Biogeography

Biogeography is an interdisciplinary science that applies concepts and methodologies from biology, geography, systems ecology, evolutionary theory, conservation, statistics, and paleontology, among other fields, to study the features of the physical environment that affect the distribution of living organisms across space and time.

Distribution Areas and Species Distributions

The fundamental question that drives biogeographical study is: How is life distributed over the earth? This is a question that can be answered at many different levels. For example, any given taxon—a group of living organisms at a particular classification level, such as a species, genus, family, or class—has a geographic range within which it is likely to be found. The plant Yucca brevifolia, a species of yucca also known as the Joshua tree, is only found growing in the dry desert soils, open plains, and mesas of the southwestern United States. This is what biogeographers refer to as the tree's distribution area. A species like Yucca brevifolia is said to have an endemic distribution area that is restricted to a particular region of the world. In contrast, Drosophila melanogaster (the common fruit fly), found in virtually all types of habitats across almost the globe, is said to have a cosmopolitan distribution area.

Many taxons have what are known as disjunct distribution areas. This occurs when very closely related species are found in widely geographically separated habitats. For example, many similar species of mosses, small mammals, and butterflies are found in the mountain ranges of the far northerly Arctic and in the more southerly mountain ranges of Europe, such as the Alps—but nowhere in between. There are several scientific hypotheses regarding historical reasons for the existence of disjunct biogeographic distribution areas.

Under the vicariance hypothesis, environmental events cause the range of a particular taxon to split into parts. For example, changes in land distribution can cause impassable fractures to form in what was once a continuous habitat. Plate tectonic shifts, continental drift, and the formation of mountains all have the potential to break apart a formerly intact habitat region. When this happens, allopatric speciation can occur. This is the evolution of new and distinct species of animals or plants due to a single species being separated into more than one non-overlapping distribution area. (Another form of vicariance occurs when individual members of a family that was once spread out over a large, continuous geographic range go extinct. This, too, can cause fragmentation in the distribution area.) In contrast, some disjunct biogeographic distributions are believed to have been caused by dispersal or the successful migration of species across an existing geographic barrier. For example, birds can cross oceans to colonize new habitats; other non-motile species, such as plants, may be transported with them as seeds.

Besides studying how organisms range globally, biogeographers examine how individual organisms within a species are spatially arranged within the area where they are found. This is a concept known as species distribution. Three major types of species distribution are identified—clumped, random, and uniform. In a clumped species distribution, multiple groups of individuals tend to form. This is a common behavioral pattern found, for instance, in herding animals like deer or cattle. For such species, a clumped distribution is beneficial because it offers protection from predators. In a random species distribution, individuals are found in scattered locations with no particular organization or method—such as wildflowers whose seeds have been dispersed by the wind over a field and grow wherever they fall. In a uniform species distribution, individuals are spread out at a roughly even distance from each other. Many desert plants, for instance, are found in a uniform distribution because this pattern enables each of them to absorb the most water from the arid environment.

Ecogeographic Rules

When biogeographic principles are used to look at the distribution of living organisms on a macroscopic scale, many striking principles emerge regarding the relative degree of biodiversity found in different regions of the earth. One significant “ecogeographic rule” is the latitudinal gradient in species diversity or richness. This term describes a striking yet not fully understood pattern in global biodiversity—the tremendous increase in the number of unique species that can be found when traveling from high latitudes (colder regions) to low latitudes (warmer regions). In other words, the equatorial tropics are home to a far greater diversity of life than the poles. This pattern applies to nearly every possible life form, including birds, mammals, freshwater fish, marine invertebrates, insects, and flowering plants and trees. For example, more than two hundred species of ants are known in Brazil, but Alaska has fewer than ten. The 50,000 square kilometers (approximately 19,305 square miles) of land in the Malaysian peninsular is home to nearly 1,500 species of trees and shrubs; the same area in the southeastern part of Russia holds fewer than fifty different species.

Paleobiogeographic data—data derived from analyzing the relationships between fossilized animals and plants and the locations on the earth where those fossils were found—indicates that species richness latitude gradients are not a recent phenomenon. It has been shown, for instance, that during the Triassic (a period spanning about 245 million to 208 million years ago), ammonoids (a now-extinct group of mollusks) were more diverse at lower latitudes than at higher ones and that the same was true of brachiopods, another group of marine invertebrates, during the Paleozoic ice age (about 570 million to 245 million years ago).

Dozens of competing hypotheses have been put forth regarding the potential mechanism or mechanisms responsible for the existence of species richness latitude gradients. Among them are the time and area hypothesis, the diversification rate hypothesis, and the hypothesis of effective evolutionary time. The time and area hypothesis argues that the tropical regions of the earth are both older and larger than the polar regions, giving them a longer span of evolutionary time in which speciation could have occurred and a greater amount of available habitat—both of which could have resulted in the higher degree of species richness that is observed at lower latitudes. (The time and area hypothesis does not account for certain anomalies, such as why the Southeast Asian sea—which is not as large as other bodies of water in the Indo-Pacific region—has the greatest degree of species richness.) The diversification rate hypothesis argues that speciation occurs faster and more frequently in tropical regions than in polar regions, either because of lower extinction rates or other factors, such as the increased likelihood of achieving reproductive isolation. The hypothesis of effective evolutionary time considers the longer amount of time ecosystems are believed to have existed in the tropics but also gives weight to the effects of temperature on the rate at which evolution can occur. At higher temperatures, such as those associated with lower latitudes, genetic mutation rates, among other factors influencing speciation, are higher. No single hypothesis about the cause of species richness latitude gradients has gained widespread acceptance within the scientific community.

Another important ecogeographic principle related to latitudes is Bergmann's rule. The nineteenth-century biologist Carl Bergmann noted that when the same species (or closely related species) of mammals and other warm-blooded animals were found in different parts of the earth, those that lived closer to the equator were comparatively smaller than those that lived at higher latitudes. For example, polar bears are much more massive than related bear species in the tropics. Bergmann's rule is often explained as a result of the direct relationship between body size and heat. Larger animals generate a greater amount of heat as a result of cell metabolic activities, and they also lose less heat to their surroundings since the ratio of their surface area to overall mass is comparatively low. These adaptations make them well-suited to living in colder, high-latitude environments.

Biogeographic Divisions

Biogeographers use a series of imaginary lines to divide the earth into six major divisions, or realms. A distinct biota, or group of plants, animals, and microbes, inhabits each zone. The most famous of these imaginary divides runs between the islands of Borneo and Sulawesi in the Indonesian archipelago, and is known as Wallace's line, after Alfred Russel Wallace. Wallace was a nineteenth-century British naturalist—a contemporary of Charles Darwin's who also developed a theory of evolution based on natural selection. It was Wallace who first noticed the sharp distinction between birds and animals on the western side of the line—which mostly came from taxons common in Asia—and those on the eastern side of the line—which mostly came from taxons common in Australia.

To the west of the Wallace line, for instance, one would find woodpeckers, pheasants, primates, squirrels, and big cats; to the east, cockatoos, bats, eucalyptus trees, and marsupials. The Wallace line is the most well-known of the world's biogeographical divisions not only because of the extreme contrasts between the zones on either side of it but also because the entire region surrounding it is a significant biodiversity hot spot. A biodiversity hot spot is a region characterized by a high density of species richness that is under threat of extinction due to human activity. Many of the species found in the region known as Wallacea have endemic distribution areas, including the caerulean paradise flycatcher (Eutrichomyias rowleyi) and the pig-like mammal known as the babirusa (Babyrousa babyrussa). Logging, mining, and agricultural development have all had a dramatic negative impact on the Wallacea ecosystem.

Biomes are another way in which biographers organize the earth's living communities. A biome is a large, naturally occurring ecosystem. The particular grouping of plants and animals that inhabit it are determined by its climate (especially temperature and precipitation) and geography. Scientists classify biomes in different ways, but five of the most common terms include aquatic (freshwater and marine habitats), desert (arid habitats), forest (densely wooded habitats), grassland (open plain habitats dominated by grasses), and tundra (open plain habitats with a permanently frozen layer of subsoil).

Theory of Island Biogeography

The theory of island biogeography was developed in the late 1960s by the ecologists E. O. Wilson, of Harvard University, and Robert MacArthur, of Princeton University, as a way of organizing and explaining the factors contributing to the relative richness of species biodiversity that occurs in the world's islands. In its most basic form, MacArthur and Wilson's theory states that the number of unique living species on any given island is primarily shaped by two competing forces: the rate at which new species move to the island and establish successful breeding populations there (the rate of immigration), and the rate at which existing populations of successfully breeding species disappear from the island (the rate of extinction).

Various other factors are responsible for determining the rate of immigration and extinction. For example, an island's rate of immigration is closely related to the distance of the island from other ecosystems from which potential colonizers could travel, such as other islands or a mainland. The theory predicts that the more geographically isolated an island is, the lower its rate of colonization will be; this pattern is known as the distance effect. Similarly, an island's rate of extinction is related to the size of the island is, the number of species that already inhabit it, and the availability of resources on it. According to the theory, a larger island will have more habitable area and more resources, like food, available—and will thus be able to support a greater number of species, lowering the extinction rate. As the number of existing species on an island increases, competition between these species for habitat and food will also cause its extinction rate to rise. By comparison, islands that have a variety of habitats (for example, mountainous regions as well as flat forested regions) will be able to support a larger number of different species.

Assuming that these factors remain unchanged, MacArthur and Wilson's theory predicts that for any newly created island, the rate of immigration will start high and drop over time, while the rate of extinction will start at zero and rise over time. Eventually, the number of unique species that exist on a newly created island will become stable, as a balance, or equilibrium, is reached between the rate of immigration and the rate of extinction. (Even after equilibrium has been reached, the specific composition of the island's flora and fauna will be dynamic, rather than static, as different species continue to replace each other.) For example, the 1883 volcanic eruption on the tiny island of Krakatoa in Indonesia caused the total extinction of all its plant and animal species. Observations over the following hundred years showed that the rates of immigration and extinction on what was now effectively a “new” island followed the pattern predicted by MacArthur and Wallace: Between 1883 and 1934, thirty-four new species moved to and colonized the island, five of which went extinct. But between 1934 and 1985, an additional fourteen species began to successfully breed on the island (a lower immigration rate than in the previous period) and an additional eight became extinct (a higher extinction rate than in the previous period).

The theory of island biogeography has since expanded to estimate the probable degree of species richness in any isolated habitat, such as a river or lake surrounded by desert, or a wetland region surrounded by dry woodlands. It is a useful tool for predicting the effects of habitat fragmentation due to human activity. For example, much of the once continuous deciduous forest of the eastern United States has been cut up into smaller, geographically separated, patches of trees, with roads, farms, and cities taking up the spaces in between. The distance effect predicts that these isolated patches of forest will have a relatively low rate of immigration, since it is difficult for plants and animals to move across the gaps between them. Additionally, their small size will reduce the available habitat area and resources they contain, raising the rate of extinction. Overall, the theory of island biogeography predicts that habitat fragmentation will reduce the level of species richness in fragmented areas. Studies of avian populations in forest preserves surrounded by human development support this prediction.

Principal Terms

alien: a species of plant or animal that is not native to a particular place, but is found to have successfully colonized it, especially one that has been introduced as a result of human activity

allopatric: of animals or plants, occupying separate, non-overlapping distribution areas; of speciation, occurring as a result of such geographic separation

biota: the totality of the flora, fauna, and microbes that inhabit a particular region, habitat type, or time period

colonization: the long-term establishment of a successfully reproducing population of a particular species of plant or animal, in a habitat where it was not previously found

cosmopolitan: of animals or plants, occupying distribution areas that span the entire or almost the entire globe

disjunct: of animals or plants, having a fragmented distribution area that includes two or more, geographically separate, regions

dispersal: for any given species, the movement out of its distribution area of the minimum number of individuals it will take to colonize a new area

distribution area: the geographic range of a particular group of organisms at any level of classification, such as species, genus, family, or class

ecogeographic rule: a principle describing correlations between the morphology of birds and mammals and the climatic or latitudinal gradients at which they are found

endemic: of animals or plants, being restricted to a particular region of the world

latitudinal gradient: of plant or animal species, an increase in overall numbers found when moving from higher to lower latitudes

speciation: the process by which new species arise or populations of the same species become reproductively isolated from each other, either by geographical separation or some other mechanism

Bibliography

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Lomolino, Mark V., et al. Foundations of Biogeography: Classic Papers with Commentary. University of Chicago Press, 2014.

Losos, Jonathan B., et al. The Theory of Island Geography Revisited. Princeton University Press, 2010.

Lomolino, Mark V. Biogeography: A Very Short Introduction. Oxford University Press, 2020.

Maestre, Fernando T., et al. "Biogeography of Global Drylands." New Phytologist, vol. 231, no. 2, 2021, pp. 540-558. doi.org/10.1111/nph.17395.

Morrone, Juan J. Evolutionary Biogeography: An Integrative Approach with Case Studies. CRC Press, 2021.

Newton, Ian. Speciation and Biogeography of Birds. Elsevier Science, 2014.

Panagiotakopulu, Eva, and Jon P. Sadler. Biogeography in the Sub-Arctic the Past and Future of North Atlantic Biotas. John Wiley & Sons, 2021.

Whittaker, Robert J., et al. Island Biogeography: Geo-Environmental Dynamics, Ecology, Evolution, Human Impact, and Conservation. Oxford University Press, 2023.