by DK
Rockpools in a wave-cut platform form a metacommunity on Eysturoy in the Faroe Islands. The rockpools are separate between tides, but become joined as one when the tide comes in.
Blurred communities
Leibold’s 2004 paper acknowledged that the metacommunities with blurred boundaries are perhaps the hardest to define. Coral reefs, for example, may look neatly separate, but many of the species that live among them swim freely and respond to a host of changing outside influences, such as shifts in ocean currents.
Since most of the world’s life exists within such vaguely defined patches, theorists have attempted further clarification. Leibold and his colleagues have suggested two different ways of identifying metacommunities for study: distinct communities embedded within a “matrix” habitat, such as clearings in a forest rich in resources; and arbitrary sampling patches in a continuous habitat, such as a random circle of trees within a forest.
The work is still at an early stage. The world is entering a biodiversity crisis, and countless species and communities appear to be under threat from the effects of human activity. Metacommunity theory may, in time, help to provide a better understanding of how natural communities will respond, and how local changes to habitats may ripple through a region, either adversely or positively.
Wildlife crossings
Many different species cross naturally between separate habitat patches. This movement can be seasonal, as in annual migrations, or prompted by natural disasters, such as fire or flood, or may take place over long timespans. It creates connections that are often essential for the health and survival of species and communities, providing renewal or new resources at pivotal moments. Increasingly, however, manmade barriers, such as clearances for agriculture, road, railroads, and urban sprawl, are breaking up this natural interflow from one habitat to another. The idea of providing wildlife with ways through is not new. For example, fishways for fish to bypass dams go back centuries. Wildlife crossings—from bridges for bears in Canada to tunnels for California’s desert tortoises—are becoming an increasingly common feature of construction work. Thousands of crossings, among them bridges, viaducts, and underpasses—often planted with vegetation—have been built to conserve habitats and to avoid fatal collisions between animals and vehicles.
See also: Competitive exclusion principle • The ecosystem • The neutral theory of biodiversity • Metapopulations
INTRODUCTION
For centuries, scientists in the Western world tried to reconcile the findings of geologists and fossil hunters with literal interpretations of biblical stories about Creation and the Great Flood. In 1654, for example, Archbishop Ussher dated Earth’s creation to October 22, 4004 BCE. A series of discoveries challenged this narrative and led to new ideas about the dynamic history of life on Earth.
Evidence in the rocks
Two Scottish geologists—James Hutton and Charles Lyell—advanced our understanding of Earth’s age. In Theory of the Earth (1795), Hutton argued that the repeated cycles of sedimentation and erosion necessary to create thousands of feet of rock strata must indicate a truly ancient origin for the planet—an idea which Lyell developed further in the 1830s. Soon after, Swiss-American geologist Louis Agassiz proposed that the topography of some regions had been shaped by glaciations. Hutton and Lyell also noted that fossils of animals and plants vanished from the geological record. Lyell believed this to be evidence of extinction, challenging the prevailing belief that species were immutable.
Fossils also offered clues to movements of Earth’s continents. German meteorologist Alfred Wegener noted that similar fossils could be found on both sides of the South Atlantic, even though they were thousands of miles apart. In his 1912 theory of continental drift, Wegener cited this as evidence that continents were once joined and had broken away. It was not until the 1960s that a viable mechanism was found for such movement. Geophysicists discovered patterns of magnetic anomalies running in parallel stripes on either side of ocean ridges and identified the process of seafloor spreading—hot magma bubbling up through cracks in the oceanic crust and forming new crust as it cools and moves away. This gradual process shifts and shapes continents.
The birth of biogeography
In the Age of Exploration from the 16th century on, scientists began to study the geographical distribution of plants and animals. By the 1860s, Alfred Russel Wallace viewed these patterns, clearly defined by physical barriers such as mountains and seas as a key supporting argument for evolution. Wallace noted, for example, the ocean straits that produced a sharp division between the flora and fauna of Australasia and Southeast Asia.
With a better understanding of Earth’s biogeography, 20th-century ecologists divided the planet into biomes—broad communities of flora and fauna that interact in different habitats, such as tropical rain forests or tundra. Botanist Leslie Holdridge refined the concept in 1947 with his life zone classification, in which he mapped zones based on the two crucial influences on vegetation: temperature and rainfall.
A “whole Earth” approach
The word “biosphere” was coined by Austrian geologist Edward Suess in 1875 to signify all the areas at or near the surface of the Earth where organic life can exist. In 1926, the Russian geochemist Vladimir Vernadsky explained the biosphere’s close interaction with the planet’s rock (lithosphere), water (hydrosphere) and air (atmosphere). This in turn led American biologist Eugene Odum to advocate a holistic approach to ecology. Odum argued that it was not possible to understand a single organism, or a group of organisms, without studying the ecosystem in which they live. He described this view as “the new ecology.”
In 1974, British scientist James Lovelock advanced the Gaia hypothesis that the interaction of living and nonliving elements in the biosphere reveal Earth to be a complex, self-regulating system that perpetuates the conditions for life. Almost two centuries earlier Hutton had articulated a similar idea—that biological and geological processes are interlinked and that Earth could be viewed as a superorganism. In Hutton’s words, “The globe of this earth is not just a machine but also an organized body as it has a regenerative power.”
Heading for extinction?
Life has survived on Earth for billions of years, despite the ravages of five mass extinctions. However, environmentalists now question whether it will survive another. Indeed, some contend that a sixth mass extinction has already started, as a result of human activity. Yet, if Lovelock’s Gaia theory is correct, it seems likely that the planet will endure—even if humans and many other current life forms do not.
IN CONTEXT
KEY FIGURE
Louis Agassiz (1807–73)
BEFORE
1795 Scottish geologist James Hutton argues that erratic boulders (rock fragments that are different from the underlying rock) in the Alps were transported by moving glaciers.
1818 In Sweden, naturalist Göran Wahlenburg publishes his theory that ice once covered Scandinavia.
1824 Danish–Norwegian mineralogist Jens Esmark theorizes that glaciers were once larger and thicker and had covered much of Norway and the adjacent seafloor.
AFTER
1938 Serbian mathematician Milutin Milankovic´ publishes a theory to explain the recurrence of ice ages based on changes in Earth’s orbit around the Sun.
In the early 19th century, there were contradictory explanations for the development of Earth’s landforms, plants, and animals. Supporters of catastrophism argued that a series of destructive shocks, such as the Great Flood described in the Bible, had re-formed the surface of the planet many times, reshaping existing mountains, lakes, and rivers and wiping out many plant and animal species. In contrast, followers of uniformitarianism contended that Earth’s features were the result of continuous and uniform natural processes of erosion, sedimentation (the depositing of particles carried by fluid flows), and volcanism.
Detailed geological studies demonstrated that neither camp was right. They established that Earth’s history has been a p
rocess of slow change, punctuated by catastrophic events. The study of glaciers, and the landforms they create, informed these ideas. After observing parallel striations in rocks of the Swiss Alps, German–Swiss geologist Jean de Charpentier (or Johann von Charpentier) postulated that glaciers in the Alps had once been more extensive and had caused the scratches as they moved and their sediment cut into the rock. Geologist Jens Esmark drew similar conclusions in Norway.
Animals enter Noah’s ark in a depiction of the Great Flood described in the Bible. Catastrophists believed that the Great Flood was one of the formative shocks that shaped the geology of Earth.
Glacier movements
Swiss zoologist Louis Agassiz developed Charpentier’s and Esmark’s ideas further. In 1837, he proposed that vast sheets of ice had once covered much of the northern hemisphere, from the North Pole to the Mediterranean and Caspian coastlines. Agassiz also undertook some detailed studies of glacier movement in Switzerland and published his Études sur les glaciers in 1840. The same year, he visited geologist William Buckland in Scotland to investigate glacial features there, prompting Scottish glaciologist James Forbes to begin similar research in the French Alps.
Some quarters, such as the Catholic Church, still argued that glacial striations had been caused by a great flood or that large silt and rock deposits had been transported by icebergs swept along by the flood. From the 1860s, however, there was wide support for Agassiz’s glaciation theory and the idea that glaciers in the Swiss Alps and Norway had once extended much further. It was also accepted that a sheet of ice had once spread across Europe, and south from the Arctic through much of North America, with catastrophic implications for plants and animals.
By the late 1800s and early 1900s, as more expeditions to both Greenland and Antarctica were undertaken, it became known that both areas were still covered in ice. Aerial surveys in the 1920s and 1930s confirmed the extent of their vast ice sheets—now defined as areas of glacier ice exceeding 19,300 sq miles (50,000 sq km); ice caps, such as Iceland’s Vatnajökull, are smaller.
Further evidence revealed that there had not been one single ice age, but at least five major ice ages in Earth’s long history. The most recent, the Quaternary Ice Age, began 2.58 million years ago and is ongoing. In the last 750,000 years, there have been eight ice advances (glacial periods) and retreats (interglacial periods). During the last glacial period, which ended 10,000–15,000 years ago, ice sheets were up to 21⁄2 miles (4 km) thick, and the sea level was 390 ft (120 m) lower.
Glaciers converge on Piz Argient, a mountain in the Swiss Alps. Like others in the Alps, these glaciers were once much more extensive than they are now, and they continue to shrink.
Receding glaciers and bird migration
A male Baltimore oriole perches on a tree fern in Costa Rica. The species flies north to breed in March and returns to the tropics in August or September.
When the last glacial period began to end, around 26,500 years ago, Earth was much colder than it is today. Much of North America and northern Eurasia was covered with ice sheets. The environment was so harsh that most birds tended to live in subtropical and tropical regions where there was more food.
As temperatures began rising, the ice sheets started to shrink, uncovering a new landscape. Ice-free ground and short, wet summers were ideal for insects, and birds began to move in, too, to take advantage of this food supply. When days got shorter in fall, some birds stayed on for the winter, but others returned south.
The distances flown by birds returning to their homes grew longer as the ice sheets retreated farther, eventually developing into long-distance spring and fall migrations between the tropics and northern latitudes. Common birds that undertake the journey include swallows, warblers, and cuckoos.
See also: Evolution by natural selection • Global warming • The Keeling Curve • Ozone depletion • Spring creep
IN CONTEXT
KEY FIGURE
Alfred Russel Wallace (1823–1913)
BEFORE
1831–36 Darwin’s studies on the voyage of HMS Beagle confirm that many animals living in one area are not found in similar habitats elsewhere.
AFTER
1874 British zoologist Philip Sclater categorizes birds by zoogeographic regions.
1876 Alfred Russel Wallace publishes The Geographical Distribution of Animals—the first extensive publication on biogeography.
1975 Hungarian biogeographer Miklos Udvardy proposes dividing biogeographic realms into biogeographic provinces.
2015 Mexican evolutionary biologist J.J. Morrone proposes an International Code of Area Nomenclature for biogeography.
The places where animals and plants live often vary in a regular manner along geographic gradients of latitude, elevation, and habitat type. The study of this variation is known as biogeography. One branch (phytogeography) examines the distribution of plants, whereas the other (zoogeography) analyzes the distribution of animals. British naturalist and biologist Alfred Russel Wallace is widely regarded as the “father of biogeography.”
In the 18th century, as explorers recorded the plants and animals they saw, a picture of geographic change had begun to emerge. On the great 1831–36 expedition of HMS Beagle, Charles Darwin saw species of birds on the Falkland Islands that did not live on mainland South America, giant tortoises that were unique to the Galapagos Islands, and marsupials such as Australia’s kangaroos. New pieces of the biogeographic jigsaw were falling into place.
From 1848, Wallace conducted years of fieldwork in South America and Southeast Asia. He researched the feeding and breeding behavior and migratory habits of thousands of species, paying specific attention to animal distribution compared with the presence or absence of geographical barriers, such as seas between islands. He concluded that the number of organisms living in a community depends on the food available in that specific habitat.
Wallace’s six zoogeographic regions began with the line he proposed in 1859 to mark the division of fauna between Southeast Asia and Australasia.
Wallace’s Line
During his 1854–62 expedition to the Malay Archipelago, Wallace collected an astonishing 126,000 specimens, many of them from species unknown to Western science, including 2 percent of the world’s bird species. He regarded biogeography as support for the theory of evolution by means of natural selection. One of Wallace’s important findings was the marked difference in bird species on either side of what was to become known as the Wallace Line, which runs along the Makassar Strait (between the islands of Borneo and Sulawesi) and the Lombok Strait (between Bali and Lombok); this separates Asian fauna from the Australasian. He found that larger mammals and most birds did not cross the line. For example, tigers and rhinos live only on the Asian side; babirusas, marsupials, and sulfur-breasted cockatoos only on the other side. He also highlighted the sharp differences between animals in North and South America.
In 1876, Wallace proposed six separate zoogeographic regions: Nearctic (North America); Neotropics (South America); Palaearctic (Europe, Africa north of the Sahara Desert, and Central, North, and East Asia); Afrotropics (Africa south of the Sahara Desert); Indomalaya (South and Southeast Asia); and Australasia (Australia, New Guinea, and New Zealand). Today Wallace’s regions, with the addition of Oceania (the islands of the Pacific Ocean) and Antarctica, are known as biogeographic realms.
All of Siberia is in the Palearctic region, and the Siberian white birch trees depicted here are part of a subdivision called the East Siberian taiga.
ALFRED RUSSEL WALLACE
Explorer, naturalist, biologist, geographer, and social reformer Alfred Russel Wallace left school at 14, and trained as a surveyor in London before becoming a teacher. He became fascinated with insects after meeting British entomologist Henry Bates. The pair ventured to the Amazon Basin in 1848 on a four-year collecting expedition. Trips to the Orinoco River and the Malay Archipelago followed. Wallace arrived at the same conclusion as Charles Darwin on the origin of species by natural s
election, and they presented their papers jointly in 1858. A world authority on fauna distribution, Wallace also raised awareness about problems caused by human impact on the environment.
Key works
1869 The Malay Archipelago
1870 Contributions to the Theory of Natural Selection
1876 The Geographical Distribution of Animals
1878 Tropical Nature, and Other Essays
1880 Island Life
See also: Evolution by natural selection • Island biogeography • The distribution of species over space and time • Biomes
IN CONTEXT
KEY FIGURE
Svante Arrhenius (1859–1927)
BEFORE
1824 French physicist Joseph Fourier suggests that Earth’s atmosphere traps the Sun’s heat like a greenhouse.
1859 Irish physicist John Tyndall provides experimental evidence to support earlier hypotheses that atmospheric gases absorb radiant heat.
AFTER
1976 American scientist Charles Keeling proves that between 1959 and 1971 carbon dioxide levels in the atmosphere increased by about 3.4 percent each year.