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Biogeography, Overview
Biogeography, Overview Mark V Lomolino, Oklahoma Natural Heritage Inventory, and University of Oklahoma, Norman, OK, USA Published by Elsevier Inc. This article is reproduced from the previous edition, volume 1, pp 455–469, r 2001, Elsevier Inc.
Glossary Biogeography Study of the geographic variation of nature, including variation in any biological characteristics (e.g., body size, population density, or species richness) on a geographic scale. Continental drift Model first proposed by Alfred Wegener that states that the continents were once united and then were displaced over the surface of the globe. Plate tectonics Study of the origin, movement, and destruction of the plates and how these processes have been involved in the evolution of Earth’s crust.
Pleistocene Geologic period from 2 million to 10,000 years before the present, which was characterized by alternating periods of glaciation events and global warming. Species composition Types of species that constitute a given community or sample. Species richness Number of species in a given community or sample.
Introduction
Early History of the Field
Traditionally, biogeography has been defined as the study of patterns in distributions of geographic ranges (Brown and Gibson, 1983). During the past three decades, however, this field has experienced a great surge in development and sophistication, and with this development the scope of the field has broadened to include an impressive diversity of patterns. Simply put, modern biogeographers now study nearly all aspects of the ‘‘geography of nature.’’ Biogeography now includes studies of variation in any biological feature (genetic, morphological, behavioral, physiological, demographic, or ecological) across geographic dimensions such as distance among sites or along gradients of area, elevation, or depth (see Brown and Lomolino, 1998).
Biogeography has a long and distinguished history, and one inextricably woven into the historical development of evolutionary biology and ecology. The historical development of biogeography had its origins coincident with the Age of World Explorations by Europeans during the eighteenth and nineteenth centuries. Yet the study of geography of nature must be an ancient one. The European explorers were not the first to ask ‘‘Where did life come from, and how did it diversify and spread across the earth?’’ Aristotle asked these same questions, as did many others before and after him, when faced with accounts of strange forms of life from foreign lands. The development of biogeography into a mature and respected field of science, however, required a much better understanding of variation in what we now call the geographic template and the associated variation in the natural world. It is by no minor coincidence that both evolutionary biology and biogeography developed in earnest during the Age of Exploration. Prior to this time, biologists had ‘‘discovered’’ and described less than 1% of plant and animal forms that we know today. Each new voyage or expedition added to the accumulated information on the earth’s environments and life-forms, and would eventually provide the raw material for the disciplines of evolution and biogeography. These disciplines are interconnected by the knowledge that selective pressures vary across space, and that all life-forms and their distributions are the product of natural selection. The early explorers and naturalists did far more than just label and catalog their specimens. They soon, perhaps irresistibly took to the task of comparing their collections across regions, elevations, and other gradients of the geographic template. At the same time, they began to develop explanations for the similarities and differences among the biotas they studied. In fact, most of the persistent themes of the field of biogeography (Table 1) were well established during the eighteenth and nineteenth centuries. To be sure, biogeography has made great strides during
Fundamentals of Biogeography Despite the sometimes overwhelming complexity of the natural world, all biogeographic patterns ultimately derive from two very general features of nature. First, as we move along any dimension of the geographic template, environmental conditions tend to vary in a predictable manner. For example, more distant sites tend to be more dissimilar than adjacent sites, environments at higher elevations tend to be cooler and wetter than those at lower elevations, and larger areas tend to capture more solar energy and a greater diversity of environmental conditions than smaller areas. Second, all forms of life differ in their abilities to adapt to geographic variation in their environment. These differences, while including a great diversity of responses (e.g., physiological, behavioral, developmental, and evolutionary), ultimately influence the three fundamental processes of biogeography: immigration, extinction, and evolution. All the biogeographic patterns we study derive from nonrandom variation in these processes across geographic gradients and across individuals, populations, and species.
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Table 1
Persistent themes of biogeography
1. Comparing and classifying geographic regions based on their biotas. 2. Reconstructing the historical development of biotas, including their origin, spread, and diversification. 3. Explaining the differences in numbers as well as types of species among geographic areas. 4. Explaining geographic variation in the characteristics of individuals and populations of closely related species, including trends in morphology, behavior, and demography. Source: Reproduced with permission from Brown JH and Lomolino MV (1998) Biogeography. Sunderland, Massachusetts: Sinauer Associates.
the twentieth century to become a mature and sophisticated science. It is important to acknowledge, however, that we owe a great deal to the many visionary explorers and naturalists who shared our fascination and asked the same questions about the geographic variation of nature.
Historic Explorations of the Eighteenth Century While motivated to a large degree by the quest for money and power, the Age of European Exploration was also fueled by the call to serve God. It was widely believed that the Creator had placed on this earth a still unknown diversity of organismsFa divine zoo or garden of life. Accordingly, early European explorers believed that there was perhaps no greater way to worship God than to unlock the mysteries of creation. Yet with each new account of some distant biota came information that challenged the prevailing views of creation. Eventually, the growing body of knowledge would overturn the long accepted view that the earth, its climate, and its species were immutable, unchanging in both space and time. More immediately, however, biologists of the eighteenth century were struck by the astounding diversity of species. Such diversity presented them with two serious problems, one practical and the other conceptual. First, biologists urgently needed a systematic and generally accepted scheme for classifying the burgeoning wealth of specimens, one that would reflect the similarities and differences among the species. Second, it quickly became clear that there were just too many species to be carried by Noah’s Ark. How could all the forms of life, now adapted to many distant and distinct regions across the globe, have originated and then spread from that one landing point? Carolus Linnaeus (1707–1778), certainly one of the most prominent biologists of all time, took on both of these challenges. In fact, his system of binomial nomenclature is the system we continue to use today to classify organisms. Linnaeus also attempted to rectify the Biblical doctrine with what he and his contemporaries knew about the diversity and geography of nature. This was especially challenging because, like most of his colleagues, Linnaeus was sure that species were immutable. Given this, how could species adapted to a single site and climate (Noah’s landing) have spread and become perfectly adapted to a suite of different environments (e.g., alpine tundra, coniferous forests, lowland forests, and grasslands)? Linnaeus’ answer: Noah’s landing had occurred along the slopes of Mount Ararat, a high mountain near the border of Turkey and Armenia. This mountain is so tall (reaching 16,853 ft above sea level) that along its slopes could be found a succession of environments and communities ranging from subtropical grasslands at the lower elevations to
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forests and alpine tundra at its summit. According to Linnaeus’ hypothesis, each elevational zone harbored a distinct assemblage of animals, each immutable but perfectly adapted to their local environment. When the Flood finally receded, these animals then dispersed to eventually colonize their respective environments across the globe. One of the foremost challenges to Linnaeus’ views came from his contemporary Comte de Buffon (1707–1788), who believed that not only were climates mutable, but species were as well. How else could animals disperse across what are now inhospitable habitats to occupy their present ranges in such isolated regions of the globe? Buffon’s theory of the origin and spread of life stemmed largely from his studies of living and fossil mammals, especially those of the Old and New World tropics. He was the first to realize that different regions of the globe, even those with the same environmental conditions, had distinct biotas. This observation was so fundamental that it eventually became biogeography’s first law: Buffon’s Law. Like Linnaeus, Buffon also concluded that there was one ‘‘landing point,’’ one site where all animals originated. However, this site, or region, was located far to the north of Mount Ararat, somewhere in the Arctic Circle where the early animals and their descendants could gain ready access to both the Old and New Worlds. This is where these life-forms survived the Flood during some earlier period when the earth’s climate was much warmer, warm enough such that tropical environments could extend far poleward. Once the floods receded, animals spread southward into the continents and began to diverge in form as they became increasingly isolated on different landmasses. Other biologists of the eighteenth century, including Joseph Banks and Johan Reinhold Forster, both of whom served as naturalist on voyages of Captain James Cook, were quick to confirm the generality of Buffon’s Law: environmentally similar but isolated regions have distinct assemblages of plants and animals. Forster also discussed the relationship between regional floras and environmental conditions and, in turn, between plant and animal associations: two cornerstones of the field now known as ecology. Forster was also one of the first scientists to report that plant diversity increases as we move toward the equator, that islands have fewer plants than the mainland, and that the diversity of insular plants increases with island size and available resources. Later in the eighteenth century, Karl Wildenow (1765–1812) and one of his students, Alexander von Humboldt (1769–1859), confirmed and further generalized both Buffon’s Law and Forster’s. Toward the end of the century, Augustin P. de Candolle added the important insight that, not only is the distribution of organisms influenced by geographic variation in environments, but they also compete for limiting resources such as food, light, and water. Therefore, by the beginning of the nineteenth century, biogeographers already had their first ‘‘law,’’ they described and tested the generality of a number of related patterns about the geography of nature, and they offered some testable theories regarding those patterns. They were actively working on at least three of the four persistent themes of biogeography (Themes 1–3 in Table 1). A number of biogeographers had abandoned the notion that species and climates were immutable. But for the field to advance and become a mature science, two additional, fundamental
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insights were needed. First, it required a mechanism for the mutability and adaptation of species. As many of us realize, Candolle’s observations about competition and the struggle for existence were central to the development of the theory of evolution by natural selection. These advances were to come in the latter part of the nineteenth century. Second, scientists had to recognize that the geographic template (i.e., the foundation for all of these patterns) also was mutable. That is, the size and relative positions of the continents and ocean basins have changed throughout the history of our planet.
Advances of the Nineteenth Century Many of the most fundamental advances in biogeography, and evolutionary biology as well, have required advances in geology. Until the nineteenth century, the age of the earth was typically assumed to be just a few thousand years, way too brief to allow what we now know to be the requisite time for evolution of its plates and the species that have rafted on those plates. The collective work of nineteenth-century paleobiologists would push the age of the earth back hundreds of thousands and eventually millions of years before the present. Legendary geologists and paleobiologists such as George Lyell (1797–1875) and Adolphe Brongniart (1801–1876), through their studies of fossils, provided incontrovertible evidence for extinction and for changes in regional climate and the elevation of land. How else could they explain the existence of fossils that have no contemporary forms, of fossils from tropical speies found in regions that are now temperate, and of shells and other marine fossils on present-day mountains? A theory of floating and drifting continents (now known as plate tectonics) would await discoveries of twentieth-century marine geologists, but their nineteenth-century forerunners understood that the earth was very old indeed, and it was mutable. Furthermore, if species (and many thousands of them) went extinct, then there had to be multiple periods of creation (or evolution) to compensate for those losses. Again, these views of a mutable earth, mutable climate, and mutable species were essential for those attempting to classify biogeographic regions based on their respective assemblage of species (Theme 1), to reconstruct the origin, spread, and diversification of life (Theme 2), and to explain differences in numbers and types of species among geographic regions (Theme 3). However, well into the middle of the nineteenth century many scientists held stubbornly to the idea that not only were species immutable, but so were their distributions. Perhaps the most distinguished champion of this static view of biogeography was Louis Agassiz (1807–1873), who argued that species remain essentially unchanged at or near their sites of creation. The static view was eventually overturned by the passionate and persuasive arguments of no one less than Charles Darwin. Not only did he propose a general theory for the diversification and adaptation of biotas (i.e., natural selection), but he was one of the world’s first and foremost dispersalists and champions of dynamic biogeography. Through his observations during his circumnavigation of the globe on the HMS Beagle (1831–1836), his later experiments on dispersal of seeds by animals, and his general synthesis on the origin and distribution of life, Darwin convinced many of his colleagues that long-distance dispersal could account for many of the
otherwise perplexing patterns of biogeography. Once he was joined by the likes of Asa Gray and Alfred Russell Wallace, Darwin and his colleagues were able to pull off a major paradigm shift in the fieldFfrom the static view of the earth and its species to the dynamic view of biogeography. Yet among all of these visionary scientists it is Wallace who is generally recognized as the father of zoogeography, and biogeography in general. While Darwin argued passionately regarding long-distance dispersal (even to the point of soundly criticizing his mentor, Charles Lyell), most of his energies were devoted toward developing and substantiating his theory of natural selection. On the other hand, biogeography was Wallace’s life’s work. Brown and Lomolino (1998) listed 17 tenets of the field that were developed by Wallace and included in his seminal monographs The Malay Archipelago (published in 1869 and dedicated to Darwin), The Geographic Distribution of Animals (1876), and Island Life (1880). Five of these tenets of biogeography are listed here: 1. Climate has a strong effect on the taxonomic similarity between two regions, but the relationship is not always linear. 2. The present biota of an area is strongly influenced by the last series of geological and climatic events. 3. Competition, predation, and other biotic factors play determining roles in the distribution, dispersal, and extinction of animals and plants. 4. When two large landmasses are united after a long period of separation, extinctions may occur because many organisms will encounter new competitors. 5. To analyze the biota of any particular region, one must determine the distributions of its organisms beyond that region as well as the distributions of their closest relatives. Using the latter approach and information provided by over a century of naturalists, Wallace developed a scheme of biogeographic regions (Figure 1) that accurately reflected the similarities and differences among biotas. This same scheme, largely unchanged, is still used today. For obvious reasons, exploration and biogeographic study of the marine realm have always lagged far behind that of terrestrial systems. Yet by the middle of the nineteenth century, biogeographers had made some significant strides in studying this new frontier. Charles Lyell discussed patterns of distribution of marine algae in his seminal work Principles of Geology, first published in 1830. Edward Forbes wrote the first comprehensive monograph on marine biogeography in 1856, in which he divided the marine realm into zoogeographic regions based on latitude, depth, and animal assemblages. In 1897 the great British ornithologist and biogeographer Philip Sclater, who produced a predecessor to Wallace’s biogeographic scheme, also developed a scheme for the marine realm based on distributions of marine mammals. Following the lead of earlier biogeographers and also based on his own extensive field studies in southwestern North America, C. Hart Merriam (1894) developed a system of what he termed ‘‘life zones’’ that confirmed earlier observations that elevational changes in vegetation were equivalent to those along latitudinal gradients. Finally, the countless specimens collected during the late eighteenth and early nineteenth centuries enabled others to begin to analyze geographic variation in characteristics of
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Figure 1 Alfred Wallace’s (1876) scheme of biogeographic regions, which attempts to divide the landmasses into classes reflecting affinities and differences among terrestrial biotas. The regions shown are still widely accepted today. Numbers identify subregions. (Reproduced from Wallace AR (1876) The Geographic Distribution of Animals. London: Macmillan.)
individuals and populations (Theme 4). C. L. Gloger reported in 1833 that, within a species, individuals from more humid habitats tend to be darker than those from drier habitats (Gloger’s Rule). C. Bergmann (1847) found that in birds and mammals, populations from cooler environments tended to have larger bodies than those from warmer environments (Bergmann’s Rule). Also, J. A. Allen reported in 1878 that birds and mammals inhabiting cooler environments also tend to have shorter appendages (Allens’ Rule).
Biogeography in the Twentieth Century Dynamics of the Geographic Template Even the earliest human explorers appreciated the fact that abiotic conditions vary as one moves from one point on the globe to another. On land, precipitation, temperature, seasonality, prevailing winds, soil conditions, and a host of other important factors vary as we move along transects of latitude, longitude, or altitude. Similarly, in the aquatic realm, temperature, currents, pressure, solar radiation, and concentrations of oxygen and dissolved nutrients vary markedly within and among ecosystems. Together, the variation in all of
these environmental characteristics combine to form the geographic template, which influences all biogeographic patterns. Although a complete understanding of all aspects of the geographic template may be a daunting and truly impossible challenge, at a regional to global scale, geographic variation in environmental conditions is quite regular and interpretable. On land, climatic conditions vary in an orderly manner with latitude, elevation, and proximity to mountain ranges or oceans (Figure 2). Major soil types (Figure 3) also vary in a similar fashion, partially because soil development is strongly influenced by local climatic conditions, especially precipitation and temperature. In the aquatic realm, although the great volume of ocean waters tends to buffer variation in temperature, surface waters still exhibit a latitudinal gradient in temperature (Figure 4). In addition, throughout most of the world’s oceans light availability and water temperatures tend to decrease while pressure increases with increasing depth. As we shall see in later sections, such regular variation in environmental characteristics translates into nonrandom variation in biogeographic patterns of organisms, with each one adapted to slightly different environmental conditions. Such adaptations are, of course, the product of a long and
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Figure 1 (Continued.)
complex evolutionary history: a series of innumerable interactions between organisms and their environments. With each successive generation, the abilities of descendants to respond and adapt to local environmental conditions change. Evolutionary change, however, is part of a never-ending battle because environmental conditions include other species, which are also evolving. Just as important, the geographic template has evolved throughout earth’s 4.5-billion-year history. Because species distributions and other aspects of their geographic variation are influenced by their interaction with the geographic template, a thorough understanding of its dynamics in space and time is essential if we are to understand any major biogeographic patterns. By the opening of the twentieth century, each of the four persistent themes of biogeography was well established. Explanations for the major biogeographic patterns could now draw on insights from the rapidly growing field of evolutionary biology, as well as our knowledge of the other two fundamental biogeographic processesFimmigration and extinction. In addition, biogeographers of the early twentieth century could tap a great wealth of information on geographic variation of biotas and of the environments that they inhabited. Obviously, a thorough knowledge of this underlying geographic template was essential for understanding patterns in distribution and variation among regions and isolated
ecosystems. Yet to develop a more accurate and more comprehensive understanding of the major patterns and processes of biogeography, another major scientific revolution was required. Biogeographers and most other natural scientists knew a great deal about contemporary environments, and most of them appreciated the fact that climatic conditions had changed, sometimes dramatically, during earlier periods of earth’s history. Yet until the 1960s, most biogeographers clung to the belief that earth’s landforms and ocean basins remained fixed. During the twentieth century, acceptance of the theory of continental drift and plate tectonics revolutionized the field of biogeography as much as acceptance of the theory of natural selection and evolution had in the previous century.
Continental Drift and Plate Tectonics Although imperceptible to most of us, the earth’s continents have moved, colliding at times and drifting apart at others: mountain ranges have formed and eroded away, seas have expanded and contracted, and islands have appeared and disappeared. These changes must have had profound effects on local and regional climates and, in turn, on the geographic distributions and variations of all forms of life on earth. As we will see in a subsequent section, the theory of continental drift and plate tectonics is a relatively recent advance. Yet, with the
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Figure 2 Major climatic regions of the world. Note that these regions occur in distinct patterns with respect to latitude and the positions of continents, oceans, and mountain ranges. (Reproduced from Strahler AN (1973) Introduction to Physical Geography, 3rd edn., 468 pp. New York: Wiley.)
Figure 3 World distribution of major soil types. Note the close correlation of these soil types with the climatic zones shown in Figure 2, reflecting the influence of temperature and precipitation on soil formation.
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Figure 4 Climatic regions based on mean monthly water temperatures: A, arctic; NB, northern boreal; SB, southern boreal; T, tropical waters; E, equatorial region; NN, northern notal; SN, southern notal; ANT, antarctic. (Reproduced from Rass TS (1986) Vicariance icthyogeography of the Atlantic ocean pelagial. In: Pelagic Biogeography, pp. 237–241. UNESCO. Technical Papers in Marine Science, 49.)
possible exception of the acceptance of the theory of natural selection, no other contribution has had more of an impact on the field of biogeography. Plate tectonics is defined as the study of the origin, movement, and destruction of the earth’s plates and how these processes have been involved in the evolution of the earth’s crust. The theory of plate tectonics has achieved general acceptance among nearly all scientists and reigns as a unifying paradigm of both geology and biogeography. Yet until just three decades ago, relatively late in the development of these fields, champions of this theory were viewed as oddballs and heretics. As with any other revolutionary theory in science, it is extremely difficult to pinpoint the origins of the theory of plate tectonics. The great geologist Charles Lyell entertained the idea during the 1830s and 1840s, but then abandoned it in favor of the accepted doctrine of the fixity of the continents and ocean basins. In their attempts to explain the affinities of biotas of now isolated continents, Lyell, Joseph Dalton Hooker, and other ‘‘extensionists’’ of the nineteenth century hypothesized the periodic emergence of great land bridges that then allowed biotic exchange. Darwin, Wallace, and other members of the dispersal camp soundly criticized such views: nothing vexed Darwin more than those extensionists who created land bridges ‘‘as easy as a cook does pancakes.’’ In an uncharacteristically critical passage in one of his letters, Darwin complained to Charles Lyell of ‘‘the geological strides which many of your disciples are takingy. If you do not stop this, if there
be a lower region of punishment for geologists, I believe, my great master, you will go there.’’ As it turns out, neither the extensionists nor the dispersalists were correct. In most cases, the similarities among now isolated biotas were instead the result of ‘‘dispersal’’ of the continents themselves. Perhaps the first important evidence for what was at first referred to as the theory of continental drift was the configuration of the continents. That is, once geographers had developed relatively accurate maps, it became clear to some that opposite coastlines seemed to fit. In 1858 one of Lyell’s contemporaries, Antonio Snider-Pelligrini, may have been the first to demonstrate the geometric fit of the coastlines of continents on opposite sides of the Atlantic Ocean. Yet it wasn’t until 1908 and 1910 that an American geologist, F. B. Taylor, and a German meteorologist, Alfred L. Wegener, independently developed models describing the movements of the earth’s crust, along with the formation of mountain chains, island arcs, and related geologic features. Wegener continued to develop his model into a more comprehensive theory of continental drift, publishing his treatise in the 1920s. Wegener’s theory, however, included too many assumptions about geologic processes and patterns that would not be well established for another three or four decades. His theory also included factual errors, such as overestimating the rate of movement of the earth’s plates by perhaps two orders of magnitude. Finally, although he speculated on a potential mechanism, Wegener’s theory really lacked a plausible one that could somehow drive the massive plates about the earth
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like bits of ice on a pond in spring. It is perhaps one of history’s most tragic ironies that, in his quest to discover this mechanism by exploring a volcanically active region of Greenland, Wegener perished in a snow storm. Wegener’s insights would not be widely appreciated for another three decades. Acceptance of the theory of continental drift and its maturation to become the more comprehensive theory of plate tectonics would require many additional insights from geographers, paleontologists, and especially marine geologists during the 1940s and 1950s. These scientists found that when they ‘‘rejoined’’ the continents based on their geometric fit, not only did their biotas seem to match up, but so did topographic features such as mountain chains, rock strata, and fossil and glacial deposits. Perhaps most critical to the acceptance of the theory of continental drift were the efforts by marine geologists following World War II to map the surface of the ocean basins. It soon became clear that beneath each ocean lay a system of ridges that were situated far offshore. As one moved away from these ridges, the seafloor became deeper and more ancient as well. Provided with these and related clues, Herman Hess and his colleagues developed the theory of seafloor spreading: continental drift finally had an underlying mechanism (Figure 5). Eventually, paleomagnetic evidence would allow marine geologists to estimate the previous positions of the continents and develop reconstructions of the sequences of their movements and creation and the dissolution of previous continents. Biogeographers were now armed with not just the evidence, but also the mechanisms responsible for the dynamics of biotas and the geographic template itself (i.e., immigration, extinction, evolution, and plate tectonics).
Glacial Cycles of the Pleistocene The great shifting, collision, and separation of earth’s plates profoundly affected the distribution of its biota, both directly and indirectly. Not only did plate tectonics alter major dispersal routes among biotas, but it substantially changed both global and regional climates. As plates shifted across different latitudes, their local biota was exposed to major shifts in climatic conditions. Areas of what is now tropical Africa, South America, and Australia once were situated over the south pole and exposed to severe antarctic climates. On a global scale, drifting continents also triggered great shifts from periods of global warming to those dominated by glacial conditions. Land absorbs substantially more solar energy than does water. Thus, global climates tended to be warmer when landmasses were situated near the equator, but cooled as they shifted poleward. Yet global climates can change substantially even during periods too short for substantial shifting of earth’s plates. For example, during the Pleistocene (roughly the past 2 million years), earth experienced many climatic upheavals. Rather than being caused by any shifts in plates (which must have been minor given the relatively short period), these climatic shifts were caused by periodic changes in characteristics of the earth’s orbit (referred to as Milankovitch cycles; Figure 6). These changes significantly altered the total amount of solar energy intercepted by the earth, ultimately causing the climatic reversals of the Pleistocene. During full glacial periods, global temperatures dropped by as much as 6 1C and most
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landmasses beyond 451 latitude were covered with glaciers often 2 to 3 km thick. Because so much water was tied up in the glaciers, sea levels dropped by 100 to 200 m, thus uniting long-isolated biotas via temporary land bridges. For example, during the last glacial maximum, the region of Southeast Asia and Malaysia was united with Sumatra, Java, and Borneo to form Greater Sunda, while Australia and New Guinea formed the island continent of Sahul (Figure 7). Winds, ocean currents, and precipitation patterns also changed substantially between interglacial and glacial periods. With each climatic upheaval, environmental regimes shifted across both latitudes and elevations. Regions such as the American Southwest, which is now dominated by desert and xeric grasslands, were once covered with coniferous forests. Warming and drying conditions that led to the current interglacial period dramatically reduced these forests and caused them to shift toward the mountain peaks, where cool and relatively humid conditions prevail. These and other events must have profoundly influenced the distributions of most if not all biotas. As Brown and Lomolino (1998) summarize, however, all the complex biogeographic dynamics of the Pleistocene were triggered by three fundamental changes in the geographic template: 1. Changes in the location, extent, and configuration of principal habitats. 2. Changes in the nature of environmental regimes (combinations of temperature, seasonality, precipitation, and soil conditions). 3. The creation and dissolution of barriers associated with changes in sea level or elevational shifts in habitats. The responses of both terrestrial and aquatic biotas, while no doubt complex, also were of three types: 1. Some species shifted geographically with their optimal habitats. 2. Some species remained and adapted to the altered local environment. 3. Other species, unable to modify their ranges or ecological associations, suffered range contraction and eventual extinction. The biogeographic dynamics of the Pleistocene remains one of the field’s most active and interesting study areas. Recent advances in analyzing and dating fossil material continue to add to our ability to reconstruct the historical development of biotas (Theme 2) and, in turn, understand major episodes of biotic interchange and recent extinctions, as well as the current distributions of species.
Current Trends in Biogeography Gradients in Species Diversity and Composition In addition to biogeographic reconstructions, modern biogeographers continue to study an impressive diversity of patterns encompassing each of the four persistent themes of the field. During the middle of the twentieth century, many biogeographers focused on more general questions. Rather than dissecting and reconstructing the range of selected species,
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Figure 5 (A) During seafloor spreading, reversals in the earth’s magnetic field are recorded as the magnetically sensitive, iron-rich crust cools. Differences in the widths of the magnetic stripes reveal differences in the duration of these polarity episodes and in the rate of seafloor spreading over time and among regions. (B) The current model of plate tectonics includes the possibility that at least three forces may be responsible for crustal movements: (1) ridge push, or the force generated by molten rock rising from the earth’s core through the mantle at the midoceanic ridges; (2) mantle drag, the tendency of the crust to ride the mantle much like boxes on a conveyor belt; and (3) slab pull, the force generated as subducting crust tends to pull trailing crust after it along the surface. (Reproduced from Stanley SM (1987) Extinction. New York: Scientific American Books Inc. with permission from Nature.)
they examined trends in the total number of species, or what is often termed species richness. Though many patterns in richness have been studied, two have received the lion’s share of attention: the species–area and species–latitude relationships. Early explorers and naturalist of the seventeenth and eighteenth centuries noted the tendencies for species richness to increase with area of a region or island, and be higher for tropical versus temperate, subarctic, and arctic biotas. Armed with data from many hundreds of additional surveys and with a battery of sophisticated statistical tools, twentieth-century biogeographers confirmed the great generality of these patterns and developed some relatively simple models to explain those patterns. Often these models were equilibrial, assuming that species richness resulted from the combined but opposing effects of processes such as
immigration into an area (which added species) and extinction (which decreased species richness). MacArthur and Wilson’s equilibrium theory of island biogeography is perhaps the prototypic example of such a theory, and one that has dominated the field since its first articulation in the 1960s. Their theory was developed to explain both the species–area relationship and the species–isolation relationship (i.e., the tendency for species richness to decrease as one moves from near to more isolated islands). Simply stated, because immigration rates (the number of species new to an island) should decrease while extinction rate (loss of species already present) should increase as the island accumulates species, the island should eventually reach a level of richness at which immigrations balance extinctions. This equilibrial level of richness should vary among islands: decreasing with isolation because
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Figure 6 Milankovitch cycles are periodic changes in the eccentricity, obliquity, and precession of the earth’s orbit. Each of these changes influences the earth’s interception of solar radiation; therefore, these cycles may have been largely responsible for the glacial cycles of the Pleistocene. (Reproduced from Gates DM (1993) Climate Change and Its Biological Consequences. Sunderland, Massachusetts: Sinauer Association, with permission from Sinauer Associates.)
immigration rates are lower for more isolated islands, and increasing with island area because populations on larger islands should be less prone to extinction.
Biogeography in the Twenty-first Century The equilibrium theory stimulated many studies in biogeography and related fields of ecology, and has served as the
paradigm of island biogeography for some four decades. Yet an increasing number of biogeographers are beginning to question its utility as a modern paradigm. Species richness is often influenced by speciation and disturbances (e.g., major storms and tectonic events), processes not included in MacArthur and Wilson’s original theory. Either the theory has to be expanded to include these processes, or it will be
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Figure 7 The lowering of sea levels during glacial maxima of the Pleistocene caused the exposure of continental shelves and the formation of dispersal routes across four regions of the eastern Pacific: Sunda, Wallacea, Sahul, and Oceania. (White areas ¼ land exposed during glacial maxima; dark shading ¼ deep water (4200 m); possible dispersal routes are indicated by arrows). Drawn by Simon SS. Driver from The Journey from Eden by Brian M. Fagan, Thames & Hudson Ltd., London.
replaced by an alternative modelFone that may eventually become the new paradigm of the field. Whatever form such a model takes, it must be sophisticated enough to address the growing complexity of questions and patterns that we now study. MacArthur and Wilson’s model was primarily developed to explain patterns in richness along gradients of area and isolation. Yet modern biogeographers are now searching for a theory to explain patterns across other geographic gradients and, perhaps more important, to explain geographic trends in the types rather than just the numbers of species. Why do the proportions of large versus small, endothermic versus ectothermic, herbivorous versus carnivorous animals, woody versus herbaceous, or annual versus perennial plant species vary across geographic gradients? How are biogeographic reconstructions related to phylogenies? In what manner do gene frequencies vary with isolation or across other geographic gradients? How do the size and shape of geographic ranges vary with latitude and among taxonomic
groups, and how do population density and other demographic parameters vary across the range of a species? We may have reached a point at which our questions and appreciation for the complexity of nature have become too sophisticated for the relatively simple models that have dominated the field since the 1960s. If this is true, biogeography may be on the verge of a major scientific revolution, one that may well rival those triggered by the seminal insights of scientists such as Charles Darwin, Alfred Russell Wallace, Alfred Wegener, Robert H. MacArthur, and E. O. Wilson.
Biogeography and the Conservation of Biodiversity Biogeographers study both the patterns and processes influencing the geographic variation of nature. We study not just how many species occur in a particular area, but why more are there than somewhere else and which ones are likely to be
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shared among areas. We study and attempt to develop explanations for what are now termed ‘‘hotspots,’’ regions of relatively high numbers and high endemicity of species. Biogeographers also study variation in the geographic template, including that associated with anthropogenic disturbances such as the spread of exotic species or the spatial patterns of deforestation. Many biogeographers study extinction and have demonstrated that it has a geographic signature: loss of species tends to be highest for the smallest and most isolated sites, namely, oceanic islands and fragments of once expansive habitats on the mainland. Given this, it becomes obvious that few disciplines could be any more relevant to understanding and preserving biological diversity than biogeography. Our task, however, is far from a simple exercise of just applying what we already know. Indeed, only a small fraction (perhaps just 2 or 3%) of all extant species have been described, and we know precious little about the geographic distributions of most of those. What we do know often comes down to just general patterns for common species, but conservation biologists require detailed information on the rare speciesFthose that may be the exceptions to most rules. We have, however, made great progress in recent years in mapping and measuring the intensity (number of endemic species) of hotspots of biological diversity. In theory, these hotspots of biodiversity should receive the highest priority from conservation biologists, especially when they coincide with high levels of human activity. Yet even these approaches, generated by existing survey information and sophisticated geographic analyses, are based on a relatively limited number of surveys. To develop more effective strategies for conserving global diversity, we still require a much more thorough understanding of the geographic variation of nature. A number of distinguished ecologists and biogeographers, including E. O. Wilson, have called for greatly accelerated efforts to map the diversity of life. With adequate support, within just a few decades we could greatly expand our knowledge of the distributions of most life-forms and, eventually, contribute to their conservation as well.
See also: Darwin, Charles (Darwinism). Dispersal Biogeography. Historical Awareness of Biodiversity. Hotspots. Island Biogeography. Species–Area Relationships. Vicariance Biogeography
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References Briggs JC (1987) Biogeography and Plate Tectonics. Amsterdam: Elsevier. Brown JH (1995) Macroecology. Chicago: Chicago University Press. Brown JH and Lomolino MV (1998) Biogeography. Sunderland, Massachusetts: Sinauer Associates. Carlquist S (1974) Island Biology. New York: Columbia University Press. Darlington PJ Jr. (1957) Zoogeography: The Geographic Distribution of Animals. New York: John Wiley & Sons. Fagan BM (1990) The Journey from Eden: The Peopling of Our World. London: Thames and Hudson. Gates DM (1993) Climate Change and Its Biological Consequences. Sunderland, Massachusetts: Sinauer Association. Guilaine J (1991) Prehistory: The World of Early Man. New York: Facts on File Publishers. MacArthur RH and Wilson EO (1963) The Theory of Island Biogeography. Monographs in Population Biology. Princeton, New Jersey: Princeton University Press. Maurer B (1994) Geographic Population Analysis: Tools for Analysis of Biodiversity. London: Blackwell Scientific Publications. Rass TS (1986) Vicariance icthyogeography of the Atlantic ocean pelagial. In: Pelagic Biogeography, pp. 237–241. UNESCO. Technical Papers in Marine Science, 49. Rosenzweig ML (1995) Species Diversity in Time and Space. New York: Cambridge University Press. Sauer JD (1988) Plant Migration: The Dynamics of Geographic Patterning in Seed Plants. Berkeley: University of California Press. Sclater PL (1858) On the general geographical distribution of the members of the class Aves. Zoology 2: 130–145. Simberloff DS (1988) Contribution of population and community biology to conservation science. Annu. Rev. Ecol. Systematics 19: 473–511. Simpson GG (1980) Splendid Isolation: The Curious History of Mammals in South America. Oxford, United Kingdom: Pergamon Press. Stanley SM (1987) Extinction. New York: Scientific American Books Inc. Strahler AN (1973) Introduction to Physical Geography, 3rd edn., 468 pp. New York: Wiley. Udvardy MDF (1969) Dynamic Zoogeography. New York: Van Nostrand-Reinhold. Wallace AR (1869) The Malay Archipelago: The Land of the Orangutan, and the Bird of Paradise. New York: Harper. Wallace AR (1876) The Geographic Distribution of Animals. London: Macmillan. Wallace AR (1880) Island Life, or the Phenomena and Causes of Insular Faunas and Floras. London: Macmillan. Wegener A (1966) The Origin of the Continents and the Oceans (translation of the 1929 edition by J. Biram). New York: Dover Publications.
Glossary Biogeography Study of the geographic variation of nature, including variation in any biological characteristics (e.g., body size, population density, or species richness) on a geographic scale. Continental drift Model first proposed by Alfred Wegener that states that the continents were once united and then were displaced over the surface of the globe. Plate tectonics Study of the origin, movement, and destruction of the plates and how these processes have been involved in the evolution of Earth’s crust.
Pleistocene Geologic period from 2 million to 10,000 years before the present, which was characterized by alternating periods of glaciation events and global warming. Species composition Types of species that constitute a given community or sample. Species richness Number of species in a given community or sample.
Introduction
Early History of the Field
Traditionally, biogeography has been defined as the study of patterns in distributions of geographic ranges (Brown and Gibson, 1983). During the past three decades, however, this field has experienced a great surge in development and sophistication, and with this development the scope of the field has broadened to include an impressive diversity of patterns. Simply put, modern biogeographers now study nearly all aspects of the ‘‘geography of nature.’’ Biogeography now includes studies of variation in any biological feature (genetic, morphological, behavioral, physiological, demographic, or ecological) across geographic dimensions such as distance among sites or along gradients of area, elevation, or depth (see Brown and Lomolino, 1998).
Biogeography has a long and distinguished history, and one inextricably woven into the historical development of evolutionary biology and ecology. The historical development of biogeography had its origins coincident with the Age of World Explorations by Europeans during the eighteenth and nineteenth centuries. Yet the study of geography of nature must be an ancient one. The European explorers were not the first to ask ‘‘Where did life come from, and how did it diversify and spread across the earth?’’ Aristotle asked these same questions, as did many others before and after him, when faced with accounts of strange forms of life from foreign lands. The development of biogeography into a mature and respected field of science, however, required a much better understanding of variation in what we now call the geographic template and the associated variation in the natural world. It is by no minor coincidence that both evolutionary biology and biogeography developed in earnest during the Age of Exploration. Prior to this time, biologists had ‘‘discovered’’ and described less than 1% of plant and animal forms that we know today. Each new voyage or expedition added to the accumulated information on the earth’s environments and life-forms, and would eventually provide the raw material for the disciplines of evolution and biogeography. These disciplines are interconnected by the knowledge that selective pressures vary across space, and that all life-forms and their distributions are the product of natural selection. The early explorers and naturalists did far more than just label and catalog their specimens. They soon, perhaps irresistibly took to the task of comparing their collections across regions, elevations, and other gradients of the geographic template. At the same time, they began to develop explanations for the similarities and differences among the biotas they studied. In fact, most of the persistent themes of the field of biogeography (Table 1) were well established during the eighteenth and nineteenth centuries. To be sure, biogeography has made great strides during
Fundamentals of Biogeography Despite the sometimes overwhelming complexity of the natural world, all biogeographic patterns ultimately derive from two very general features of nature. First, as we move along any dimension of the geographic template, environmental conditions tend to vary in a predictable manner. For example, more distant sites tend to be more dissimilar than adjacent sites, environments at higher elevations tend to be cooler and wetter than those at lower elevations, and larger areas tend to capture more solar energy and a greater diversity of environmental conditions than smaller areas. Second, all forms of life differ in their abilities to adapt to geographic variation in their environment. These differences, while including a great diversity of responses (e.g., physiological, behavioral, developmental, and evolutionary), ultimately influence the three fundamental processes of biogeography: immigration, extinction, and evolution. All the biogeographic patterns we study derive from nonrandom variation in these processes across geographic gradients and across individuals, populations, and species.
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Table 1
Persistent themes of biogeography
1. Comparing and classifying geographic regions based on their biotas. 2. Reconstructing the historical development of biotas, including their origin, spread, and diversification. 3. Explaining the differences in numbers as well as types of species among geographic areas. 4. Explaining geographic variation in the characteristics of individuals and populations of closely related species, including trends in morphology, behavior, and demography. Source: Reproduced with permission from Brown JH and Lomolino MV (1998) Biogeography. Sunderland, Massachusetts: Sinauer Associates.
the twentieth century to become a mature and sophisticated science. It is important to acknowledge, however, that we owe a great deal to the many visionary explorers and naturalists who shared our fascination and asked the same questions about the geographic variation of nature.
Historic Explorations of the Eighteenth Century While motivated to a large degree by the quest for money and power, the Age of European Exploration was also fueled by the call to serve God. It was widely believed that the Creator had placed on this earth a still unknown diversity of organismsFa divine zoo or garden of life. Accordingly, early European explorers believed that there was perhaps no greater way to worship God than to unlock the mysteries of creation. Yet with each new account of some distant biota came information that challenged the prevailing views of creation. Eventually, the growing body of knowledge would overturn the long accepted view that the earth, its climate, and its species were immutable, unchanging in both space and time. More immediately, however, biologists of the eighteenth century were struck by the astounding diversity of species. Such diversity presented them with two serious problems, one practical and the other conceptual. First, biologists urgently needed a systematic and generally accepted scheme for classifying the burgeoning wealth of specimens, one that would reflect the similarities and differences among the species. Second, it quickly became clear that there were just too many species to be carried by Noah’s Ark. How could all the forms of life, now adapted to many distant and distinct regions across the globe, have originated and then spread from that one landing point? Carolus Linnaeus (1707–1778), certainly one of the most prominent biologists of all time, took on both of these challenges. In fact, his system of binomial nomenclature is the system we continue to use today to classify organisms. Linnaeus also attempted to rectify the Biblical doctrine with what he and his contemporaries knew about the diversity and geography of nature. This was especially challenging because, like most of his colleagues, Linnaeus was sure that species were immutable. Given this, how could species adapted to a single site and climate (Noah’s landing) have spread and become perfectly adapted to a suite of different environments (e.g., alpine tundra, coniferous forests, lowland forests, and grasslands)? Linnaeus’ answer: Noah’s landing had occurred along the slopes of Mount Ararat, a high mountain near the border of Turkey and Armenia. This mountain is so tall (reaching 16,853 ft above sea level) that along its slopes could be found a succession of environments and communities ranging from subtropical grasslands at the lower elevations to
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forests and alpine tundra at its summit. According to Linnaeus’ hypothesis, each elevational zone harbored a distinct assemblage of animals, each immutable but perfectly adapted to their local environment. When the Flood finally receded, these animals then dispersed to eventually colonize their respective environments across the globe. One of the foremost challenges to Linnaeus’ views came from his contemporary Comte de Buffon (1707–1788), who believed that not only were climates mutable, but species were as well. How else could animals disperse across what are now inhospitable habitats to occupy their present ranges in such isolated regions of the globe? Buffon’s theory of the origin and spread of life stemmed largely from his studies of living and fossil mammals, especially those of the Old and New World tropics. He was the first to realize that different regions of the globe, even those with the same environmental conditions, had distinct biotas. This observation was so fundamental that it eventually became biogeography’s first law: Buffon’s Law. Like Linnaeus, Buffon also concluded that there was one ‘‘landing point,’’ one site where all animals originated. However, this site, or region, was located far to the north of Mount Ararat, somewhere in the Arctic Circle where the early animals and their descendants could gain ready access to both the Old and New Worlds. This is where these life-forms survived the Flood during some earlier period when the earth’s climate was much warmer, warm enough such that tropical environments could extend far poleward. Once the floods receded, animals spread southward into the continents and began to diverge in form as they became increasingly isolated on different landmasses. Other biologists of the eighteenth century, including Joseph Banks and Johan Reinhold Forster, both of whom served as naturalist on voyages of Captain James Cook, were quick to confirm the generality of Buffon’s Law: environmentally similar but isolated regions have distinct assemblages of plants and animals. Forster also discussed the relationship between regional floras and environmental conditions and, in turn, between plant and animal associations: two cornerstones of the field now known as ecology. Forster was also one of the first scientists to report that plant diversity increases as we move toward the equator, that islands have fewer plants than the mainland, and that the diversity of insular plants increases with island size and available resources. Later in the eighteenth century, Karl Wildenow (1765–1812) and one of his students, Alexander von Humboldt (1769–1859), confirmed and further generalized both Buffon’s Law and Forster’s. Toward the end of the century, Augustin P. de Candolle added the important insight that, not only is the distribution of organisms influenced by geographic variation in environments, but they also compete for limiting resources such as food, light, and water. Therefore, by the beginning of the nineteenth century, biogeographers already had their first ‘‘law,’’ they described and tested the generality of a number of related patterns about the geography of nature, and they offered some testable theories regarding those patterns. They were actively working on at least three of the four persistent themes of biogeography (Themes 1–3 in Table 1). A number of biogeographers had abandoned the notion that species and climates were immutable. But for the field to advance and become a mature science, two additional, fundamental
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insights were needed. First, it required a mechanism for the mutability and adaptation of species. As many of us realize, Candolle’s observations about competition and the struggle for existence were central to the development of the theory of evolution by natural selection. These advances were to come in the latter part of the nineteenth century. Second, scientists had to recognize that the geographic template (i.e., the foundation for all of these patterns) also was mutable. That is, the size and relative positions of the continents and ocean basins have changed throughout the history of our planet.
Advances of the Nineteenth Century Many of the most fundamental advances in biogeography, and evolutionary biology as well, have required advances in geology. Until the nineteenth century, the age of the earth was typically assumed to be just a few thousand years, way too brief to allow what we now know to be the requisite time for evolution of its plates and the species that have rafted on those plates. The collective work of nineteenth-century paleobiologists would push the age of the earth back hundreds of thousands and eventually millions of years before the present. Legendary geologists and paleobiologists such as George Lyell (1797–1875) and Adolphe Brongniart (1801–1876), through their studies of fossils, provided incontrovertible evidence for extinction and for changes in regional climate and the elevation of land. How else could they explain the existence of fossils that have no contemporary forms, of fossils from tropical speies found in regions that are now temperate, and of shells and other marine fossils on present-day mountains? A theory of floating and drifting continents (now known as plate tectonics) would await discoveries of twentieth-century marine geologists, but their nineteenth-century forerunners understood that the earth was very old indeed, and it was mutable. Furthermore, if species (and many thousands of them) went extinct, then there had to be multiple periods of creation (or evolution) to compensate for those losses. Again, these views of a mutable earth, mutable climate, and mutable species were essential for those attempting to classify biogeographic regions based on their respective assemblage of species (Theme 1), to reconstruct the origin, spread, and diversification of life (Theme 2), and to explain differences in numbers and types of species among geographic regions (Theme 3). However, well into the middle of the nineteenth century many scientists held stubbornly to the idea that not only were species immutable, but so were their distributions. Perhaps the most distinguished champion of this static view of biogeography was Louis Agassiz (1807–1873), who argued that species remain essentially unchanged at or near their sites of creation. The static view was eventually overturned by the passionate and persuasive arguments of no one less than Charles Darwin. Not only did he propose a general theory for the diversification and adaptation of biotas (i.e., natural selection), but he was one of the world’s first and foremost dispersalists and champions of dynamic biogeography. Through his observations during his circumnavigation of the globe on the HMS Beagle (1831–1836), his later experiments on dispersal of seeds by animals, and his general synthesis on the origin and distribution of life, Darwin convinced many of his colleagues that long-distance dispersal could account for many of the
otherwise perplexing patterns of biogeography. Once he was joined by the likes of Asa Gray and Alfred Russell Wallace, Darwin and his colleagues were able to pull off a major paradigm shift in the fieldFfrom the static view of the earth and its species to the dynamic view of biogeography. Yet among all of these visionary scientists it is Wallace who is generally recognized as the father of zoogeography, and biogeography in general. While Darwin argued passionately regarding long-distance dispersal (even to the point of soundly criticizing his mentor, Charles Lyell), most of his energies were devoted toward developing and substantiating his theory of natural selection. On the other hand, biogeography was Wallace’s life’s work. Brown and Lomolino (1998) listed 17 tenets of the field that were developed by Wallace and included in his seminal monographs The Malay Archipelago (published in 1869 and dedicated to Darwin), The Geographic Distribution of Animals (1876), and Island Life (1880). Five of these tenets of biogeography are listed here: 1. Climate has a strong effect on the taxonomic similarity between two regions, but the relationship is not always linear. 2. The present biota of an area is strongly influenced by the last series of geological and climatic events. 3. Competition, predation, and other biotic factors play determining roles in the distribution, dispersal, and extinction of animals and plants. 4. When two large landmasses are united after a long period of separation, extinctions may occur because many organisms will encounter new competitors. 5. To analyze the biota of any particular region, one must determine the distributions of its organisms beyond that region as well as the distributions of their closest relatives. Using the latter approach and information provided by over a century of naturalists, Wallace developed a scheme of biogeographic regions (Figure 1) that accurately reflected the similarities and differences among biotas. This same scheme, largely unchanged, is still used today. For obvious reasons, exploration and biogeographic study of the marine realm have always lagged far behind that of terrestrial systems. Yet by the middle of the nineteenth century, biogeographers had made some significant strides in studying this new frontier. Charles Lyell discussed patterns of distribution of marine algae in his seminal work Principles of Geology, first published in 1830. Edward Forbes wrote the first comprehensive monograph on marine biogeography in 1856, in which he divided the marine realm into zoogeographic regions based on latitude, depth, and animal assemblages. In 1897 the great British ornithologist and biogeographer Philip Sclater, who produced a predecessor to Wallace’s biogeographic scheme, also developed a scheme for the marine realm based on distributions of marine mammals. Following the lead of earlier biogeographers and also based on his own extensive field studies in southwestern North America, C. Hart Merriam (1894) developed a system of what he termed ‘‘life zones’’ that confirmed earlier observations that elevational changes in vegetation were equivalent to those along latitudinal gradients. Finally, the countless specimens collected during the late eighteenth and early nineteenth centuries enabled others to begin to analyze geographic variation in characteristics of
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Figure 1 Alfred Wallace’s (1876) scheme of biogeographic regions, which attempts to divide the landmasses into classes reflecting affinities and differences among terrestrial biotas. The regions shown are still widely accepted today. Numbers identify subregions. (Reproduced from Wallace AR (1876) The Geographic Distribution of Animals. London: Macmillan.)
individuals and populations (Theme 4). C. L. Gloger reported in 1833 that, within a species, individuals from more humid habitats tend to be darker than those from drier habitats (Gloger’s Rule). C. Bergmann (1847) found that in birds and mammals, populations from cooler environments tended to have larger bodies than those from warmer environments (Bergmann’s Rule). Also, J. A. Allen reported in 1878 that birds and mammals inhabiting cooler environments also tend to have shorter appendages (Allens’ Rule).
Biogeography in the Twentieth Century Dynamics of the Geographic Template Even the earliest human explorers appreciated the fact that abiotic conditions vary as one moves from one point on the globe to another. On land, precipitation, temperature, seasonality, prevailing winds, soil conditions, and a host of other important factors vary as we move along transects of latitude, longitude, or altitude. Similarly, in the aquatic realm, temperature, currents, pressure, solar radiation, and concentrations of oxygen and dissolved nutrients vary markedly within and among ecosystems. Together, the variation in all of
these environmental characteristics combine to form the geographic template, which influences all biogeographic patterns. Although a complete understanding of all aspects of the geographic template may be a daunting and truly impossible challenge, at a regional to global scale, geographic variation in environmental conditions is quite regular and interpretable. On land, climatic conditions vary in an orderly manner with latitude, elevation, and proximity to mountain ranges or oceans (Figure 2). Major soil types (Figure 3) also vary in a similar fashion, partially because soil development is strongly influenced by local climatic conditions, especially precipitation and temperature. In the aquatic realm, although the great volume of ocean waters tends to buffer variation in temperature, surface waters still exhibit a latitudinal gradient in temperature (Figure 4). In addition, throughout most of the world’s oceans light availability and water temperatures tend to decrease while pressure increases with increasing depth. As we shall see in later sections, such regular variation in environmental characteristics translates into nonrandom variation in biogeographic patterns of organisms, with each one adapted to slightly different environmental conditions. Such adaptations are, of course, the product of a long and
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Figure 1 (Continued.)
complex evolutionary history: a series of innumerable interactions between organisms and their environments. With each successive generation, the abilities of descendants to respond and adapt to local environmental conditions change. Evolutionary change, however, is part of a never-ending battle because environmental conditions include other species, which are also evolving. Just as important, the geographic template has evolved throughout earth’s 4.5-billion-year history. Because species distributions and other aspects of their geographic variation are influenced by their interaction with the geographic template, a thorough understanding of its dynamics in space and time is essential if we are to understand any major biogeographic patterns. By the opening of the twentieth century, each of the four persistent themes of biogeography was well established. Explanations for the major biogeographic patterns could now draw on insights from the rapidly growing field of evolutionary biology, as well as our knowledge of the other two fundamental biogeographic processesFimmigration and extinction. In addition, biogeographers of the early twentieth century could tap a great wealth of information on geographic variation of biotas and of the environments that they inhabited. Obviously, a thorough knowledge of this underlying geographic template was essential for understanding patterns in distribution and variation among regions and isolated
ecosystems. Yet to develop a more accurate and more comprehensive understanding of the major patterns and processes of biogeography, another major scientific revolution was required. Biogeographers and most other natural scientists knew a great deal about contemporary environments, and most of them appreciated the fact that climatic conditions had changed, sometimes dramatically, during earlier periods of earth’s history. Yet until the 1960s, most biogeographers clung to the belief that earth’s landforms and ocean basins remained fixed. During the twentieth century, acceptance of the theory of continental drift and plate tectonics revolutionized the field of biogeography as much as acceptance of the theory of natural selection and evolution had in the previous century.
Continental Drift and Plate Tectonics Although imperceptible to most of us, the earth’s continents have moved, colliding at times and drifting apart at others: mountain ranges have formed and eroded away, seas have expanded and contracted, and islands have appeared and disappeared. These changes must have had profound effects on local and regional climates and, in turn, on the geographic distributions and variations of all forms of life on earth. As we will see in a subsequent section, the theory of continental drift and plate tectonics is a relatively recent advance. Yet, with the
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Figure 2 Major climatic regions of the world. Note that these regions occur in distinct patterns with respect to latitude and the positions of continents, oceans, and mountain ranges. (Reproduced from Strahler AN (1973) Introduction to Physical Geography, 3rd edn., 468 pp. New York: Wiley.)
Figure 3 World distribution of major soil types. Note the close correlation of these soil types with the climatic zones shown in Figure 2, reflecting the influence of temperature and precipitation on soil formation.
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Figure 4 Climatic regions based on mean monthly water temperatures: A, arctic; NB, northern boreal; SB, southern boreal; T, tropical waters; E, equatorial region; NN, northern notal; SN, southern notal; ANT, antarctic. (Reproduced from Rass TS (1986) Vicariance icthyogeography of the Atlantic ocean pelagial. In: Pelagic Biogeography, pp. 237–241. UNESCO. Technical Papers in Marine Science, 49.)
possible exception of the acceptance of the theory of natural selection, no other contribution has had more of an impact on the field of biogeography. Plate tectonics is defined as the study of the origin, movement, and destruction of the earth’s plates and how these processes have been involved in the evolution of the earth’s crust. The theory of plate tectonics has achieved general acceptance among nearly all scientists and reigns as a unifying paradigm of both geology and biogeography. Yet until just three decades ago, relatively late in the development of these fields, champions of this theory were viewed as oddballs and heretics. As with any other revolutionary theory in science, it is extremely difficult to pinpoint the origins of the theory of plate tectonics. The great geologist Charles Lyell entertained the idea during the 1830s and 1840s, but then abandoned it in favor of the accepted doctrine of the fixity of the continents and ocean basins. In their attempts to explain the affinities of biotas of now isolated continents, Lyell, Joseph Dalton Hooker, and other ‘‘extensionists’’ of the nineteenth century hypothesized the periodic emergence of great land bridges that then allowed biotic exchange. Darwin, Wallace, and other members of the dispersal camp soundly criticized such views: nothing vexed Darwin more than those extensionists who created land bridges ‘‘as easy as a cook does pancakes.’’ In an uncharacteristically critical passage in one of his letters, Darwin complained to Charles Lyell of ‘‘the geological strides which many of your disciples are takingy. If you do not stop this, if there
be a lower region of punishment for geologists, I believe, my great master, you will go there.’’ As it turns out, neither the extensionists nor the dispersalists were correct. In most cases, the similarities among now isolated biotas were instead the result of ‘‘dispersal’’ of the continents themselves. Perhaps the first important evidence for what was at first referred to as the theory of continental drift was the configuration of the continents. That is, once geographers had developed relatively accurate maps, it became clear to some that opposite coastlines seemed to fit. In 1858 one of Lyell’s contemporaries, Antonio Snider-Pelligrini, may have been the first to demonstrate the geometric fit of the coastlines of continents on opposite sides of the Atlantic Ocean. Yet it wasn’t until 1908 and 1910 that an American geologist, F. B. Taylor, and a German meteorologist, Alfred L. Wegener, independently developed models describing the movements of the earth’s crust, along with the formation of mountain chains, island arcs, and related geologic features. Wegener continued to develop his model into a more comprehensive theory of continental drift, publishing his treatise in the 1920s. Wegener’s theory, however, included too many assumptions about geologic processes and patterns that would not be well established for another three or four decades. His theory also included factual errors, such as overestimating the rate of movement of the earth’s plates by perhaps two orders of magnitude. Finally, although he speculated on a potential mechanism, Wegener’s theory really lacked a plausible one that could somehow drive the massive plates about the earth
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like bits of ice on a pond in spring. It is perhaps one of history’s most tragic ironies that, in his quest to discover this mechanism by exploring a volcanically active region of Greenland, Wegener perished in a snow storm. Wegener’s insights would not be widely appreciated for another three decades. Acceptance of the theory of continental drift and its maturation to become the more comprehensive theory of plate tectonics would require many additional insights from geographers, paleontologists, and especially marine geologists during the 1940s and 1950s. These scientists found that when they ‘‘rejoined’’ the continents based on their geometric fit, not only did their biotas seem to match up, but so did topographic features such as mountain chains, rock strata, and fossil and glacial deposits. Perhaps most critical to the acceptance of the theory of continental drift were the efforts by marine geologists following World War II to map the surface of the ocean basins. It soon became clear that beneath each ocean lay a system of ridges that were situated far offshore. As one moved away from these ridges, the seafloor became deeper and more ancient as well. Provided with these and related clues, Herman Hess and his colleagues developed the theory of seafloor spreading: continental drift finally had an underlying mechanism (Figure 5). Eventually, paleomagnetic evidence would allow marine geologists to estimate the previous positions of the continents and develop reconstructions of the sequences of their movements and creation and the dissolution of previous continents. Biogeographers were now armed with not just the evidence, but also the mechanisms responsible for the dynamics of biotas and the geographic template itself (i.e., immigration, extinction, evolution, and plate tectonics).
Glacial Cycles of the Pleistocene The great shifting, collision, and separation of earth’s plates profoundly affected the distribution of its biota, both directly and indirectly. Not only did plate tectonics alter major dispersal routes among biotas, but it substantially changed both global and regional climates. As plates shifted across different latitudes, their local biota was exposed to major shifts in climatic conditions. Areas of what is now tropical Africa, South America, and Australia once were situated over the south pole and exposed to severe antarctic climates. On a global scale, drifting continents also triggered great shifts from periods of global warming to those dominated by glacial conditions. Land absorbs substantially more solar energy than does water. Thus, global climates tended to be warmer when landmasses were situated near the equator, but cooled as they shifted poleward. Yet global climates can change substantially even during periods too short for substantial shifting of earth’s plates. For example, during the Pleistocene (roughly the past 2 million years), earth experienced many climatic upheavals. Rather than being caused by any shifts in plates (which must have been minor given the relatively short period), these climatic shifts were caused by periodic changes in characteristics of the earth’s orbit (referred to as Milankovitch cycles; Figure 6). These changes significantly altered the total amount of solar energy intercepted by the earth, ultimately causing the climatic reversals of the Pleistocene. During full glacial periods, global temperatures dropped by as much as 6 1C and most
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landmasses beyond 451 latitude were covered with glaciers often 2 to 3 km thick. Because so much water was tied up in the glaciers, sea levels dropped by 100 to 200 m, thus uniting long-isolated biotas via temporary land bridges. For example, during the last glacial maximum, the region of Southeast Asia and Malaysia was united with Sumatra, Java, and Borneo to form Greater Sunda, while Australia and New Guinea formed the island continent of Sahul (Figure 7). Winds, ocean currents, and precipitation patterns also changed substantially between interglacial and glacial periods. With each climatic upheaval, environmental regimes shifted across both latitudes and elevations. Regions such as the American Southwest, which is now dominated by desert and xeric grasslands, were once covered with coniferous forests. Warming and drying conditions that led to the current interglacial period dramatically reduced these forests and caused them to shift toward the mountain peaks, where cool and relatively humid conditions prevail. These and other events must have profoundly influenced the distributions of most if not all biotas. As Brown and Lomolino (1998) summarize, however, all the complex biogeographic dynamics of the Pleistocene were triggered by three fundamental changes in the geographic template: 1. Changes in the location, extent, and configuration of principal habitats. 2. Changes in the nature of environmental regimes (combinations of temperature, seasonality, precipitation, and soil conditions). 3. The creation and dissolution of barriers associated with changes in sea level or elevational shifts in habitats. The responses of both terrestrial and aquatic biotas, while no doubt complex, also were of three types: 1. Some species shifted geographically with their optimal habitats. 2. Some species remained and adapted to the altered local environment. 3. Other species, unable to modify their ranges or ecological associations, suffered range contraction and eventual extinction. The biogeographic dynamics of the Pleistocene remains one of the field’s most active and interesting study areas. Recent advances in analyzing and dating fossil material continue to add to our ability to reconstruct the historical development of biotas (Theme 2) and, in turn, understand major episodes of biotic interchange and recent extinctions, as well as the current distributions of species.
Current Trends in Biogeography Gradients in Species Diversity and Composition In addition to biogeographic reconstructions, modern biogeographers continue to study an impressive diversity of patterns encompassing each of the four persistent themes of the field. During the middle of the twentieth century, many biogeographers focused on more general questions. Rather than dissecting and reconstructing the range of selected species,
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Figure 5 (A) During seafloor spreading, reversals in the earth’s magnetic field are recorded as the magnetically sensitive, iron-rich crust cools. Differences in the widths of the magnetic stripes reveal differences in the duration of these polarity episodes and in the rate of seafloor spreading over time and among regions. (B) The current model of plate tectonics includes the possibility that at least three forces may be responsible for crustal movements: (1) ridge push, or the force generated by molten rock rising from the earth’s core through the mantle at the midoceanic ridges; (2) mantle drag, the tendency of the crust to ride the mantle much like boxes on a conveyor belt; and (3) slab pull, the force generated as subducting crust tends to pull trailing crust after it along the surface. (Reproduced from Stanley SM (1987) Extinction. New York: Scientific American Books Inc. with permission from Nature.)
they examined trends in the total number of species, or what is often termed species richness. Though many patterns in richness have been studied, two have received the lion’s share of attention: the species–area and species–latitude relationships. Early explorers and naturalist of the seventeenth and eighteenth centuries noted the tendencies for species richness to increase with area of a region or island, and be higher for tropical versus temperate, subarctic, and arctic biotas. Armed with data from many hundreds of additional surveys and with a battery of sophisticated statistical tools, twentieth-century biogeographers confirmed the great generality of these patterns and developed some relatively simple models to explain those patterns. Often these models were equilibrial, assuming that species richness resulted from the combined but opposing effects of processes such as
immigration into an area (which added species) and extinction (which decreased species richness). MacArthur and Wilson’s equilibrium theory of island biogeography is perhaps the prototypic example of such a theory, and one that has dominated the field since its first articulation in the 1960s. Their theory was developed to explain both the species–area relationship and the species–isolation relationship (i.e., the tendency for species richness to decrease as one moves from near to more isolated islands). Simply stated, because immigration rates (the number of species new to an island) should decrease while extinction rate (loss of species already present) should increase as the island accumulates species, the island should eventually reach a level of richness at which immigrations balance extinctions. This equilibrial level of richness should vary among islands: decreasing with isolation because
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Figure 6 Milankovitch cycles are periodic changes in the eccentricity, obliquity, and precession of the earth’s orbit. Each of these changes influences the earth’s interception of solar radiation; therefore, these cycles may have been largely responsible for the glacial cycles of the Pleistocene. (Reproduced from Gates DM (1993) Climate Change and Its Biological Consequences. Sunderland, Massachusetts: Sinauer Association, with permission from Sinauer Associates.)
immigration rates are lower for more isolated islands, and increasing with island area because populations on larger islands should be less prone to extinction.
Biogeography in the Twenty-first Century The equilibrium theory stimulated many studies in biogeography and related fields of ecology, and has served as the
paradigm of island biogeography for some four decades. Yet an increasing number of biogeographers are beginning to question its utility as a modern paradigm. Species richness is often influenced by speciation and disturbances (e.g., major storms and tectonic events), processes not included in MacArthur and Wilson’s original theory. Either the theory has to be expanded to include these processes, or it will be
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Biogeography, Overview
Figure 7 The lowering of sea levels during glacial maxima of the Pleistocene caused the exposure of continental shelves and the formation of dispersal routes across four regions of the eastern Pacific: Sunda, Wallacea, Sahul, and Oceania. (White areas ¼ land exposed during glacial maxima; dark shading ¼ deep water (4200 m); possible dispersal routes are indicated by arrows). Drawn by Simon SS. Driver from The Journey from Eden by Brian M. Fagan, Thames & Hudson Ltd., London.
replaced by an alternative modelFone that may eventually become the new paradigm of the field. Whatever form such a model takes, it must be sophisticated enough to address the growing complexity of questions and patterns that we now study. MacArthur and Wilson’s model was primarily developed to explain patterns in richness along gradients of area and isolation. Yet modern biogeographers are now searching for a theory to explain patterns across other geographic gradients and, perhaps more important, to explain geographic trends in the types rather than just the numbers of species. Why do the proportions of large versus small, endothermic versus ectothermic, herbivorous versus carnivorous animals, woody versus herbaceous, or annual versus perennial plant species vary across geographic gradients? How are biogeographic reconstructions related to phylogenies? In what manner do gene frequencies vary with isolation or across other geographic gradients? How do the size and shape of geographic ranges vary with latitude and among taxonomic
groups, and how do population density and other demographic parameters vary across the range of a species? We may have reached a point at which our questions and appreciation for the complexity of nature have become too sophisticated for the relatively simple models that have dominated the field since the 1960s. If this is true, biogeography may be on the verge of a major scientific revolution, one that may well rival those triggered by the seminal insights of scientists such as Charles Darwin, Alfred Russell Wallace, Alfred Wegener, Robert H. MacArthur, and E. O. Wilson.
Biogeography and the Conservation of Biodiversity Biogeographers study both the patterns and processes influencing the geographic variation of nature. We study not just how many species occur in a particular area, but why more are there than somewhere else and which ones are likely to be
Biogeography, Overview
shared among areas. We study and attempt to develop explanations for what are now termed ‘‘hotspots,’’ regions of relatively high numbers and high endemicity of species. Biogeographers also study variation in the geographic template, including that associated with anthropogenic disturbances such as the spread of exotic species or the spatial patterns of deforestation. Many biogeographers study extinction and have demonstrated that it has a geographic signature: loss of species tends to be highest for the smallest and most isolated sites, namely, oceanic islands and fragments of once expansive habitats on the mainland. Given this, it becomes obvious that few disciplines could be any more relevant to understanding and preserving biological diversity than biogeography. Our task, however, is far from a simple exercise of just applying what we already know. Indeed, only a small fraction (perhaps just 2 or 3%) of all extant species have been described, and we know precious little about the geographic distributions of most of those. What we do know often comes down to just general patterns for common species, but conservation biologists require detailed information on the rare speciesFthose that may be the exceptions to most rules. We have, however, made great progress in recent years in mapping and measuring the intensity (number of endemic species) of hotspots of biological diversity. In theory, these hotspots of biodiversity should receive the highest priority from conservation biologists, especially when they coincide with high levels of human activity. Yet even these approaches, generated by existing survey information and sophisticated geographic analyses, are based on a relatively limited number of surveys. To develop more effective strategies for conserving global diversity, we still require a much more thorough understanding of the geographic variation of nature. A number of distinguished ecologists and biogeographers, including E. O. Wilson, have called for greatly accelerated efforts to map the diversity of life. With adequate support, within just a few decades we could greatly expand our knowledge of the distributions of most life-forms and, eventually, contribute to their conservation as well.
See also: Darwin, Charles (Darwinism). Dispersal Biogeography. Historical Awareness of Biodiversity. Hotspots. Island Biogeography. Species–Area Relationships. Vicariance Biogeography
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