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Biogeography

Biogeography

Why do different species occur in the places they do? Biogeography is the study of why animal species (and also plants) live in different regions on Earth. This includes both organisms alive today as well as those that have become extinct. Any particular animal species is found where it is because that species either evolved and originated there or came there from some other place. The two divisions of biogeography reflect these two ways that animals come to occupy an area. Biogeography can be broken down into historical biogeography, which studies the past history and evolution of a species, and ecological biogeography, which studies the environment of a species.

Ecological Biogeography

Ecological biogeography studies how animal species are distributed in relation to the environment. The environment that influences what animals are present in a region includes both nonliving, abiotic factors (such as climate or soil composition) as well as living, biotic factors (such as other plants and animals). Earth is divided into major ecological areas called biomes . Biomes are regions of distinct climate and plant life. There are several kinds of biomes. Examples include the dry, hot desert in which cactuses and other plants are adapted to low water conditions, and the tropical evergreen forest with heavy year-round rainfall and lush plant life.

Dispersal occurs when an animal moves away from the area in which it was born and lives in another area. Dispersal increases the biogeographic range of a species, spreading the population. However, the extent to which an animal can disperse may be limited by ecological factors. Animals that disperse into areas for which they are not adapted will not survive. For example, alligators cannot disperse into central North America because it is too cold during the winter. These ecological limits to dispersal help determine the range of an animal species.

Historical Biogeography

Historical biogeography is the study of how animals that are present in a geographical region today relate to the animals that lived there in the past. A major factor explaining why a species is present in a region today is the presence of the same species in the past, or the presence of a closely related species that once lived there and from which the current species has descended. That is to say, a species is located somewhere because it was there in the past, or because an ancestor of the species lived there.

Continental drift is a major factor in determining current species distributions. All the continents on Earth were once part of one single land mass called Pangaea. About 200 million years ago, this landmass began to drift apart to form the continents of today. There are correspondingly six major biogeographic regions. They are the Neararctic, covering North America; the Neotropical, covering South America; the Ethiopian, covering Africa; the Oriental, covering India and southeastern Asia; the Palearctic, covering Europe and northern Asia; and the Australian, covering Australia.

Each of these regions has a group of animals that are more closely related to each other than to animals in other biogeographic regions. This is because of local diversification by speciation (the forming of new species) and the radiation (spread) of species within a biogeographical region; animals in a region are descendants from the ancestors that were previously there. The same is true for plants. Many animal species that are closely related stay in the same biogeographical region because it is hard to disperse or move between these regions. These regions are isolated from one another by an ocean or a very large mountain range, or are connected by only a narrow landmass (an isthmus ). This isolation serves as a barrier to dispersal; most animals simply can not swim across the ocean to colonize another continent. Likewise, most animals that live in the Pacific Ocean cannot cross the land bridge that joins North and South America to reach the Atlantic Ocean, and vice versa.

Sometimes a population of animals is split into two populations by the sudden appearance of a physical barrier across which no individual can disperse; this is called a vicariant event. These two populations can become separate species over time because of isolation. An example of a natural vicariant event is an earthquake making a new canyon that is too wide for mice on either side to disperse across. Humans create obstacles that can also cause vicariance, such as highways that would stop mice from dispersing.

Humans can help promote dispersal. As technology has increased worldwide travel and transportation in the nineteenth and twentieth centuries, some animals have been able to disperse into new biogeographic regions on boats, trucks, or planes. How all the organisms in one place interact with each other and their environment is called the community ecology of an area. Biogeographic regions strongly determine the community ecology of an area. As a consequence, species that successfully disperse to new biogeographic areas can cause huge ecological impacts. For example, the brown tree snake began invading Pacific islands late in the twentieth century. The local animals, especially birds, are easy prey for brown tree snakes because they have not adapted to snake predators. The snakes can quickly wipe out the bird populations that can not adapt fast enough.

see also Living Fossils.

Laura A. Higgins

Bibliography

Brown, James H., and Mark V. Lomolino. Biogeography, 2nd ed. Sunderland, MA: Sinauer Associates, Inc., 1998.

Purves, William K. et al. Life: The Science of Biology, 5th ed. Sunderland, MA: Sinauer Associates, Inc., 1998.

An example of a species that created an ecological impact through its migration is the brown tree snake. This species began invading Pacific islands late in the twentieth century by stowing away on shipping boats. As no snakes were native to the islands, the local animals, especially birds, became easy prey for them, and the native bird populations on the islands were negatively affected.

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Biogeography

Biogeography

An enormous variety of species live in the thin layer on Earth's surface that makes up the biosphere. None of these species is found everywhere on Earth's surface. Instead, the number and kinds of species change dramatically as one moves from one place to the next. The science that studies the past and present distribution patterns of organisms and seeks to understand the mechanisms that underlie these patterns is called biogeography.

Biogeographers explain the distributions of species using four basic principles regarding the nature of Earth and the organisms that live on it:

  1. Environmental variability: For a variety of reasons, the conditions that organisms experience change dramatically across Earth's surface. Climate and elevation are two major influences.
  2. Ecological limitation: Every organism has a limited range of conditions that must be met in order to allow it to live and reproduce. Since a species is a population of reproductively compatible organisms that have similar biological properties, no species can be found everywhere.
  3. Continental drift: The locations of landmasses across Earth's surface have not remained the same, but have changed slowly over the course of Earth's history. Therefore, the conditions experienced by organisms change over long periods of Earth's history.
  4. Evolutionary change: Species do not stay the same over time, but are in a constant state of change as individuals best able to survive and reproduce within certain environments become more frequent, while others less capable die or fail to produce offspring. The ability of a species to evolve allows it to persist over long periods of time and track the changes occurring on Earth's surface.

The first two principles indicate that the current geographic distribution of a species is determined by how its ecological limitations are related to the environmental conditions it encounters. Species with similar requirements will be found together in the same locations. Regions on continents or in oceans where the species share similar ecological limitations are called biomes. For example, deserts are biomes where the species are all able to withstand relatively hot, dry climates.

The third and fourth principles indicate that as continents move about across the face of Earth, they carry with them the species that inhabit them. When continents that were once connected separate, populations are fragmented, and subsequent evolutionary changes in related species will occur independently. The timing of such independent evolutionary changes provides clues about the timing of Earth's history. Much of the history of continental drift, for example, can be reconstructed by examining the geographical distribution of fossils and of related groups of living species.

All four principles suggest that as the conditions on Earth change over long periods of time, each species will respond to these changes in one of three distinct ways. First, a species may change its geographic distribution to track changes in the location of its favored set of ecological conditions. For instance, during ice ages, many species moved southward. Second, a species may undergo evolutionary change to adapt to changing conditions. Third, if a species cannot shift its geographic range or undergo evolutionary change, the species will go extinct. Over the history of Earth, no species has been able to persist unchanged as the biosphere has changed.

see also Biodiversity; Biome; Evolution

Brian Maurer

Bibliography

Brown, J. H., and M. V. Lomolino. Biogeography. Sunderland, MA: Sinauer Associates, Inc., 1998.

Cox, B. C., and P. D. Moore. Biogeography: An Ecological and Evolutionary Approach. Boston: Blackwell Scientific Publications, 1985.

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Biogeography

Biogeography

Biogeography is the study of the patterns of distribution of the world's living organisms. It tries to determine where plants and animals occur, why they occur where they do, and when and how the patterns developed. Bio-geographic patterns are largely determined by climate, geology, soil conditions, and historical events. Individual plant species are generally restricted to particular habitats, but many plants have widely overlapping ecological requirements so that many different kinds grow together in communities.

Impact of Climate

Rainfall has a significant impact on the distribution of plant types. Savannas, steppes, and prairies occur where rainfall patterns result in long, dry periods at certain times of the year. During the dry season, fires often sweep through these areas. Woody plants, with buds for future vegetative growth borne above ground, are killed by the flames. Grasses and other herbaceous plants, whose reproductive buds are produced on underground shoots, and, therefore, protected from fires survive and thrive. Where annual rainfall is greater and more uniform throughout the year, fires are less frequent and woodlands develop. In contrast, deserts develop where rainfall is severely limited.

Vegetation is also influenced by temperature and length of growing season. In the Arctic, where the ground is frozen for several months of the year and the growing season is measured in weeks, only a relatively few, specialized species of dwarf plants are able to grow. Diversity under such conditions is considerably less than in the tropics, where annual temperatures and rainfall often remain favorable, and the growing season extends throughout the year. Trees in the tropics can grow to a large size and provide further habitats for epiphytic plants and animals among their branches in the forest canopy.

Other Factors

While climate is a major force in determining the patterns of biogeography, other factors are also important, including the physiological requirements and tolerances of individual species. Although many plants overlap in their ecological tolerances, they all vary from each other. Individual species of plants rarely occur continuously in the landscape to the exclusion of all others. For instance, the red maple (Acer rubrum ) in eastern North America is a plant of acid soils and commonly grows with other wetland species in lowlands from eastern Canada to Florida and from the Atlantic to the Mississippi, but it can also grow on dry ridges and hilltops with a different association of plants within the same geographic region. When we examine the distribution of red maple carefully, we also see that the individual plants are not continuous, but occur only where growing conditions are favorable. Some individuals may occur next to each other, but others live some distance away. The individuals within a reasonably close distance to each other, and which are capable of interbreeding, are called populations. Populations, just like individuals, may occur next to each other or be widely separated. Populations occurring far from the main range of distribution of a species are generally referred to as disjunct populations, or simply as disjuncts.

Highly specialized habitats such as bogs, barrens, rock outcrops, and vernal pools, which themselves occur in a scattered fashion across the landscape, are frequently home to disjunct species that are especially adapted to those particular ecological conditions. These habitats can be further divided by soil types. Barrens may occur over serpentine, limestone, sandstone, granite, and other less common types of rocks, and each supports a different group of plants particularly adapted to that specific habitat. One particular plant that has a wide distribution in North America from the southeastern United States to eastern and central Canada is the pitcher plant (Sarracenia purpurea ). When the populations are plotted on a map the species appears to have a continuous distribution throughout its range, but in reality, individual populations occur only in scattered, highly acidic, boggy situations.

Intercontinental Disjunctions

Looking at the distribution of plants today, we see it only as a single slice of time. Studying historical data, we find a very different picture of the position of continents and the distribution of plants and animals. One of the most challenging problems faced by biogeographers is to explain intercontinental disjunctions, in which closely related plants are found on opposite sides of the world from each other and separated by major oceans. One intercontinental disjunction that has attracted particular attention is the one between eastern Asia and eastern North America. About seventy-five genera of plants are restricted to these two areas and occur nowhere else in the world. These plants have no or few close relatives in their respective regions, and there is no confusion over their close relationship to their disjunct sister taxa . Swedish botanist Carl Linnaeus first noticed that closely related plants grew in these two areas in 1750, but it was not until Asa Gray published a series of papers between 1840 and 1860 that this disjunction was brought into prominence. In fact, Gray's series of papers, which were written in response to requests by Charles Darwin for statistics on the North American flora, are often considered to be the seminal papers in the field of biogeography.

The genera belonging to this pattern often occur in what are considered to be ancient lineages , and include Magnoliaceae, Berberidaceae, Schisandraceae, Illiciaceae, Hamamelidaceae, and Saururaceae, but some more modern groups such as Rubiaceae and Asteraceae also contain a few genera with close relatives on opposite sides of the world and nowhere else. Gray tried to explain this pattern by proposing migrations across a Bering land bridge connecting the Asian and American continents during periods when sea levels were lower and corridors for migration were available in the center of North America. This was a simple and plausible explanation, but in reality the origin of this pattern of distribution has proven to be much more complex.

Later, the German botanist Adolf Engler (1844-1930), in writing about the vegetational history of Earth, made use of rapidly accumulating fossil evidence, particularly from the Arctic, to show that the forests in which these disjunct plants now occurred had once been more widespread and continuous at higher latitudes in the northern hemisphere: they essentially circled the globe in a zone where boreal forests now exist. Engler believed that deteriorating climates and the uplifting of mountains worldwide, and increasing aridity in the western part of North America, led to the extinction of this vegetation type in large portions of the world during the latter portion of the Tertiary period . (This group of plants is still frequently referred to as the Arcto-Tertiary geoflora .) It was also believed that the Pleistocene glaciations of the last two million years further contributed to the extinction of many of the plants in North America and Europe. It has been postulated that the major east-west mountain ranges in Europethe Pyrenees, Alps, Carpathians, and Balkanswould have blocked the migration of plants in front of the southwardly moving ice sheet, thereby resulting in extinction. In contrast, north-south ranges, such as the Appalachians, allowed southerly migration and survival.

Many of the plants and animals restricted to eastern Asia and eastern North America today are known from fossils in Europe and western Asia, and geological evidence indicates that the Bering land bridge was not the only route available for migration between Eurasia and America. Until about forty-nine million years ago and perhaps as recently as about thirty-seven million years ago, North America, Greenland, Iceland, and Europe existed close enough to each other to allow direct migration of plants and animals across the North Atlantic. At that same time, the connection across the Bering Straight was also at a higher latitude than it is today, and climate may have been a controlling factor in plant and animal migrations. It is interesting to note that many genera of plants Magnolia, Liriodendron, Juglans, Sassafras, Acer, and so forththat now occur primarily or have their greatest diversity in eastern and southeast Asia and eastern North America are known from the Miocene of Iceland. According to Malcolm McKenna in the Annals of the Missouri Botanical Garden (1983), the closest relatives of Iceland's plants at that time were in North America. Since the end of the Pleistocene glaciations, the composition of Iceland's flora has become more European in character.

Island Biogeography

Islands require special examination. In a sense, islands are like isolated laboratories where long-term experiments in adaptation and evolution are taking place. Many factors have to be considered to understand the origin and development of an island's biota . Such factors include size and elevation of the island, latitude, distance from nearest landmass, age of the island and how long it has remained above sea level, past connections to mainlands, source of migrants, frequency of arrival of new colonists, wind direction, and rainfall patterns. Extinctions and recolonizations, too, have to be analyzed to understand the biological patterns present on islands.

Hawaii and the Galapagos Islands are classic examples where processes of island biogeography have been studied. Hawaii has never been connected to another landmass but instead sits over one of Earth's geological hot spots. As the Pacific plate moves to the west-northwest in conveyor belt fashion, new islands are created as magma flows up through Earth's crust to form volcanoes that eventually reach far above sea level. Activity of the hot spot is apparently intermittent, since the volcanoes are separated by gaps of varying sizes. As the islands move away from the hot spot, they are gradually eroded by the elements and eventually consumed as the Pacific plate dives under the Asian continent. This process has been going for at least seventy million years. Since the islands were barren at their creation, the plants and animals on the Hawaiian islands must have originally come from elsewhere. The nearest major landmasses to Hawaii, and the most likely sources of plant and animal colonists, are more than 4,000 kilometers away.

The first colonists to reach Hawaii would have encountered a rich diversity of wide-open ecological niches ranging from sea level to the tops of mountains (some of which exceed 4,000 meters). The diversity of unoccupied habitats is thought to have promoted rapid speciation . Because of the great distance from the major sources of colonists, the number of successful colonizations is estimated to be only 270 to 280 species of plants. These have evolved to about 1,000 native species today, although some botanists who place greater emphasis on minor variations consider the number to be much higher. The Hawaiian flora is also considered to be disharmonious, meaning its species distribution differs from that of similar mainland regions. For example, only three native orchids are found on Hawaii, although one would expect many more because of the archipelago's tropical location and wide range of habitats. Conversely, the Campanulaceae (bluebell family) is the most speciose family in the islands, with 110 species of native plants. In other regions of the tropics, the family is an insignificant portion of the flora.

see also Biodiversity; Evolution of Plants; Gray, Asa; Orchidaceae.

David E. Boufford

Bibliography

Carlquist, S. Island Biology. New York: Columbia University Press, 1974.

Cox, C. B., and P. D. Moore. Biogeography: An Ecological and Evolutionary Approach. New York: John Wiley & Sons, 1973.

Daubenmire, R. Plant Geography: With Special Reference to North America. New York:Academic Press, 1978.

De Laubenfels, D. J. "Botany of Japan and Its Relations to That of Central and Northern Asia, Europe, and North America." Proceedings of the American Academy of Arts and Sciences 4 (1860): 130-35.

. A Geography of Plants and Animals. Dubuque, IA: William C. Brown Co.,1972.

McKenna, M. C. "Holarctic Landmass Rearrangement, Cosmic Events, and Cenezoic Terrestrial Organisms." Annals of the Missouri Botanical Garden 70 (1983): 459-89.

Pears, N. Basic Biogeography. Whitstable, KY: Whitstable Litho Ltd., 1977.

Pielou, E. C. Biogeography. New York: John Wiley & Sons, 1979.

Stott, P. Historical Plant Geography. London: George Allen & Unwin, 1981.

Wagner, W. L., D. R. Herbst, and S. H. Sohmer. Manual of the Flowering Plants of Hawaii, rev. ed. Honolulu, HI: University of Hawaii Press, Bishop Museum Press, 1999.

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biogeographical region

biogeographical region A biological subdivision of the Earth's surface that is delineated on the same general principles as a biogeographical province but has superior taxonomic status. The provinces are grouped into regions, of which the following are generally recognized: Antarctic, Australasian, Ethiopian, Nearctic, Neotropical, Oceanian, Oriental, and Palaearctic. A variant of this grouping has been proposed for mammals (see mammal regions). See also faunal region; floral province; and floristic region.

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biogeographical region

biogeographical region A biological subdivision of the Earth's surface that is delineated on the same general principles as a biogeographical province but has superior taxonomic status. The provinces are grouped into regions, of which the following are generally recognized: Antarctic, Australasian, Ethiopian, Nearctic, Neotropical, Oceanian, Oriental, and Palaearctic. A variant to this grouping has been proposed for mammals (see MAMMAL REGIONS).

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biogeographical region

biogeographical region A biological subdivision of the Earth's surface, delineated on the same general principles as a biogeographical province, but having superior taxonomic status. The provinces are grouped into regions, of which the following are generally recognized: Antarctic, Australasian, Ethiopian, Nearctic, Neotropical, Oceanian, Oriental, and Palaearctic.

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biogeographical province

biogeographical province A biological subdivision of the Earth's surface, on the basis of taxonomic rather than ecological criteria, and embracing both faunal and floral characteristics. The hierarchical status of such a unit, and the total number of such units, varies from one authority to another. See faunal region; floral province; and floristic region.

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biogeographical barrier

biogeographical barrier The various disjunctive geographical groupings of plants and animals are usually delimited by one or more barriers to migration, which prevent faunal and/or floral mixing. Such barriers may be climatic, involving temperature and the availability of water, or physical, involving for example mountain ranges or expanses of sea water.

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biogeographical barrier

biogeographical barrier A barrier that prevents the migration of species. The various disjunctive geographical groupings of plants and animals are usually delimited by one or more such barriers which may be climatic, involving temperature and the availability of water, or physical, involving, for example, mountain ranges or expanses of sea water.

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biogeographical barrier

biogeographical barrier A barrier that prevents the migration of species. The various disjunctive geographical groupings of plants and animals are usually delimited by one or more such barriers which may be climatic, involving temperature and the availability of water, or physical, involving e.g. mountain ranges or expanses of sea water.

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biogeography

biogeography The scientific study of the past and present geographical distribution of plants and animals at different taxonomic levels. Modern biogeography also lays great stress on the ecological character of the world vegetation types, and on the evolving relationship between humans and their environment. See also PALAEOBIOGEOGRAPHY.

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biogeography

biogeography A diverse subject, that focused traditionally on the geographical distribution of plants and animals at different taxonomic levels, past and present. Modern biogeography also lays great stress on the ecological character of the world vegetation types, and on the evolving relationship between humans and their environment.

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biogeography

biogeography A diverse subject, traditionally focusing on the distribution of plants and animals at different taxonomic levels, past and present. Modern biogeography, however, also lays great stress on the ecological character of the world vegetation types, and on the evolving relationship between humans and their environment.

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biogeographical province

biogeographical province A biological subdivision of the Earth's surface, usually on the basis of taxonomic rather than ecological criteria, that embraces both faunal and floral characteristics. The hierarchical status of such a unit, and the total number of such units, varies from one authority to another.

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"biogeographical province." A Dictionary of Plant Sciences. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/biogeographical-province-0

biogeographical province

biogeographical province A biological subdivision of the Earth's surface, usually on the basis of taxonomic rather than ecological criteria, and embracing both faunal and floral characteristics. The hierarchical status of such a unit, and the total number of such units, varies from one authority to another.

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"biogeographical province." A Dictionary of Zoology. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

"biogeographical province." A Dictionary of Zoology. . Encyclopedia.com. (August 17, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/biogeographical-province-1

"biogeographical province." A Dictionary of Zoology. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/biogeographical-province-1

biogeography

biogeography The study of the geographical distribution of plants and animals at different taxonomic levels, past and present, the habitats in which they occur, and the ecological relationships involved.

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"biogeography." A Dictionary of Ecology. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

"biogeography." A Dictionary of Ecology. . Encyclopedia.com. (August 17, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/biogeography-0

"biogeography." A Dictionary of Ecology. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/biogeography-0

biogeography

biogeography The branch of biology that deals with the geographical distribution of plants and animals. See plant geography; zoogeography.

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"biogeography." A Dictionary of Biology. . Encyclopedia.com. 17 Aug. 2017 <http://www.encyclopedia.com>.

"biogeography." A Dictionary of Biology. . Encyclopedia.com. (August 17, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/biogeography-3

"biogeography." A Dictionary of Biology. . Retrieved August 17, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/biogeography-3