BIOGEOGRAPHY, OVERVIEW

Mark V. Lomolino Oklahoma Biological Survey, Oklahoma Natural Heritage Inventory, and University of Oklahoma

I. Introduction BIOGEOGRAPHY HAS A LONG AND DISTIN- II. Biogeography in the Twentieth Century GUISHED HISTORY, and one inextricably woven into III. Biogeography and the Conservation of the historical development of evolutionary biology and Biodiversity ecology. Modern biogeography now includes an impres- sive diversity of patterns, each of which dealing with some aspect of the spatial variation of nature. Given this, few disciplines can be any more relevant to under- GLOSSARY standing and conserving biological diversity than bioge- ography. biogeography Study of the geographic variation of na- ture, including variation in any biological character- istics (e.g., body size, population density, or species richness) on a geographic scale. continental drift Model first proposed by Alfred Weg- I. INTRODUCTION ener that states that the continents were once united and then were displaced over the surface of the Traditionally, biogeography has been defined as the globe. study of patterns in distributions of geographic ranges plate tectonics Study of the origin, movement, and de- (Brown and Gibson, 1983). During the past three de- struction of the plates and how these processes have cades, however, this field has experienced a great surge been involved in the evolution of Earth’s crust. in development and sophistication, and with this devel- Pleistocene Geologic period from 2 million to 10,000 opment the scope of the field has broadened to include years before the present, which was characterized an impressive diversity of patterns. Simply put, modern by alternating periods of glaciation events and biogeographers now study nearly all aspects of the ‘‘ge- global warming. ography of nature.’’ Biogeography now includes studies species composition Types of species that constitute of variation in any biological feature (genetic, morpho- a given community or sample. logical, behavioral, physiological, demographic, or eco- species richness Number of species in a given commu- logical) across geographic dimensions such as distance nity or sample. among sites or along gradients of area, elevation, or depth (see Brown and Lomolino, 1998).

Encyclopedia of Biodiversity, Volume 1 Copyright  2001 by Academic Press. All rights of reproduction in any form reserved. 455 456 BIOGEOGRAPHY, OVERVIEW

A. Fundamentals of Biogeography selective pressures vary across space, and that all life- forms and their distributions are the product of natu- Despite the sometimes overwhelming complexity of the ral selection. natural world, all biogeographic patterns ultimately de- The early explorers and naturalists did far more than rive from two very general features of nature. First, just label and catalog their specimens. They soon, per- as we move along any dimension of the geographic haps irresistibly took to the task of comparing their template, environmental conditions tend to vary in a collections across regions, elevations, and other gradi- predictable manner. For example, more distant sites ents of the geographic template. At the same time, they tend to be more dissimilar than adjacent sites, environ- began to develop explanations for the similarities and ments at higher elevations tend to be cooler and wetter differences among the biotas they studied. In fact, most than those at lower elevations, and larger areas tend to of the persistent themes of the field of biogeography capture more solar energy and a greater diversity of (Table I) were well established during the eighteenth environmental conditions than smaller areas. Second, and nineteenth centuries. To be sure, biogeography has all forms of life differ in their abilities to adapt to geo- made great strides during the twentieth century to be- graphic variation in their environment. These differ- come a mature and sophisticated science. It is important ences, while including a great diversity of responses to acknowledge, however, that we owe a great deal to (e.g., physiological, behavioral, developmental, and the many visionary explorers and naturalists who evolutionary), ultimately influence the three fundamen- shared our fascination and asked the same questions tal processes of biogeography: immigration, extinction, about the geographic variation of nature. and evolution. All the biogeographic patterns we study derive from nonrandom variation in these processes 1. Historic Explorations of the across geographic gradients and across individuals, Eighteenth Century populations, and species. While motivated to a large degree by the quest for money and power, the Age of European Exploration B. Early History of the Field was also fueled by the call to serve God. It was widely believed that the Creator had placed on this earth a Biogeography has a long and distinguished history, and still unknown diversity of organisms—a divine zoo or one inextricably woven into the historical development garden of life. Accordingly, early European explorers of evolutionary biology and ecology. The historical de- believed that there was perhaps no greater way to wor- velopment of biogeography had its origins coincident ship God than to unlock the mysteries of creation. with the Age of World Explorations by Europeans dur- Yet with each new account of some distant biota came ing the eighteenth and nineteenth centuries. Yet the information that challenged the prevailing views of cre- study of geography of nature must be an ancient one. ation. Eventually, the growing body of knowledge The European explorers were not the first to ask ‘‘Where would overturn the long accepted view that the earth, did life come from, and how did it diversify and spread its climate, and its species were immutable, unchanging across the earth?’’ Aristotle asked these same questions, in both space and time. More immediately, however, as did many others before and after him, when faced biologists of the eighteenth century were struck by the 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 TABLE I the geographic template and the associated variation in Persistent Themes of Biogeographya the natural world. It is by no minor coincidence that both evolutionary biology and biogeography developed 1. Comparing and classifying geographic regions based on their biotas. in earnest during the Age of Exploration. Prior to this 2. Reconstructing the historical development of biotas, including time, biologists had ‘‘discovered’’ and described less their origin, spread, and diversification. than 1% of plant and animal forms that we know today. 3. Explaining the differences in numbers as well as types of spe- Each new voyage or expedition added to the accumu- cies among geographic areas. lated information on the earth’s environments and life- 4. Explaining geographic variation in the characteristics of individ- uals and populations of closely related species, including trends forms, and would eventually provide the raw material in morphology, behavior, and demography. for the disciplines of evolution and biogeography. These disciplines are interconnected by the knowledge that a After Brown and Lomolino (1998). BIOGEOGRAPHY, OVERVIEW 457 astounding diversity of species. Such diversity pre- Like Linnaeus, Buffon also concluded that there was sented them with two serious problems, one practical one ‘‘landing point,’’ one site where all animals origi- and the other conceptual. First, biologists urgently nated. However, this site, or region, was located far to needed a systematic and generally accepted scheme for the north of Mount Ararat, somewhere in the Arctic classifying the burgeoning wealth of specimens, one Circle where the early animals and their descendants that would reflect the similarities and differences among could gain ready access to both the Old and New the species. Second, it quickly became clear that there Worlds. This is where these life-forms survived the were just too many species to be carried by Noah’s Ark. Flood during some earlier period when the earth’s cli- How could all the forms of life, now adapted to many mate was much warmer, warm enough such that tropi- distant and distinct regions across the globe, have origi- cal environments could extend far poleward. Once the nated and then spread from that one landing point? floods receded, animals spread southward into the con- Carolus Linnaeus (1707–1778), certainly one of the tinents and began to diverge in form as they became most prominent biologists of all time, took on both of increasingly isolated on different landmasses. these challenges. In fact, his system of binomial nomen- Other biologists of the eighteenth century, including clature is the system we continue to use today to classify Joseph Banks and Johan Reinhold Forster, both of organisms. Linnaeus also attempted to rectify the Bibli- whom served as naturalist on voyages of Captain James cal doctrine with what he and his contemporaries knew Cook, were quick to confirm the generality of Buffon’s about the diversity and geography of nature. This was Law: environmentally similar but isolated regions have especially challenging because, like most of his col- distinct assemblages of plants and animals. Forster also leagues, Linnaeus was sure that species were immuta- discussed the relationship between regional floras and ble. Given this, how could species adapted to a single environmental conditions and, in turn, between plant site and climate (Noah’s landing) have spread and be- and animal associations: two cornerstones of the field come perfectly adapted to a suite of different environ- now known as ecology. Forster was also one of the first ments (e.g., alpine tundra, coniferous forests, lowland scientists to report that plant diversity increases as we forests, and grasslands)? Linnaeus’ answer: Noah’s land- move toward the equator, that islands have fewer plants ing had occurred along the slopes of Mount Ararat, a than the mainland, and that the diversity of insular high mountain near the border of Turkey and Armenia. plants increases with island size and available resources. This mountain is so tall (reaching 16,853 ft above sea Later in the eighteenth century, Karl Wildenow (1765– level) that along its slopes could be found a succession 1812) and one of his students, Alexander von Humboldt of environments and communities ranging from sub- (1769–1859), confirmed and further generalized both tropical grasslands at the lower elevations to forests and Buffon’s Law and Forster’s. Toward the end of the cen- alpine tundra at its summit. According to Linnaeus’ tury, Augustin P. de Candolle added the important in- hypothesis, each elevational zone harbored a distinct sight that, not only is the distribution of organisms assemblage of animals, each immutable but perfectly influenced by geographic variation in environments, adapted to their local environment. When the Flood but they also compete for limiting resources such as finally receded, these animals then dispersed to eventu- food, light, and water. ally colonize their respective environments across the Therefore, by the beginning of the nineteenth cen- globe. tury, biogeographers already had their first ‘‘law,’’ they One of the foremost challenges to Linnaeus’ views described and tested the generality of a number of came from his contemporary Comte de Buffon (1707– related patterns about the geography of nature, and 1788), who believed that not only were climates muta- they offered some testable theories regarding those ble, but species were as well. How else could animals patterns. They were actively working on at least three disperse across what are now inhospitable habitats to of the four persistent themes of biogeography (Themes occupy their present ranges in such isolated regions of 1–3 in Table I). A number of biogeographers had the globe? Buffon’s theory of the origin and spread of abandoned the notion that species and climates were life stemmed largely from his studies of living and fossil immutable. But for the field to advance and become mammals, especially those of the Old and New World a mature science, two additional, fundamental insights tropics. He was the first to realize that different regions were needed. First, it required a mechanism for the of the globe, even those with the same environmental mutability and adaptation of species. As many of us conditions, had distinct biotas. This observation was realize, Candolle’s observations about competition and so fundamental that it eventually became biogeogra- the struggle for existence were central to the develop- phy’s first law: Buffon’s Law. ment of the theory of evolution by natural selection. 458 BIOGEOGRAPHY, OVERVIEW

These advances were to come in the latter part of collective work of nineteenth-century paleobiologists the nineteenth century. Second, scientists had to would push the age of the earth back hundreds of thou- recognize that the geographic template (i.e., the foun- sands and eventually millions of years before the pres- dation for all of these patterns) also was mutable. ent. Legendary geologists and paleobiologists such as That is, the size and relative positions of the continents George Lyell (1797–1875) and Adolphe Brongniart and ocean basins have changed throughout the history (1801–1876), through their studies of fossils, provided of our planet. incontrovertible evidence for extinction and for changes in regional climate and the elevation of land. How else 2. Advances of the Nineteenth Century could they explain the existence of fossils that have Many of the most fundamental advances in biogeogra- no contemporary forms, of fossils from tropical speies phy, and evolutionary biology as well, have required found in regions that are now temperate, and of shells advances in geology. Until the nineteenth century, the and other marine fossils on present-day mountains? A age of the earth was typically assumed to be just a few theory of floating and drifting continents (now known thousand years, way too brief to allow what we now as plate tectonics) would await discoveries of twentieth- know to be the requisite time for evolution of its plates century marine geologists, but their nineteenth-century and the species that have rafted on those plates. The forerunners understood that the earth was very old

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. (From Wallace, 1876.) BIOGEOGRAPHY, OVERVIEW 459 indeed, and it was mutable. Furthermore, if species The static view was eventually overturned by the (and many thousands of them) went extinct, then there passionate and persuasive arguments of no one less had to be multiple periods of creation (or evolution) than Charles Darwin. Not only did he propose a general to compensate for those losses. theory for the diversification and adaptation of biotas Again, these views of a mutable earth, mutable cli- (i.e., natural selection), but he was one of the world’s mate, and mutable species were essential for those at- first and foremost dispersalists and champions of dy- tempting to classify biogeographic regions based on namic biogeography. Through his observations during their respective assemblage of species (Theme 1), to his circumnavigation of the globe on the HMS Beagle reconstruct the origin, spread, and diversification of life (1831–1836), his later experiments on dispersal of (Theme 2), and to explain differences in numbers and seeds by animals, and his general synthesis on the origin types of species among geographic regions (Theme 3). and distribution of life, Darwin convinced many of his However, well into the middle of the nineteenth century colleagues that long-distance dispersal could account many scientists held stubbornly to the idea that not only for many of the otherwise perplexing patterns of bioge- were species immutable, but so were their distributions. ography. Once he was joined by the likes of Asa Gray Perhaps the most distinguished champion of this static and Alfred Russell Wallace, Darwin and his colleagues view of biogeography was Louis Agassiz (1807–1873), were able to pull off a major paradigm shift in the who argued that species remain essentially unchanged field—from the static view of the earth and its species at or near their sites of creation. to the dynamic view of biogeography.

FIGURE 1 (continued) 460 BIOGEOGRAPHY, OVERVIEW

Yet among all of these visionary scientists it is Wal- biogeographic scheme, also developed a scheme for the lace who is generally recognized as the father of zooge- marine realm based on distributions of marine mam- ography, and biogeography in general. While Darwin mals. Following the lead of earlier biogeographers and argued passionately regarding long-distance dispersal also based on his own extensive field studies in south- (even to the point of soundly criticizing his mentor, western North America, C. Hart Merriam (1894) devel- Charles Lyell), most of his energies were devoted to- oped a system of what he termed ‘‘life zones’’ that con- ward developing and substantiating his theory of natu- firmed earlier observations that elevational changes in ral selection. On the other hand, biogeography was vegetation were equivalent to those along latitudinal Wallace’s life’s work. Brown and Lomolino (1998) listed gradients. 17 tenets of the field that were developed by Wallace Finally, the countless specimens collected during the and included in his seminal monographs The Malay late eighteenth and early nineteenth centuries enabled Archipelago (published in 1869 and dedicated to Dar- others to begin to analyze geographic variation in char- win), The Geographic Distribution of Animals (1876), acteristics of individuals and populations (Theme 4). and Island Life (1880). Five of these tenets of biogeogra- C. L. Gloger reported in 1833 that, within a species, phy are listed here: individuals from more humid habitats tend to be darker than those from drier habitats (Gloger’s Rule). C. Berg- 1. Climate has a strong effect on the taxonomic simi- mann (1847) found that in birds and mammals, popula- larity between two regions, but the relationship is tions from cooler environments tended to have larger not always linear. bodies than those from warmer environments (Berg- 2. The present biota of an area is strongly influenced mann’s Rule). Also, J. A. Allen reported in 1878 that by the last series of geological and climatic events. birds and mammals inhabiting cooler environments 3. Competition, predation, and other biotic factors also tend to have shorter appendages (Allens’ Rule). play determining roles in the distribution, dis- persal, and extinction of animals and plants. 4. When two large landmasses are united after a long II. BIOGEOGRAPHY IN THE period of separation, extinctions may occur be- cause many organisms will encounter new compet- TWENTIETH CENTURY itors. 5. To analyze the biota of any particular region, one A. Dynamics of the Geographic Template must determine the distributions of its organisms Even the earliest human explorers appreciated the fact beyond that region as well as the distributions of that abiotic conditions vary as one moves from one their closest relatives. point on the globe to another. On land, precipitation, temperature, seasonality, prevailing winds, soil condi- Using the latter approach and information provided tions, and a host of other important factors vary as we by over a century of naturalists, Wallace developed a move along transects of latitude, longitude, or altitude. scheme of biogeographic regions (Fig. 1) that accurately Similarly, in the aquatic realm, temperature, currents, reflected the similarities and differences among biotas. pressure, solar radiation, and concentrations of oxygen This same scheme, largely unchanged, is still used and dissolved nutrients vary markedly within and today. among ecosystems. Together, the variation in all of For obvious reasons, exploration and biogeographic these environmental characteristics combine to form study of the marine realm have always lagged far behind the geographic template, which influences all biogeo- that of terrestrial systems. Yet by the middle of the graphic patterns. nineteenth century, biogeographers had made some sig- Although a complete understanding of all aspects of nificant strides in studying this new frontier. Charles the geographic template may be a daunting and truly Lyell discussed patterns of distribution of marine algae impossible challenge, at a regional to global scale, geo- in his seminal work Principles of Geology, first published graphic variation in environmental conditions is quite in 1830. Edward Forbes wrote the first comprehensive regular and interpretable. On land, climatic conditions monograph on marine biogeography in 1856, in which vary in an orderly manner with latitude, elevation, and he divided the marine realm into zoogeographic regions proximity to mountain ranges or oceans (Fig. 2). Major based on latitude, depth, and animal assemblages. In soil types (Fig. 3) also vary in a similar fashion, partially 1897 the great British ornithologist and biogeographer because soil development is strongly influenced by local Philip Sclater, who produced a predecessor to Wallace’s climatic conditions, especially precipitation and tem- BIOGEOGRAPHY, OVERVIEW 461

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. (After Strahler, 1973.)

perature. In the aquatic realm, although the great vol- fluenced by their interaction with the geographic tem- ume of ocean waters tends to buffer variation in temper- plate, a thorough understanding of its dynamics in space ature, surface waters still exhibit a latitudinal gradient and time is essential if we are to understand any major in temperature (Fig. 4). In addition, throughout most biogeographic patterns. of the world’s oceans light availability and water temper- By the opening of the twentieth century, each of atures tend to decrease while pressure increases with the four persistent themes of biogeography was well increasing depth. established. Explanations for the major biogeographic As we shall see in later sections, such regular varia- patterns could now draw on insights from the rapidly tion in environmental characteristics translates into growing field of evolutionary biology, as well as our nonrandom variation in biogeographic patterns of or- knowledge of the other two fundamental biogeographic ganisms, with each one adapted to slightly different processes—immigration and extinction. In addition, environmental conditions. Such adaptations are, of biogeographers of the early twentieth century could tap course, the product of a long and complex evolutionary a great wealth of information on geographic variation history: a series of innumerable interactions between of biotas and of the environments that they inhabited. organisms and their environments. With each succes- Obviously, a thorough knowledge of this underlying sive generation, the abilities of descendants to respond geographic template was essential for understanding and adapt to local environmental conditions change. patterns in distribution and variation among regions Evolutionary change, however, is part of a never-ending and isolated ecosystems. Yet to develop a more accurate battle because environmental conditions include other and more comprehensive understanding of the major species, which are also evolving. Just as important, the patterns and processes of biogeography, another major geographic template has evolved throughout earth’s scientific revolution was required. 4.5-billion-year history. Because species distributions Biogeographers and most other natural scientists and other aspects of their geographic variation are in- knew a great deal about contemporary environments, 462 BIOGEOGRAPHY, OVERVIEW

FIGURE 3 World distribution of major soil types. Note the close correlation of these soil types with the climatic zones shown in Fig. 2, reflecting the influence of temperature and precipitation on soil formation.

and most of them appreciated the fact that climatic advance. Yet, with the possible exception of the conditions had changed, sometimes dramatically, dur- acceptance of the theory of natural selection, no other ing earlier periods of earth’s history. Yet until the 1960s, contribution has had more of an impact on the field most biogeographers clung to the belief that earth’s of biogeography. landforms and ocean basins remained fixed. During the Plate tectonics is defined as the study of the origin, twentieth century, acceptance of the theory of continen- movement, and destruction of the earth’s plates and tal drift and plate tectonics revolutionized the field of how these processes have been involved in the evolu- biogeography as much as acceptance of the theory of tion of the earth’s crust. The theory of plate tectonics natural selection and evolution had in the previous has achieved general acceptance among nearly all century. scientists and reigns as a unifying paradigm of both geology and biogeography. Yet until just three decades 1. Continental Drift and Plate Tectonics ago, relatively late in the development of these fields, Although imperceptible to most of us, the earth’s champions of this theory were viewed as oddballs continents have moved, colliding at times and drifting and heretics. apart at others: mountain ranges have formed and As with any other revolutionary theory in science, eroded away, seas have expanded and contracted, and it is extremely difficult to pinpoint the origins of the islands have appeared and disappeared. These changes theory of plate tectonics. The great geologist Charles must have had profound effects on local and regional Lyell entertained the idea during the 1830s and 1840s, climates and, in turn, on the geographic distributions but then abandoned it in favor of the accepted doctrine and variations of all forms of life on earth. As we of the fixity of the continents and ocean basins. In will see in a subsequent section, the theory of conti- their attempts to explain the affinities of biotas of nental drift and plate tectonics is a relatively recent now isolated continents, Lyell, Joseph Dalton Hooker, BIOGEOGRAPHY, OVERVIEW 463

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. (After Rass, 1986.)

and other ‘‘extensionists’’ of the nineteenth century some that opposite coastlines seemed to fit. In 1858 hypothesized the periodic emergence of great land one of Lyell’s contemporaries, Antonio Snider-Pelli- bridges that then allowed biotic exchange. Darwin, grini, may have been the first to demonstrate the geo- Wallace, and other members of the dispersal camp metric fit of the coastlines of continents on opposite soundly criticized such views: nothing vexed Darwin sides of the Atlantic Ocean. Yet it wasn’t until 1908 more than those extensionists who created land and 1910 that an American geologist, F. B. Taylor, and bridges ‘‘as easy as a cook does pancakes.’’ In an a German meteorologist, Alfred L. Wegener, indepen- uncharacteristically critical passage in one of his let- dently developed models describing the movements of ters, Darwin complained to Charles Lyell of ‘‘the the earth’s crust, along with the formation of mountain geological strides which many of your disciples are chains, island arcs, and related geologic features. Weg- taking....Ifyoudonotstop this, if there be a ener continued to develop his model into a more com- lower region of punishment for geologists, I believe, prehensive theory of continental drift, publishing his my great master, you will go there.’’ treatise in the 1920s. Wegener’s theory, however, in- As it turns out, neither the extensionists nor the cluded too many assumptions about geologic processes dispersalists were correct. In most cases, the similarities and patterns that would not be well established for among now isolated biotas were instead the result of another three or four decades. His theory also included ‘‘dispersal’’ of the continents themselves. Perhaps the factual errors, such as overestimating the rate of move- first important evidence for what was at first referred ment of the earth’s plates by perhaps two orders of to as the theory of continental drift was the configura- magnitude. Finally, although he speculated on a poten- tion of the continents. That is, once geographers had tial mechanism, Wegener’s theory really lacked a plausi- developed relatively accurate maps, it became clear to ble one that could somehow drive the massive plates 464 BIOGEOGRAPHY, OVERVIEW about the earth like bits of ice on a pond in spring. It situated near the equator, but cooled as they shifted is perhaps one of history’s most tragic ironies that, in poleward. his quest to discover this mechanism by exploring a Yet global climates can change substantially even volcanically active region of Greenland, Wegener per- during periods too short for substantial shifting of ished in a snow storm. earth’s plates. For example, during the Pleistocene Wegener’s insights would not be widely appreciated (roughly the past 2 million years), earth experienced for another three decades. Acceptance of the theory of many climatic upheavals. Rather than being caused by continental drift and its maturation to become the more any shifts in plates (which must have been minor given comprehensive theory of plate tectonics would require the relatively short period), these climatic shifts were many additional insights from geographers, paleontolo- caused by periodic changes in characteristics of the gists, and especially marine geologists during the 1940s earth’s orbit (referred to as Milankovitch cycles; Fig. and 1950s. These scientists found that when they ‘‘re- 6). These changes significantly altered the total amount joined’’ the continents based on their geometric fit, not of solar energy intercepted by the earth, ultimately caus- only did their biotas seem to match up, but so did ing the climatic reversals of the Pleistocene. During topographic features such as mountain chains, rock full glacial periods, global temperatures dropped by as strata, and fossil and glacial deposits. Perhaps most much as 6ЊC and most landmasses beyond 45Њ latitude critical to the acceptance of the theory of continental were covered with glaciers often 2 to 3 km thick. Be- drift were the efforts by marine geologists following cause so much water was tied up in the glaciers, sea World War II to map the surface of the ocean basins. levels dropped by 100 to 200 m, thus uniting long- It soon became clear that beneath each ocean lay a isolated biotas via temporary land bridges. For example, system of ridges that were situated far offshore. As one during the last glacial maximum, the region of South- moved away from these ridges, the seafloor became east Asia and Malaysia was united with Sumatra, Java, deeper and more ancient as well. Provided with these and Borneo to form Greater Sunda, while Australia and and related clues, Herman Hess and his colleagues de- New Guinea formed the island continent of Sahul veloped the theory of seafloor spreading: continental (Fig. 7). drift finally had an underlying mechanism (Fig. 5). Winds, ocean currents, and precipitation patterns Eventually, paleomagnetic evidence would allow ma- also changed substantially between interglacial and gla- rine geologists to estimate the previous positions of the cial periods. With each climatic upheaval, environmen- continents and develop reconstructions of the se- tal regimes shifted across both latitudes and elevations. quences of their movements and creation and the disso- Regions such as the American Southwest, which is now lution of previous continents. Biogeographers were now dominated by desert and xeric grasslands, were once armed with not just the evidence, but also the mecha- covered with coniferous forests. Warming and drying nisms responsible for the dynamics of biotas and the conditions that led to the current interglacial period geographic template itself (i.e., immigration, extinction, dramatically reduced these forests and caused them evolution, and plate tectonics). to shift toward the mountain peaks, where cool and relatively humid conditions prevail. 2. Glacial Cycles of the Pleistocene These and other events must have profoundly influ- The great shifting, collision, and separation of earth’s enced the distributions of most if not all biotas. As plates profoundly affected the distribution of its biota, Brown and Lomolino (1998) summarize, however, all both directly and indirectly. Not only did plate tectonics the complex biogeographic dynamics of the Pleistocene alter major dispersal routes among biotas, but it sub- were triggered by three fundamental changes in the stantially changed both global and regional climates. geographic template: 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, 1. Changes in the location, extent, and configuration and Australia once were situated over the south pole of principal habitats. and exposed to severe antarctic climates. 2. Changes in the nature of environmental regimes On a global scale, drifting continents also triggered (combinations of temperature, seasonality, precipi- great shifts from periods of global warming to those tation, and soil conditions). dominated by glacial conditions. Land absorbs substan- 3. The creation and dissolution of barriers associated tially more solar energy than does water. Thus, global with changes in sea level or elevational shifts in climates tended to be warmer when landmasses were habitats. BIOGEOGRAPHY, OVERVIEW 465

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. (From Stanley, 1987.) (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. (After Stanley, 1987.)

The responses of both terrestrial and aquatic biotas, ecological associations, suffered range contraction while no doubt complex, also were of three types: and eventual extinction.

1. Some species shifted geographically with their opti- The biogeographic dynamics of the Pleistocene re- mal habitats. mains one of the field’s most active and interesting 2. Some species remained and adapted to the altered study areas. Recent advances in analyzing and dating local environment. fossil material continue to add to our ability to recon- 3. Other species, unable to modify their ranges or struct the historical development of biotas (Theme 2) 466 BIOGEOGRAPHY, OVERVIEW

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. (After Gates, 1993.) BIOGEOGRAPHY, OVERVIEW 467

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 ϭ land exposed during glacial maxima; dark shading ϭ deep water (Ͼ200 m); possible dispersal routes are indicated by arrows). (After Fagan, 1990; Guilaine, 1991.)

and, in turn, understand major episodes of biotic inter- sity of patterns encompassing each of the four persistent change and recent extinctions, as well as the current themes of the field. During the middle of the twentieth distributions of species. century, many biogeographers focused on more general questions. Rather than dissecting and reconstructing the range of selected species, they examined trends in B. Current Trends in Biogeography the total number of species, or what is often termed species richness. 1. Gradients in Species Diversity Though many patterns in richness have been studied, and Composition two have received the lion’s share of attention: the spe- In addition to biogeographic reconstructions, modern cies–area and species–latitude relationships. Early ex- biogeographers continue to study an impressive diver- plorers and naturalist of the seventeenth and eighteenth 468 BIOGEOGRAPHY, OVERVIEW centuries noted the tendencies for species richness to a theory to explain patterns across other geographic increase with area of a region or island, and be higher for gradients and, perhaps more important, to explain geo- tropical versus temperate, subarctic, and arctic biotas. graphic trends in the types rather than just the numbers Armed with data from many hundreds of additional of species. Why do the proportions of large versus small, surveys and with a battery of sophisticated statistical endothermic versus ectothermic, herbivorous versus tools, twentieth-century biogeographers confirmed the carnivorous animals, woody versus herbaceous, or an- great generality of these patterns and developed some nual versus perennial plant species vary across geo- relatively simple models to explain those patterns. Of- graphic gradients? How are biogeographic reconstruc- ten these models were equilibrial, assuming that species tions related to phylogenies? In what manner do gene richness resulted from the combined but opposing ef- frequencies vary with isolation or across other geo- fects of processes such as immigration into an area graphic gradients? How do the size and shape of geo- (which added species) and extinction (which decreased graphic ranges vary with latitude and among taxonomic species richness). MacArthur and Wilson’s equilibrium groups, and how do population density and other demo- theory of island biogeography is perhaps the prototypic graphic parameters vary across the range of a species? example of such a theory, and one that has dominated We may have reached a point at which our questions the field since its first articulation in the 1960s. Their and appreciation for the complexity of nature have be- theory was developed to explain both the species–area come too sophisticated for the relatively simple models relationship and the species–isolation relationship (i.e., that have dominated the field since the 1960s. If this the tendency for species richness to decrease as one is true, biogeography may be on the verge of a major moves from near to more isolated islands). Simply scientific revolution, one that may well rival those trig- stated, because immigration rates (the number of spe- gered by the seminal insights of scientists such as cies new to an island) should decrease while extinction Charles Darwin, Alfred Russell Wallace, Alfred Weg- rate (loss of species already present) should increase ener, Robert H. MacArthur, and E. O. Wilson. as the island accumulates species, the island should eventually reach a level of richness at which immigra- tions balance extinctions. This equilibrial level of rich- ness should vary among islands: decreasing with isola- III. BIOGEOGRAPHY AND THE tion because immigration rates are lower for more CONSERVATION OF BIODIVERSITY isolated islands, and increasing with island area because populations on larger islands should be less prone to Biogeographers study both the patterns and processes extinction. influencing the geographic variation of nature. We study not just how many species occur in a particular 2. Biogeography in the area, but why more are there than somewhere else and Twenty-first Century which ones are likely to be shared among areas. We The equilibrium theory stimulated many studies in bio- study and attempt to develop explanations for what geography and related fields of ecology, and has served are now termed ‘‘hotspots,’’ regions of relatively high as the paradigm of island biogeography for some four numbers and high endemicity of species. Biogeogra- decades. Yet an increasing number of biogeographers phers also study variation in the geographic template, are beginning to question its utility as a modern para- including that associated with anthropogenic distur- digm. Species richness is often influenced by speciation bances such as the spread of exotic species or the spatial and disturbances (e.g., major storms and tectonic patterns of deforestation. Many biogeographers study events), processes not included in MacArthur and Wil- extinction and have demonstrated that it has a geo- son’s original theory. Either the theory has to be ex- graphic signature: loss of species tends to be highest panded to include these processes, or it will be replaced for the smallest and most isolated sites, namely, oceanic by an alternative model—one that may eventually be- islands and fragments of once expansive habitats on come the new paradigm of the field. the mainland. Given this, it becomes obvious that few Whatever form such a model takes, it must be sophis- disciplines could be any more relevant to understanding ticated enough to address the growing complexity of and preserving biological diversity than biogeography. questions and patterns that we now study. MacArthur Our task, however, is far from a simple exercise of and Wilson’s model was primarily developed to explain just applying what we already know. Indeed, only a patterns in richness along gradients of area and isola- small fraction (perhaps just 2 or 3%) of all extant species tion. Yet modern biogeographers are now searching for have been described, and we know precious little about BIOGEOGRAPHY, OVERVIEW 469 the geographic distributions of most of those. What we Brown, J. H., and Lomolino, M. V. (1998). Biogeography. Sinauer do know often comes down to just general patterns for Associates, Sunderland, Massachusetts. Carlquist, S. (1974). Island Biology. Columbia University Press, common species, but conservation biologists require New York. detailed information on the rare species—those that Darlington, P. J., Jr. (1957). Zoogeography: The Geographic Distribu- may be the exceptions to most rules. tion of Animals. John Wiley & Sons, New York. We have, however, made great progress in recent Fagan, B. M. (1990). The journey from Eden: the peopling of our world. years in mapping and measuring the intensity (number Thames and Hudson, London. Gates, D. M. (1993). Climate change and its biological consequences. of endemic species) of hotspots of biological diversity. Sinauer Association, Sunderland, Massachusetts. In theory, these hotspots of biodiversity should receive Guilaine, J. (1991). Prehistory: the world of early man. Facts on File the highest priority from conservation biologists, espe- Publishers, New York. cially when they coincide with high levels of human MacArthur, R. H., and Wilson, E. O. (1963). The Theory of Island activity. Yet even these approaches, generated by ex- Biogeography. Monographs in Population Biology. Princeton Uni- versity Press, Princeton, New Jersey. isting survey information and sophisticated geographic Maurer, B. (1994). Geographic Population Analysis: Tools for Analysis analyses, are based on a relatively limited number of of Biodiversity. Blackwell Scientific Publications, London. surveys. Rass, T. S. (1986). Vicariance icthyogeography of the Atlantic ocean To develop more effective strategies for conserving pelagial. In: Pelagic Biogeography, pp. 237–241. UNESCO Techni- global diversity, we still require a much more thorough cal Papers in Marine Science, 49. Rosenzweig, M. L. (1995). Species Diversity in Time and Space. Cam- understanding of the geographic variation of nature. A bridge University Press, New York. number of distinguished ecologists and biogeographers, Sauer, J. D. (1988). Plant Migration: The Dynamics of Geographic including E. O. Wilson, have called for greatly acceler- Patterning in Seed Plants. University of California Press, Berkeley. ated efforts to map the diversity of life. With adequate Sclater, P. L. (1858). On the general geographical distribution of the support, within just a few decades we could greatly members of the class Aves. Zoology 2, 130–145. Simberloff, D. S. (1988). Contribution of population and community expand our knowledge of the distributions of most life- biology to conservation science. Annu. Rev. Ecol. Systematics forms and, eventually, contribute to their conservation 19, 473–511. as well. Simpson, G. G. (1980). Splendid Isolation: The Curious History of Mammals in South America. Pergamon Press, Oxford, United Kingdom. Stanley, S. M. (1987). Extinction. Scientific American Books, Inc., See Also the Following Articles New York. Strahler, A. N. (1973). Introduction to Physical Geography, 3rd ed. DARWIN, CHARLES • DISPERSAL BIOGEOGRAPHY • 468 pp. Wiley, New York. HISTORICAL AWARENESS OF BIODIVERSITY • HOTSPOTS • Udvardy, M. D. F. (1969). Dynamic Zoogeography. Van Nostrand– ISLAND BIOGEOGRAPHY • SPECIES AREA RELATIONSHIPS • Reinhold, New York. VICARIANCE BIOGEOGRAPHY Wallace, A. R. (1869). The Malay Archipelago: The Land of the Orang- utan, and the Bird of Paradise. Harper, New York. Wallace, A. R. (1876). The Geographic Distribution of Animals. Macmil- Bibliography lan, London. Wallace, A. R. (1880). Island Life, or the Phenomena and Causes of Briggs, J. C. (1987). Biogeography and Plate Tectonics. Elsevier, Am- Insular Faunas and Floras. Macmillan, London. sterdam. Wegener, A. (1966). The Origin of the Continents and the Oceans Brown, J. H. (1995). Macroecology. Chicago University Press, (translation of the 1929 edition by J. Biram). Dover Publications, Chicago. New York.