The Ecology Book
Page 19
In the process, British ecologist John Odling-Smee argued, so-called “niche-constructor” species create a more favorable environment for themselves—as seen in countless examples, from ancient oxygen-producing cyanobacteria that altered the composition of the atmosphere in prehistoric times, to beavers creating wetlands.
Modern methods
Traditionally, the task of monitoring environmental change has been the responsibility of academics and professional ecologists, but millions of interested amateurs now provide enormous amounts of raw data on everything from flowering dates to butterfly numbers, and from the state of coral reefs to the breeding populations of birds. With computer power to quickly process vast amounts of data, and with Earth’s ecology changing faster than ever, this “citizen science” looks set to become an invaluable resource for ecology.
IN CONTEXT
KEY FIGURE
Alexander von Humboldt (1769–1859)
BEFORE
1750 Carl Linnaeus explains that the distribution of plants is determined by climate.
AFTER
1831–36 Charles Darwin makes various observations on the voyage of HMS Beagle, confirming that many animals living in one area are not found in similar habitats elsewhere.
1874 British zoologist Philip Sclater produces a description of the zoogeography (the geographical distribution of animals) of the world’s birds.
1876 Alfred Russel Wallace publishes his two-volume book The Geographical Distribution of Animals, which becomes the definitive biogeography text for the next 80 years.
The distribution, or range, of biological communities and species varies according to many factors—including latitude, climate, elevation, habitat, isolation, and the species’ characteristics. The study of species distribution is called biogeography. Biogeography is also concerned with how and why the patterns of distribution change over time.
Early zoologists and botanists such as Carl Linnaeus were well aware of geographical variations in species’ distributions, but the first to make detailed studies of this aspect of zoology was the Prussian polymath Alexander von Humboldt, who traveled to Latin America with French botanist Aime Bonpland in 1799. Their five-year expedition laid the basis of plant geography. Humboldt believed observation in situ to be paramount, and used sophisticated instruments to make meticulous records of both plant and animal species, noting all the factors that could influence the data. This holistic approach is best illustrated in his highly detailed map and cross section of Chimborazo mountain in Ecuador.
Wallace’s contribution
Many 19th-century naturalists contributed to biogeographical knowledge, but one of the most significant was British naturalist Alfred Russel Wallace. After reading Philip Sclater’s account of the global distribution of bird species, Wallace set out to do the same for other animals. He examined all the factors known at the time to be relevant, including changes in land bridges and the effects of glaciations. He produced maps to demonstrate how vegetation influenced animal ranges, and he summarized the distribution of all known families of vertebrates.
Wallace then proposed six zoogeographic regions, which are still largely in use today: the Nearctic (North America), Neotropics (South America), Palearctic (Europe, north Africa, and most of Asia), Afrotropics (south of the Sahara), Indomalaya (South and Southeast Asia), and Australasia (Australia, New Guinea, and New Zealand). The dividing line between these last two regions, which runs through Indonesia, is still known as “Wallace’s Line.”
“The unity of nature means the interrelationship of all physical sciences.”
Alexander von Humboldt
Plate tectonics
Wallace also made some remarkable discoveries from the fossil record. For example, he worked out that early rodents had evolved in the Northern Hemisphere, moving via Eurasia into South America. Later, in 1915, German geologist Alfred Wegener proposed the radical idea that the continents of South America and Africa were once connected, which allowed the spread of tapirs and other species.
Wegener understood that the distribution of species was in part a record of geological history. Species colonize new areas as conditions change, and over time have become separated by barriers such as new oceans or mountain ranges. Today, as human-made changes to climate and the environment gather pace—creating new barriers—this understanding has taken on a new and vital importance.
Tapirs evolved in North America at least 50 million years ago. They spread to and now live in Central and South America, as well as southeast Asia, but died out in North America.
ALEXANDER VON HUMBOLDT
Known as the “founder of plant geography,” Humboldt also made valuable contributions to geology, meteorology, and zoology. Born in Berlin in 1769, he started collecting plants, shells, and insects at an early age. His expedition to Latin America in 1799–1804 encompassed Mexico, Cuba, Venezuela, Colombia, and Ecuador, and his team broke the world altitude record when they climbed to 19,285ft (5,878m) on Chimborazo.
Humboldt also speculated that volcanoes result from deep subterranean fissures, investigated the decrease in temperature with altitude, and discovered that the strength of Earth’s magnetic field decreases away from the poles. The 23-volume work detailing his expedition set a new standard for scientific writing, cementing his fame.
Key works
1807 Essay on the Geography of Plants
1805–1829 Personal Narrative of Travels to the Equinoctial Regions of the New Continent During Years 1799–1804
See also: A modern view of diversity • Animal ecology • Island biogeography • Big ecology • Climate and vegetation
IN CONTEXT
KEY FIGURES
Thomas Malthus (1766–1834), Pierre-François Verhulst (1804–49)
BEFORE
1798 Thomas Malthus argues that populations increase exponentially, based on a common ratio, whereas food supplies grow more slowly at a constant rate, leading to potential food shortages.
1835 Belgian statistician Adolphe Quetelet suggests that population growth tends to slow down as population density increases.
AFTER
1911 Anderson McKendrick, working as an army physician, applies the Verhulst equation to bacteria populations.
1920 American biologist Raymond Pearl proposes the Verhulst equation as a “law” of population growth.
Pierre-François Verhulst was a Belgian mathematician who, after reading Thomas Malthus’s An Essay on the Principle of Population, became fascinated by human population growth. In 1845, he published his own model for population dynamics, which was later named the Verhulst equation.
Although influenced by the ideas of Malthus, Verhulst realized that there was a major flaw in his predictions. Malthus had claimed that human population tends to increase geometrically, doubling at regular time intervals. Verhulst thought this to be too simplistic, reasoning that the Malthus model did not take into account a larger population’s difficulty in finding food. He argued instead that “the population gets closer and closer to a steady state,” in which the rate of reproduction is proportionate to both the existing population and the amount of available food. In Verhulst’s model, after the point of maximum population growth—the “point of inflection”—the growth rate becomes progressively slower, gradually leveling off to reach the “carrying capacity” of an area—the number of individuals it can sustain. When visualized, Verhulst’s model produces an S-shaped curve, which was later called a logistic curve.
“The hypothesis of geometric progression can hold only in very special circumstances.”
Pierre-François Verhulst
Practical demonstrations
Verhulst’s model was ignored for several decades, partly because he himself was not entirely convinced. However, in 1911, Scottish army physician and epidemiologist Anderson McKendrick used the logistic equation to forecast growth in populations of bacteria. Then, in 1920, Verhulst’s equation was adopted and promoted in America by Raymond Pearl.
; Pearl conducted experiments with fruit flies and hens. He gave a constant quantity of food to fruit flies kept in a bottle. Initially, their fertility rate increased. However, as the population density grew, competition for resources increased, and eventually reached a bottleneck. After this, the flies’ fertility rate dropped; their numbers continued to increase but slowly, and generally the population level stabilized.
Similarly, Pearl found that when the number of hens in a pen increased, the birds struggled to find enough food. As the space between them reduced, the hens laid fewer eggs and, as their fertility rate declined, the rate of population growth slowly stabilized.
“Biologists are at the present time in no way likely to suffer ostracism if they venture to study human problems.”
Raymond Pearl
Variable strategies
The two key variables in Verhulst’s equation are the maximum capacity of a species to reproduce (r), and the carrying capacity of the area (K). Organisms are either r-strategists or K-strategists. R-strategists, such as bacteria, mice, and small birds, reproduce rapidly, mature early, and have a relatively short life. K-strategists, such as humans, elephants, and giant redwood trees, have a slower reproduction rate, take longer to mature, and tend to live longer. Ecologists study r-strategists, which are often found in unstable environments, to assess risks to their necessary high reproduction levels, and study K-strategists in more predictable environments to ensure long-term species survival.
Fruit flies are small, common flies that are attracted to ripe fruit and vegetables. They are popular for laboratory studies because they reproduce so quickly and are easy to cultivate.
THOMAS MALTHUS
Malthus was born in Surrey, UK, in 1766, the seventh child of a prosperous family. After studying languages and mathematics at the University of Cambridge, he took a post as curate of a rural church. In 1798, he published an essay arguing that the rate of increase in human populations outstrips much steadier rises in food production, leading to inevitable starvation. Malthus went on to publish six further editions of the essay, and he made a number of visits to Europe to gather population data. In 1805, he was appointed Professor of History and Political Economy at the East India Company College in Hertfordshire. He became increasingly involved in debate about economic policy, and criticized the Poor Laws for causing inflation and failing to improve life for the poor. Malthus died in 1834.
Key works
1798 An Essay on the Principle of Population
1820 Principles of Political Economy
1827 Definitions in Political Economy
See also: Distribution of species over space and time • Metapopulations • Metacommunities • Overpopulation
IN CONTEXT
KEY FIGURE
Stephen A. Forbes (1844–1930)
BEFORE
1799–1804 Alexander von Humboldt pioneers the field of biogeography in his travels in Latin America.
1866 German naturalist Ernst Haeckel coins the term “ecology” to describe the study of organisms in relation to their environments.
1876 After traveling extensively, British naturalist Alfred Russel Wallace publishes The Geographical Distribution of Animals.
AFTER
1890s Frederic Clements proposes the notion of ecological communities.
1895 In Ecology of Plants Johannes Warming describes the impact of the environment on the distribution of plants.
The notion of a naturalist—someone who studies organisms in the natural world—dates back to ancient Greece. Aristotle made copious observations of wildlife, and his work laid the foundations for later naturalists. It was not until the 19th century, however, that the potential of such surveys was really understood.
The new study of ecology
As naturalists undertook longer field trips, the global distribution of species became more apparent, and the concept of ecology as a science gained traction.
One of the first scientists to employ ecological methods was American biologist Stephen A. Forbes. In the 1880s, while studying fish in a Wisconsin lake, he realized that survey data could be interpreted to give a picture of interactions between different species—not just their abundance. Forbes extended the scope of the conventional survey, combining practical fieldwork with theoretical analysis and experiments. These rounded ecological surveys created a picture of the natural order within an environment. By shedding light on the interrelated effects of its plant and animal life, they could also help explain the distribution of species and variations over time.
Satellite images enable ecologists to observe large-scale changes easily. The green areas in this image of the Caspian Sea are evidence of algal growth—the product of nutrient enrichment.
See also: Classification of living things • Animal ecology • Biodiversity and ecosystem function
IN CONTEXT
KEY FIGURE
Johannes Eugenius Warming (1841–1924)
BEFORE
1859 Charles Darwin’s detailed descriptions of plants and animals in their natural environment mark the start of an appreciation of what is later termed “ecology.”
AFTER
1935 British botanist Arthur Tansley publishes an article in Ecology in which he defines the term “ecosystem.”
1938 American botanists John Weaver and Frederic Clements further develop the concepts of plant communities and succession.
1995 David Attenborough’s television documentary “The Private Life of Plants” depicts plants as dynamic influencers of their environment.
Plant ecology examines how plants interact with one another and with their environments. Danish botanist Johannes Eugenius Warming first brought the sciences of botany and ecology together in his book The Ecology of Plants in 1895. He described how plants react to their surroundings, and how their life cycles and structures relate to where they grow. The book introduced the concept of plant communities, and outlined how a group of species interact and develop in response to the same local conditions.
Plants and ecosystems
For many years, plant ecology and animal ecology were studied separately, but in the early 20th century a more connected perspective emerged. Important theories on plant communities and succession—the process by which an ecological community changes over time—were established during this time period. In 1926, Russian geochemist Vladimir Vernadsky introduced the idea of Earth’s biosphere, the parts of its surface and atmosphere where all living organisms exist and interact.
Plants are sensitive barometers of change within an environment. The study of their anatomy, physiology, distribution, and abundance, as well as their interactions with other organisms and their response to environmental factors, such as soil conditions, hydrology, and pollution, can provide invaluable information about the entire ecosystem.
“That land is a community is the basic concept of ecology.”
Aldo Leopold
American ecologist
See also: Climate and vegetation • Ecological succession • The biosphere • Endangered habitats • Deforestation
IN CONTEXT
KEY FIGURE
Andreas Schimper (1856–1901)
BEFORE
1737 Carl Linnaeus’s Flora Lapponica includes details of the geographical distribution of Lapland plants.
1807 Alexander von Humboldt publishes his seminal Essay on the Geography of Plants.
AFTER
1916 In Plant Succession: an Analysis of the Development of Vegetation, Frederic Clements describes how communities of species are indicators of the climate in which they have matured.
1968 “The Role of Climate in the Distribution of Vegetation,” by American geographers John Mather and Gary Yoshioka, explains how temperature and rainfall alone are not enough to define plant distributions.
That different plants grow in different climates was likely common knowledge for as long as agriculture has existed; many cultures have traded plants for thousands of years. However, the clear link betwee
n a region’s dominant vegetation type and climate was not categorically spelled out until German botanist Andreas Schimper published his ideas on plant geography in 1898.
Botanists such as Carl Linnaeus and Alexander von Humboldt had written about plant distributions in the 18th and early 19th centuries. The widely traveled Humboldt understood that climate was one of the key factors governing where plants did and did not grow. Schimper went one step further than Humboldt by explaining that similar vegetation types arise under similar climatic conditions in different parts of the world. He then produced a global classification of vegetation zones that reflected this observation.
Schimper’s 1898 book Plant-geography upon a Physiological Basis ran to 870 pages and is one of the largest ecology monographs written by a single author. A synthesis of plant geography and plant physiology (the functioning of plants), it became the foundation of the study of plant ecology. Schimper explained that the connection between the structures of plants and the external conditions they faced in different places was the key to what he described as “ecological plant-geography.” Vegetation was divided into broad tropical, temperate, arctic, mountain, and aquatic zones, then subdivided further, according to the prevailing climate. For example, tropical vegetation was divided into savanna, thorn-forest, woodland, tropical rain forest, or woodland with a pronounced dry season, according to whether the climate was wet all year round, seasonally wet, or mostly dry.