The study of the geographic distribution of living things is called biogeography. Biogeographers use observations from ecology, evolutionary biology, geology, paleontology, and climatology to examine distribution patterns of organisms at local, regional, and global levels. These studies produce insight into past ecological and geological events that in turn affect present-day biodiversity—the number of species of living things relative to a given area. Understanding how and why organisms live where they do allows scientists and conservationists to better manage and protect preserves and wilderness areas. Although the major patterns of distribution and broad principles of biogeography have been established, many specific questions remain to be answered. These questions relate primarily to the unique distribution of certain organisms. For example, hundreds of species of lungless salamanders are widespread in the Western Hemisphere, but only two species are found in the Eastern Hemisphere—in Italy and France. How do such differences in populations occur? Biogeographers also ask questions about environmental factors such as climate, and physical features such as mountain ranges, that affect the movement of organisms and the colonization of a region. Biogeography also looks at how human activities have influenced distribution patterns by causing the rapid extinction of some species and the introduction of others to regions where they never occurred before. Historical BackgroundThe earliest explorers were aware of broad global patterns of animal and plant distribution as they discovered new lands and found unknown varieties of plants and animals. Charles Darwin recognized and reported on the significance of geographic distribution patterns. In his classic book On the Origin of the Species (1859), Darwin devoted two chapters to plant and animal distribution. He noted that each continent or region had representative and distinctive species, yet each species possessed some qualities and characteristics found in other species in similar but distant environments. Darwin attributed these patterns to several factors. He presumed that the common ancestors of these species were extinct, and further noted that changes in climate had probably occurred over geologic time. He hypothesized that the extinction of species in the central part of a geographic range would isolate separate populations of the same species at either end of the range, where conditions might be different. Upon all of these ideas Darwin imposed his theory of natural selection, a process that operates on variation within a species: organisms that have traits allowing them to adapt to changes in their surroundings will thrive and reproduce. Those that cannot adapt will die or emigrate to other areas. Over time, this leads to the evolution of a new species. (See also evolution.) Alfred Russel Wallace, a contemporary of Darwin, recognized the dynamic aspects of biogeographical patterns. In his book The Malay Archipelago (1869) Wallace discussed the animals of that region. He recognized the fauna (animal life) of Celebes, New Guinea, and Australia as distinctive and separate from the fauna of Borneo, the Sunda Islands, and the Malay Archipelago. The boundary line, from the Philippines southward, that separates the two faunal units is called Wallace's Line by modern biogeographers. Wallace's Geographical Distribution of Animals (1876) was the first thorough presentation of the present distribution of animals throughout the world. During the 20th century several schemes of biogeographical distribution patterns were developed, and refinements were made on the observations of regional patterns. Although there is increased detailed knowledge of the global distribution of many species, the general ideas and hypotheses of Darwin and Wallace have withstood more than a century of scientific scrutiny and still apply to modern biogeography. (See also Darwin, Charles.) Continental DriftA key to the modern distribution of a species is the site of its ancestral origin. The origin of many organisms was a puzzle until the significance of continental drift—the separation of major landmasses over geologic time—to biogeography became apparent. PangaeaThere are numerous theories on the precise timing and order of continental drift. However, its history with respect to living things can be traced back to the breakup of a single landmass, Pangaea, over some 200 million years. The major nonoceanic habitat in the early Triassic period (roughly 248 million years ago), Pangaea was a compact landmass composed of all of today's continents. Over the next 200 million years, the landmass would fragment and its components would drift slowly apart, ultimately to the positions they occupy today. Laurasia and GondwanalandBy the beginning of the Jurassic period, about 208 million years ago, Pangaea had separated into northern (Laurasia) and southern (Gondwanaland) landmasses. Laurasia comprised what later became North America, Europe, and Asia. Gondwanaland consisted of all of the other land. Antarctica, India, and Australia began to shift away from Gondwanaland in the middle of the Jurassic period (about 135 million years ago). By the end of the Cretaceous period (about 65 million years ago) Gondwanaland separated into South America and Africa. Madagascar drifted free of the latter. Australia and Antarctica remained connected, and North America and Eurasia were still joined at the site of the present Bering Strait. The Drift Toward Modern ContinentsDuring the early Tertiary period (approximately 65–25 million years ago) the continental landmasses drifted to their present locations. South America ultimately became connected by Central America to North America. India migrated northward and crashed into Asia, an event that formed the Himalayas (see mountain). The Antarctica-Australia landmass split apart: Antarctica moved toward the South Pole, and Australia moved toward the Equator. (See also plate tectonics.) The Effect on Species DistributionUnderstanding the patterns of continental drift has greatly increased our understanding of modern distribution patterns of living things. Although the geologic changes occurred over a 200-million-year time frame, some organisms show little evidence of evolutionary change. For example, side-necked turtles (suborder Pleurodira) have changed little since the Jurassic period; scientists presume they dispersed throughout Gondwanaland before it broke apart. Today the turtles are found in parts of South America, Africa, Madagascar, and Australia—but in no other parts of the world. Groups such as the mammals evolved more rapidly in the last 100 million years. Because of this, land areas isolated from each other through the separation of continents may have widely different forms of mammals. For example, the marsupials of Australia are very distinct from mammals of other continents (except for a few Western Hemisphere representatives such as the opossum, Didelphis marsupialis). This is because the Australian marsupials evolved after Australia separated from the other continents. Climatic Life ZonesAlthough the present distribution of organisms depends on their ancestral distribution and evolution over geologic time, certain environmental factors have also played a role. The most apparent general environmental factor affecting the distribution of a species is climate. Suitable conditions of temperature and moisture are vital to all organisms. Certain species are intolerant of cold; other species cannot live in warm climates. Some have evolved to withstand extreme drought; others are adapted to excessively wet conditions. Temperature, rainfall, and other environmental factors such as soil type and day length, determine if a species can survive in a region. The major recognizable life zones of the continents are called biomes. Because vegetation is usually the dominant and most apparent feature of the landscape, a biome is characterized by its plant community. Biomes represent the large-scale general patterns of species distribution and do not include regional variations. Six major biomes are generally recognized: tundra, taiga, temperate deciduous forest, tropical rainforest, grassland and savanna, and desert. Most of these are subdivided in several ways as a result of regional variability. Each is characterized by organisms that are adapted to the climatic conditions and environmental nature of the biome. TundraThe coldest of the six major biomes, and one of the driest, tundra covers much of the land between the North Pole and the taiga forest, in some places as far south as latitude 60° N. Life abounds in the tundra during the warmer months, but only those species specially adapted to the long frozen winters can survive year-round. A layer of permanently frozen soil, called permafrost, lies under the rocky topsoil. The number of species is low—lichens, grasses, and mosses are the dominant plants; there are no trees, but some flowering plants are abundant during the short, cool summers. During the dark winter months most of the organisms become dormant or migrate to warmer regions. Animals include caribou, wolves, rodents, and, in summer, birds, insects, and fish. Amphibians and reptiles are rare. TaigaThe taiga, or boreal forest, covers a broad region of cold, but not permanently frozen, land south of the tundra, extending in a belt across North America, Europe, and Asia between the latitudes 60° N. and 50° N. Evergreen conifers—needleleafed shrubs and trees such as pine, fir, and spruce—dominate the landscape, though deciduous hardwoods such as birch and aspen are found in some areas. Species diversity is higher than in the tundra but lower than that of warmer biomes. Boreal fauna include bears, moose, wolverines, lynx, rodents, and the snowshoe hare. Migratory birds are plentiful in the summer, when insects also are abundant. Temperate Deciduous ForestIn the milder climates of the Northern Hemisphere, regionally variable rainfall supports the growth of deciduous forests. These forests are prevalent in the temperate regions of eastern North America, western and central Europe, and eastern Asia. Deciduous forests are distinct from the coastal forests of western North America and Chile, where milder temperatures and higher precipitation support different patterns of growth and diversity. Temperate forests are dominated by hardwood broadleafed trees, most of which lose their leaves each winter. Oak, beech, hickory, and maple are prevalent examples. Winters vary from cold to mild, but the growing season is several months long, and rainfall is generally high. Flowering plants are abundant in the spring. Animals include squirrels, rabbits, deer, birds, fox, and in some areas, wolves and bears. Tropical RainforestLush with lianas (vines), broadleafed trees, and abundant animal life, tropical rainforests are situated in the wet, warm areas near the Equator. Rainforests occupy parts of northern South America and of Central America, and much of equatorial Africa, southeastern Asia, the East Indies, and northeastern Australia. Species numbers and interactions among species in tropical rainforests surpass those of any other terrestrial environment. Many species found in rainforests are not found anyplace else on Earth. Among these are mammals such as gorillas, chimpanzees, and most monkeys; birds such as parrots, cockatoos, and macaws; countless species of insects; and trees such as mahogany and teak. Sadly, large tracts of rainforest are being lost as land is cleared for human activities such as logging and agriculture. (See also rainforest.) Grassland and SavannaIn temperate or tropical regions where precipitation is sparse or erratic, grasses are the dominant plants. Trees and shrubs are rare or absent in temperate grasslands, which cover large tracts of central North America, central and eastern Asia, eastern Europe, southern Africa, and northern and southern South America. Tropical grasslands, or savannas, have scattered trees. Savannas are found in northern Australia, southern India and Asia, and more than half of sub-Saharan Africa. Both savannas and grasslands exist as vast plains whose flatness is broken by occasional hills and dales that accommodate a small number of vascular plants. A wide range of animal species inhabit these biomes, from the lions and gazelles of the African Serengeti Plains, to the coyotes, skunks, badgers, and songbirds of the North American prairies. DesertMany people associate deserts with intense heat, but these biomes actually are characterized by their dryness: any area where annual rainfall is less than 20 inches (50 centimeters) is classified as a desert. Deserts are found on every continent. (See also desert). Hot deserts are found near the latitudes 30° N. and 30° S. The Sahara in northern Africa is the world's largest hot desert. Other hot deserts include the Kalahari in southern Africa and those in the southwestern United States and in the outback of Australia. Plants and animals living in hot deserts are specially adapted to the harsh environment. Plants such as cacti (Cactaceae) store moisture in their stems and have needlelike leaves that limit transpiration (See plant). The kidneys of some desert mammals, such as rodents, are uniquely adapted to retaining water. Many animals are active at night, thereby avoiding the daytime heat. Reptiles are abundant, as are coyote, fox, owls, vultures, and hawks. Cold deserts lie at higher latitudes. Aside from the polar desert that covers much of Antarctica, the largest cold deserts are the Taklimakan in western Xinjiang Uygur Autonomous Region and the Gobi in Mongolia. The Great Basin Desert in the western United States and the Atacama Desert of Chile also are cold deserts. Vegetation is limited and scattered. In the Great Basin, sagebrush is the dominant vegetation, though there are scattered clumps of juniper and piñon pine. Wild rose and elderberry are among the few flowering plants. Animals include jack rabbits and burrowing animals such as rodents, fox, lizards, and coyotes. Factors Affecting DistributionPrehistoric distribution, evolutionary history, and climate are prevalent factors influencing modern distribution patterns of plants and animals. A variety of other factors also affect regional distribution of species. These factors differ greatly in their impact, depending upon the timing of events and the biology of the species involved. The following are some of the factors considered by biogeographers. Environmental BarriersEnvironmental barriers can influence the distribution of species by making a region inaccessible to a particular group. For instance, a high mountain range may prevent migration of ground-dwelling species between two areas. Open ocean is an obvious and significant environmental barrier between landmasses. Biomes themselves can even serve as environmental barriers, one of the most famous being the Sahara, which ecologically separates central from northern Africa. Biogeographers use ecology, geology, and evolutionary biology to determine the impact of environmental barriers on species distribution. Dispersal MechanismsThe study of dispersal patterns involves examining the structure, function, and behavior of animals. The degree of mobility of organisms and the methods for dispersal of eggs or seeds are important in understanding how species overcome environmental barriers and colonize new areas. Plant seeds that are carried by wind, water, or animals can disperse over great distances. For instance, coconut palms are found on most tropical islands because their seeds—coconuts—can float across long distances. Many birds and mammals can migrate over long distances to establish new populations. Evolution and SpeciationAs populations of plants and animals adapt to new environmental circumstances over geologic time, they may evolve into new species that are considerably different from their ancestors. Information from the fossil record and from affinities between extant (living) species helps biogeographers speculate about where the latter or their ancestors lived in the past. Comparison of DNA sequences from separate populations of extant species has become a vital tool for calculating when these species may have evolved. Invasion and CompetitionCompetition between species for the same food sources or for the same habitats is significant in the distribution of organisms. Although competition is a complex biological phenomenon, many biologists consider it a key factor in modern biogeographical patterns. A classic example is the case of many South American marsupial mammals that became extinct between roughly 24 and 15 million years ago. Biologists believe that the South American mammals could not compete for resources with North American placental species, which invaded South America via the Central American land bridge and gradually replaced the southern marsupials. (See also mammal.) Human InfluenceToday's distribution patterns are in great part a product of human intervention, whether intentional or accidental. House sparrows and starlings, originally species of Europe, have become a dominant part of the bird population of the eastern United States. The European hare, introduced into Australia, has successfully colonized many areas of the continent. The brown tree snake (Boiga irregularis), a native of Papua New Guinea, rendered most of the flightless birds of Guam extinct after being transported accidentally to the island in a ship's cargo. The South American water hyacinth and the Asian kudzu vine are considered pests in the southeastern United States because of their invasion of the region following human introduction. These instances represent some of the thousands of human introductions of plants and animals to new regions. Although most species introduced from one continent to another do not fare well and eventually become extinct without human care, some are extremely successful and may replace native forms to the extent of becoming unwanted pests. Besides the transcontinental introduction of species, transplants to different regions on the same continent are also common. For example, rainbow trout, native to rivers along the northwestern coast of North America, have been successfully established in cold-water streams of the central and eastern United States. Because of its commercial importance since ancient times, the date palm has been distributed throughout North Africa and the Middle East to places, including isolated oases, where it might never have reached without human assistance. The extinction of many species has resulted from human activities as well. Over the last several hundred years humans have eliminated countless species—some, such as the dodo bird and the passenger pigeon, through hunting and others, such as the Carolina parakeet, by destroying natural habitat. Hundreds of other species including the Chinese alligator, the mountain gorilla, the Indian, Sumatran, and Javan rhinoceros, and the California condor live at the edge of extinction. Although extinction and the gradual replacement of species by others is a natural phenomenon, the rapid and global impact of human activity has greatly accelerated the process; thus evolutionary replacement has not had time to occur. Many changes incurred by the introduction of species to new areas and the relentless extinction of others are irrevocable. (See also endangered species.) Island and Marine BiogeographyBecause they are small geographic units with distinct boundaries, islands serve as useful models to illustrate the mechanisms of biogeographical phenomena. Intensive ecological studies on islands have provided key insights into invasion and colonization patterns, dispersal mechanisms, and extinction rates. Ecologists have documented that the amount of usable habitat on an island determines how many species can be supported. Mathematical formulas show that islands reach an equilibrium, or balance, when the rate at which immigrating species integrate into the island's communities equals the extinction rate of resident species. These findings have vital implications for conservation and management of actual islands, as well as mainland “islands”—areas of habitat that are isolated from each other by geographic barriers such as highways or shopping malls built in formerly intact habitat. Such construction presents a tremendous barrier to species such as turtles. (See also population biology.) Ocean environments have not received the level of attention that terrestrial habitats have from a biogeographical point of view. Environmental barriers in marine systems are seldom as abrupt as in terrestrial systems, so that distinct groups of organisms are not as recognizable. Furthermore, the world's oceans are continuous, so migration between areas and mixing of species are likelier. Because of these differences, isolation of major groups from each other is less likely to occur in marine habitats. |