Biogeography

Biogeography is the study of how, where, and why populations and species are distributed over the face of the earth; as such it relates centrally to ecology, evolution, and biodiversity. There are two primary avenues of research in biogeography: ecological biogeography, which aims to identify those ecological and climatic factors that control the distribution of many species; and phylogenetic biogeography, which aims to identify whether several different groups distributed in the same regions have spe-ciated in response to a common geological history. These two subdisciplines exist because the geographic distribution of organisms is determined partly by ecological factors and partly by a lineage's evolutionary history. The conservation of biodiversity involves recognizing the key ecological requirements of species and identifying the regions that have evolved unique biotas.

Many of the factors that determine where species are found relate to ecology and climate. For example, as one approaches the poles, biodiversity declines; the tropics are the source of most of the world's species. Organisms are also adapted to particular habitats and climates, and the availability of appropriate food items; different species vary in their range of habitat tolerance, such that some groups are broadly distributed while others are distributed narrowly. Some organisms will even migrate vast distances annually to track their preferred habitat as climate changes with the seasons. Understanding the ecological and environmental requirements of species can provide us with great insight into the distribution of organisms. Our understanding of ecological biogeography is based upon both static and dynamic patterns. For example, the term tree line describes the current elevation above which no trees can be found because of climatic extremes; below tree line, the highest elevation at which any given tree species can be found will vary. During climate changes over the last 10,000 years, North American plant species migrated at different rates and in different directions to track their preferred habitat, such that ecological communities showed no real cohe-siveness through time. Other important ecological biogeographic patterns relate to the number of species that islands, or even continents, can support. For instance, there is a well-established relationship between the size of a region and the number of species it contains. This was first quantified by J. C. Willis in 1922 and later extended in important research by Robert MacArthur and E. O. Wilson in the 1950s and 1960s. Finally, research by scientists beginning with work done by Linnaeus and including experiments conducted by Charles Darwin has documented how organisms within a species can move or disperse over large distances.

Although climatic and ecological factors clearly relate in an important way to organis-mic distribution or biogeography, regions with very similar climates can have very different types of species, and species with similar ecologies can occur in very different regions, complicating the study of ecological biogeogra-phy. Simply understanding a region's climate and the ecology of its constituent organisms does not provide a complete understanding of the biogeography. Augustin de Candolle, a French botanist who published several important works between roughly 1815 and 1825, was one of the first scientists to recognize this fact. To understand more completely where organisms are distributed, it is also necessary to know their evolutionary relationships.

Since evolution involves species descending from ancestors, a new species arises somewhere within the range of its ancestor. The ecological requirements of the ancestral species partly determine its biogeographic distribution, but geographic barriers also determine the limits of the species' range. In terrestrial species such barriers might include mountain ranges, oceans, or regions of inhospitable climate. For marine species barriers include land masses and deep ocean basins which, at least for species that live in shallow water, are analogous to regions of inhospitable climate. Many of the barriers that affect terrestrial and marine species are formed by geological processes driven by plate tectonics. Sometimes the formation of geographic barriers, mediated by plate tectonics, can trigger evolution and spe-ciation in several groups distributed in the same region, because the barriers that form often separate populations of several species from one another; one of the principal ways that speciation occurs is when populations become isolated. It is also conceivable that many species may expand their ranges together when geographic barriers fall, as sometimes happens with climatic changes or continental collisions mediated by plate tectonics.

Georges-Louis Leclerc de Buffon, Augustin de Candolle, and other scientists active in the late eighteenth and early nineteenth centuries recognized that different regions tend to have their own distinct complement of species, and this was a fundamental insight relating the history of species to the geological history of the regions they occurred in. Initially, this observation was explained by invoking the idea that the different species had been created for each of the different regions. Philip Lutley Sclater and Alfred Russell Wallace were scientists who came up with schemes in the mid dle of the nineteenth century to divide the world into a series of distinct floras and faunas. Some of the regions were equivalent to continents, but others, notably in the case of India, were within continental borders. By the time Wallace published on this topic, scientists had accepted that the differences between the floras and faunas represented differences in the regions' respective geological histories; it is now known that India was separated from Asia until about 30 million years ago, and the boundary between India and Asia, demarcated in places by the Himalayas, represents a collisional zone between what were once separate continental blocks.

Wallace's 1869 book The Malay Archipelago is a popular account of the differences between the animals and plants typically found on the Asian and Australian continents. The narrow transition zone between those biotas is found in the Malay Archipelago, and the geographic boundary between the biotas is referred to as "Wallace's Line"—though the precise position of the line across the archipelago has been debated.

The study of phylogenetic biogeography is complicated because different species do not always speciate at the same time when barriers are formed. Furthermore, barriers that determine the ranges of some species by preventing their movement may not affect other species. Finally, some species may be able to extend their ranges greatly through infrequent, long-range dispersal events, such that the range they occupy and their evolution are not determined simply by the history of the establishment of geographic barriers in the regions in which they occur. Thus biogeography of regional faunas is related to geological history, but it is also related to the unique ecological features of organisms.

The formation of geographic barriers, because it encourages speciation, can play an important role in regulating diversity. For example, plate tectonics can act either to separate continents and their respective faunas or to join them. During the last 500 million years of the history of life, global diversity was higher when the continents were well separated than when they were together.

The association between evolution and biogeography is also shown by the role that biogeography played in the development of ideas about evolution. Charles Darwin, in his 1839 book The Voyage of the HMS Beagle, described how different islands in the Galapagos had unique species that were very similar to the species distributed on other islands in the chain. One of his foremost examples involved mockingbirds—not, as commonly but mistakenly believed, finches. He later used this point in his (1859) Origin of Species to show how the separation of these islands and the establishment of oceanic barriers between them might have driven the evolution of the terrestrial elements of these island faunas. Alfred Russell Wallace, another important figure in the history of evolutionary biology, also discussed, in an article published in 1855, how the emergence of geographic barriers that separated formerly contiguous populations of a species could lead to evolution and speciation.

Just as biogeography relates to ecology and evolution, it figures in our understanding of the biodiversity crisis. Furthermore, biogeographic information relates in an important way to conserving diversity because it involves identifying where different species are concentrated and understanding the mechanisms that are leading to extinctions. The biodiversity crisis is largely engendered by humans' activities that relate to two biogeographic factors: habitat destruction and alien species. In addition, the current biodiversity crisis can at least be partly understood as a manifestation of those biogeographic factors that eliminate or generate biological diversity. These are established from study of both the modern and fossil biota. One of the primary factors tied up with biogeography that contributes to the current biodiversity crisis—and also led to mass extinctions in the past—is habitat destruction. Habitat destruction causes extinctions for several reasons, including the fact that there is a well-established relationship between the area that a species occupies and its likelihood of future extinction: as the area shrinks, species become more likely to go extinct.

Alien species are also contributing to the biodiversity crisis. These species come to occupy new, larger ranges because of accidental or purposeful introduction by humans. When they enter new regions they often no longer have to cope with predators, and they can expand unchecked, outcompeting or simply overgrowing native species. Such invaders can change the ecology of a region very rapidly, thus altering the selective environment in which organisms evolved, potentially making once adaptive traits of organisms no longer adaptive in the face of changing competitors. The typical biogeographic history of most groups did not involve novel combinations of species coming into contact with one another. Instead, the respective biotas of different regions came into contact only rarely, and in events such as plate tectonic collisions between formerly isolated regions. These ancient plate tectonic events often had prodigious biotic consequences. For example, the Great American Interchange is an event that occurred 3 million years ago, when North America and South America collided at the Isthmus of Panama, and the mammals of these continents, which formerly had been separated, were mixed. After the collision extinction rates climbed and speciation rates fell in both continents, but especially in South America.

The modern South American mammal fauna is relatively impoverished, at least compared with its state 3 million years ago, while being relatively enriched in mammals of North American origin. Therefore the biogeographic nature of the current biodiversity crisis is an acceleration of events that formerly occurred rarely and over much longer time scales. If anything, human activities leading to the introduction of alien species equal or exceed the extent to which biotas have been mixed by plate tectonics at any time in the past.

—Bruce S. Lieberman

See also: Alien Species; Ecological Niches; Ecology; Ecosystems; Evolution

Bibliography

Brooks, Dan R., and Deborah A. McLennan. 1991. Phylogeny, Ecology, and Behavior: A Research Program in Comparative Biology. Chicago: University of Chicago Press; Brown, James H., and Mark V. Lomolino. 1998. Biogeography, 2d ed. Sunderland, MA: Sinauer; Hallam, Anthony. 1994. An Outline of Phanerozoic Biogeography. Vol. 10. Oxford: Oxford University Press; Lieberman, Bruce S. 2000. Paleo-biogeography: Using Fossils to Study Global Change, Plate Tectonics, and Evolution. New York: Plenum/Kluwer; McKerrow, William S., and Christopher R. Scotese. 1990. Palaeozoic Palaeogeography and Biogeography. Geological Society (London) Memoir 2. London: Geological Society of London; Vermeij, Geerat. 1978. Biogeography and Adaptation. Cambridge: Harvard University Press.

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