Biogeography

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 Europe—the Pyrenees, Alps, Carpathians, and Balkans—would 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 forth—that 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.