and ecology
IN CONTEXT
KEY FIGURE
Frederic Clements (1874–1945)
BEFORE
1872 German botanist August Grisebach classifies the world’s vegetation patterns in relation to climate.
1874 British philosopher Herbert Spencer suggests that the human population can be thought of as a giant organism.
1899 In the US, Henry Cowles proposes that vegetation develops in stages, a process called succession.
AFTER
1926 US ecologist Henry Gleason argues that a climax community is a coincidental collection of individuals.
1939 British botanist Arthur Tansley suggests there is not a single climax community but “polyclimaxes” responding to various factors.
The term “climax community” was first proposed in 1916 by American botanist Frederic Clements. He used it to describe an enduring ecological community that has reached a steady state, such as a naturally stable forest of old-growth trees that has not undergone or been subjected to any unnatural changes, such as logging.
Regional communities
In the 19th century, German botanists August Grisebach and Oscar Drude were among those who recognized that patterns of vegetation around the world reflect factors such as climate variations. It was clear, for example, that the typical vegetation in a wet, tropical climate was
very different to that in a dry, temperate climate. Then in a landmark paper in 1899, American botanist Henry Cowles described how plants colonized sand dunes around Lake Michigan in stages—or “successions”—of increasing size and complexity.
In an influential book, Plant Succession (1916), Frederic Clements developed Cowles’s idea, which he combined with the biogeographic thinking of the two German botanists to produce a theory of the development of natural communities.
Clements suggested that the way to understand patterns of vegetation across the world is to think in terms of “formations.” A formation is a large, natural community of plants dominated by a range of life forms that reflects the regional climate. In each region, plants go through stages or successions until they reach the most complex, highly developed form of vegetation possible. Once it finally reaches this climax, the community stabilizes, in what was later termed a “steady state,” and stops changing.
Clements then proposed that climax communities are bound together. Although an ecological community is made up of a multitude of plants at different stages of growth, he argued that it can be considered as a single complex organism. A community grows toward a climax in the same way that an individual develops through life stages. Clements expanded the idea to embrace all organisms in a “biome” that comprised “all the species of plants and animals at home in a particular habitat.” From this, the idea of the ecosystem as a “superorganism” later developed.
“For Clements, climates are like genomes, and vegetation is like an organism whose characteristics its genome determines.”
Christopher Elliott
Philosopher of science
A fluctuating process
Clements’s ideas were challenged from the start, although the idea of a “steady state” proved influential and dominated thinking about ecosystems up until the 1960s. However, scientists realized that communities change constantly in response to conditions, and it is almost impossible to observe a true climax community. A $10,000 prize for identifying such a community, offered by American botanist Frank Egler in the 1950s, was never claimed. Despite the difficulties, ecologists continued to use the theory of a climax community to decide how to respond to invasive species that threatened to disturb an established native community, and in recent decades Clements’s ideas have regained support.
Succession remains a core principle of ecology. In general, early succession phases consist of fast-growing and well-dispersed species that are replaced by more competitive species. Initially, ecologists thought that ecological succession ended in what they described as the climax phase, when the ecosystem reached a stable equilibrium. However, it is now accepted that ecological succession is a dynamic process that is constantly in flux.
The Sonora Desert is often seen as an example of a climax community. It has both winter and summer rains, so its unique plants, which include the tall saguaro cactus, are unusually lush.
See also: The ecosystem • The distribution of species over space and time • Ecological succession • Open community theory • The ecological guild • Biomes
IN CONTEXT
KEY FIGURE
Henry Allan Gleason (1882–1975)
BEFORE
1793 Alexander von Humboldt uses the word “association” to sum up the range of plant types in a particular habitat.
1899 In the US, Henry Cowles states that vegetation develops in stages, in a process he calls plant succession.
1916 Frederic Clements posits the idea of a climax community as a single organism.
AFTER
1935 Arthur Tansley coins the term “ecosystem.”
1947 Robert H. Whittaker begins field studies that will refute Clements’s holistic idea of plant communities.
1959 John Curtis boosts Henry Gleason’s reputation with numerical studies of prairie plant communities.
When American plant ecologist Frederic Clements proposed the idea of climax communities in 1916, he envisioned the community as a superorganism in which all plants and animals interact to develop the community. A year later, American plant ecologist Henry Gleason dismissed the idea; he argued that plant species have no common purpose but merely pursue their own individual needs. Gleason’s hypothesis became known as the “open community” theory. The dispute initiated a debate that still rages in ecological circles today.
Gleason did not deny that plant communities could be mapped and their interactions identified, but he could see none of the integration proposed by Clements. Instead, Gleason believed that groups of plants were random growths of individuals and species, responding to local conditions.
Individual needs
Gleason maintained that the changes that occur during plant succession, as the composition of a community evolves, are not integrated stages, as in the development of a single organism. Rather, they are a combination of responses from individual species as they seek to meet their own needs within a locality. “Every species of plant,” Gleason argued, “is a law unto itself.” Gleason also denied that there is any endpoint or climax community; he believed that communities are always changing.
Diseases such as American chestnut blight challenge the idea of a fully integrated climax community, as the loss of the dominant tree species should cause the entire ecosystem to collapse.
Changing opinions
Gleason’s argument with Clements caused quite a stir at the time. Clements seemed to be creating an overview in which natural patterns of vegetation were determined by clear rules, just as in Newtonian science the movement of the planets is dictated by incontrovertible laws. Clements and his supporters were able to look at the bigger picture, while Gleason was viewed as a reductionist, myopically intent on the details and challenging the entire idea of ecology as a science controlled by laws.
Gleason appeared to be saying that there are no patterns in nature: it is all random. Worse still, he was accused by some of justifying exploitative farming, since his ideas seemed to imply that man need not worry too much about disturbing the balance of the natural environment—because there is no balance. Gleason’s ideas were therefore forgotten in the enthusiasm for developing ecology as a science. He became so frustrated that he gave up ecology during the 1930s as holism became progressively supported by the idea of the interactive “ecosystem.”
Nonetheless, as ecologists continued to study the world, they found more and more flaws in Clements’s theory. In the 1950s, the work of American plant ecologists Robert H. Whittaker and John Curtis showed how impossible it was to identify communities as neat units of holistic theory, and that the real world was more nuanced and complex. When it comes to studying ecosystems in the field, Gleason’s ideas seem to provide a better fit.
In the ensuing decades, while environmentalists continue to champion holistic ideas, ecologists have also increasingly incorporated Gleason’s concepts into their work. He is now considered to be one of the most significant figures in 20th-century ecology.
HENRY ALLAN GLEASON
Born in 1882, Henry Gleason studied biology at the University of Illinois. He held faculty posts and conducted acclaimed early ecological research in Sand Ridge State Forest, Illinois. In the 1920s, Gleason’s theory of individualistic—rather than holistic—plant communities was not accepted by ecologists. This rejection led Gleason to abandon ecology in the 1930s. He had long held posts at the New York Botanical Garden and became famed for his work on plant classification. With botanist Arthur Cronquist, he co-wrote a definitive guide to the plants of the northeastern US. He retired in 1950 but continued to write and study. He died in 1975.
Key works
1922 “On the Relation between Species and Area”
1926 “The Individualistic Concept of the Plant Association”
See also: The ecosystem • The distribution of species over space and time • Ecological succession • Climax community • The ecological guild • Biomes
IN CONTEXT
KEY FIGURE
Richard B. Root
(1936–2013)
BEFORE
1793 Alexander von Humboldt uses the word “association” to describe the mix of plant types within a particular habitat.
1917 In the US, Joseph Grinnell coins the term “niche” to describe how a species fits into its environment.
1935 British botanist Arthur Tansley identifies ecoystems—integrated biotic communities—as fundamental units of ecology.
AFTER
1989 In the US, James MacMahon suggests that it does not matter how ecological guild members use resources.
2001 Argentinian ecologists Sandra Diaz and Marcelo Cabido propose grouping species that have a similar effect on their environment.
Ecologists have long sought to understand how species in a community interact to exploit resources. A key concept in the explanation of this interplay is the idea of guilds, first developed by American biologist and ecologist Richard B. Root in 1967.
Root had researched the way the Blue-gray Gnatcatcher exploits its ecological niche for his doctoral thesis. The concept of ecological niches dates back to 1917, when American biologist Joseph Grinnell used the term to describe how the California Thrasher fitted into its dry, scrubby chaparral environment. The thrasher’s “niche” describes the aspects of its habitat for which it is suitably adapted.