Biogeography By Joy Nystrom Mast, Carthage College Biogeography is the study of the geographical distribution of living and fossil plants and animals as a result of ecological and evolutionary processes. Biogeography analyzes organism-environment relations through change over space and time, and often includes human-biota interactions. The main questions explored by biogeographers deal with organism patterns in order to understand the underlying processes. Biogeographers ponder questions such as: Why is a species present in a given area? Conversely, if a species is not present, then why is it missing from the area? What are the historical and ecological factors that help determine where a species occurs? What are the effects on evolution and plate tectonics? How have humans altered geographic distribution of organisms? The science of biogeography has been revitalized in the past 60 years due to our understanding of plate tectonics, mechanisms limiting distributions, island biogeography theories, and mathematical and technological tools. Current work in biogeography uses spatial patterns of organisms, past and present, to determine ecological processes. Biogeographers use experimental testing and quantification of biotic interactions. Vegetation dynamics is the primary focus for approximately half of the biogeographic research conducted by U.S. geographers. Other major focuses include ecosystem structure and function, zoogeography, paleoecology, and development of new biogeographic methodology. In particular, mapping and modeling spatial patterns of abundance and distribution of species of plants and animals has greatly advanced with geographic information systems and remote sensing technology. 1
Development of Biogeography To better understand the current field of biogeography, it is important to explore the foundations and history of the science. Biogeography is a synthetic study which is based in part on the subjects of community ecology, geology, systematics, evolutionary biology, and paleontology. The development of the subject of biogeography may be broken into four historical periods. 1600 1850: Age of Reason Early studies of organisms geographic distributions were focused on descriptive studies with historical explorations. These scientists focused on documenting spatial patterns of organisms, focusing on the effects of climate, latitude, and altitude. Comte de Buffon (1707-1788), also known as Georges-Louis Leclerc, determined that distant regions with similar climate and similar-appearing vegetation have different animal species. This is now referred to at Buffon s Law. He is also the author of Historie Naturelle, a 44 volume natural history encyclopedia. Carl Linnaeus (1707-1778) studied the plants and animals spread from Mount Ararat in Turkey in order to explore the idea of the biblical flood. As a result of documenting elevational zones of Ararat, he came up with the idea of biomes defined as major ecological communities. In addition, Carl Linnaeus is considered the father of the science of taxonomy, which is the science of classification. This time period is also known as a great age for exploration. Johann Reinhold Forster (1729-1798) was the naturalist on James Cook's second Pacific voyage in 1778. He advanced biogeography by creating global biotic regions for plants. Forster noted the higher species diversity in tropics, as well as species diversity being correlated with 2
island size. Alexander von Humboldt (1769-1859) created a botanical geography that was foundational to the field of biogeography. He determined that plant vegetation types are strongly correlated with local climate to create latitudinal belts of vegetation. Moreover, he developed elevational vegetation Zones for the Andes in South America. 1850 1900: Evolution by Natural Selection The idea of evolution based on natural selection greatly altered the way species distributions were explained. Charles Darwin (1809-1882) is most famous for publishing, The Origin of Species, outlining his idea of evolution through natural selection. Natural selection occurs when individuals in a population either do not survive equally well, do not breed equally well, or do not survive and breed equally well due to inherited differences. Evolution in turn can be thought of in two ways: microevolution and macroevolution. In microevolution, evolution is considered changes in the genetic composition of a population with the passage of each generation. For macroevolution, evolution is the gradual change of organisms from one form into another, with the origins of species and lineages from ancestral forms. For an example, Darwin studied the adaptation of Galapagos Island finches to specialize in tree versus ground varieties, then further evolving their bill structure (for grasping, probing, crushing) into seed, insect, cactus, or fruit eaters. This divergent evolution is a diversification over evolutionary time of a species into several different species, commonly referred to as adaptive radiation. Alfred Russel Wallace (1823-1913) is also famous for independently developing the idea of evolution by natural selection, based on his work in Indonesia. He found that the species on Sumatra and Java were very different from nearby New Guinea, even though the climates were similar. Wallace s study of biota in Southeast Asia showed 3
geographic distance is not equal taxonomic similarity, and the boundary area between these islands is now referred to as Wallace s Line. Wallace is also considered to be the originator of zoogeography, which is the biogeography focused on animals. Wallace integrated geological, fossil, and evolutionary information to consider paleoclimate influences distributions, developing six great biotic regions. Other notable contributions to biogeography during this period include mapping biotic regions and understanding limiting factors. Philip Lutley Sclater (1829-1913) advanced the subject of biogeography with his defining terrestrial biotic regions for birds and marine regions for marine mammals. Justus Liebig (1803-1876) changed the way scientists viewed restrictions on organisms away from a focus on total resources available with his law of the minimum. The law of the minimum states that the scarcest resource (or limiting factor) in the environment makes it difficult for a species to live, grow, and reproduce. 1900 1950: Continental Drift and Ecology Themes in biogeography in the first half of the twentieth century focused on links to paleontology, centers of species origins, and the biological species concept. The emphasis in the science of biogeography was on evolution, history, dispersal, and mechanisms of survival. The greatest impact on biogeography in this period was the theory of continental drift in 1912 and 1915 by the German geologist Alfred Wegener (1880-1930). Before the theory of plate tectonics, it was difficult for biogeographers to explain certain patterns of species distributions with the assumption that land masses were fixed in their geographic positions. Wegener s theory was actually not widely accepted until the 1960s when proof of continental drift came from a series of linear 4
magnetic anomalies on either side of the Mid-Atlantic Ridge. With the acceptance of the continental drift theory, biogeographers could now explain the disjunct biogeographic distribution of present day organisms found on different continents but having similar ancestors. Species can interact as continents collide. Subsequently, when the continents separate they take their new species with them. Biogeographers now ponder how plate tectonics may have affected the evolution of life. In turn, biogeographers offer evidence for plate tectonics such as dispersal of species via corridors such as the Bering land bridge or widely separated ( disjunct ) species distributions that can t be explained by dispersal. For instance, Nothofagus (southern beech) trees which only occur in southern South America and in New Zealand. In addition to historical explanations of organism distributions, biogeographers also examined ecological reasons for spatial patterns. Theories on ecological succession were formally developed in the late 1800s and early 1900s to show predictable and orderly changes in the composition or structure of ecological communities. In 1899, Henry Cowles published his study of stages of vegetation development on dunes along Lake Michigan. In 1916, Frederic Clements published his famous theory of vegetation development focusing on gradually changes over time to best fit to the local conditions. His climax theory of vegetation dominated plant ecology was later largely replaced by other theories, notably by Henry Gleason s 1926 concept of distribution of plants depending on the individual species rather than the Clements s idea of plant associations. In 1934, Christen Raunkiaer (1860-1938) helped change the way biogeographers classified species with life forms based on ecological rather than taxonomic classification. In 1935, Sir Arthur Tansley (1871-1955) refined the term ecosystem to 5
mean the whole complex natural unit in a system consisting of all plants, animals and micro-organisms (biotic factors) in an area functioning together with all of the non-living physical (abiotic) factors of the environment. 1950 Present: Ecological and Historical Theories Since 1950, the field of biogeography has been revitalized with advanced in ecological and historical theories focused on phylogenetic classification to related different species, mechanisms limiting geographic distribution, and distances and size influencing number of species in an area. During this period, the concept of new species arising due to geographic isolation was developed by the Ernst Mayr (1904-2004). Mayr is also well known for defining the biological species concept as potentially interbreeding to produce fertile offspring. In addition, Mayr helped to define the term cladistics to refer to classifications which only take into account genealogy, based on evolutionary ancestry. Cladistics, or phylogenetic classification, views a species as a group of lineage-connected individuals, compared to the traditional Linnaean taxonomy which focused on the similarities between species. Cladograms are created based on the order in which different groups branched off from their common ancestors, arranged with the most closely related species on adjacent branches of the phylogenetic tree. Theories also expanded during this time period on how a species can occur in widely geographically separated areas and the mechanisms that limit these distributions. In 1958, Leon Croizat (1932-1982) published his concept of vicariance biogeography to explain disjunction of multiple species due to the growth of barriers instead of via dispersal. Croizat s works include Manual of Phytogeography (1952), Panbiogeography (1958), Space, Time, Form (1964). Robert Harding Whittaker (1920 1980) proposed a 6
new method to analyze limits to plant distributions by comparing species abundance to environmental gradients. His gradient analyses approach focuses on abiotic factors such as light, water, temperature, and soil nutrients in plant communities. Biogeography during this period moved from observational to predictive studies with the theory of island biogeography. In 1963, R. H. MacArthur and E. O. Wilson hypothesized that species richness of an area could be predicted to explain distributions. The theory states that if one knows the rates of colonization and extinction of an island, then it is possible to predict number of equilibrium species that area could support. They based the species richness prediction on two factors: (1) distance of island from a mainland source of species for colonization pool; and (2) the size of the island for available habitat and variety of niches. With these two factors, MacArthur and Wilson predicted the number of species the area could maintain, as well as the turnover rate for the area. According to island biogeography theory, small and distant islands have a lower number of species that can be maintained compared to large and near islands. The theory also states that there would be a turnover of the species as new species colonize and old species go extinct, but the number of species overall should achieve an equilibrium number. This theory has been applied to other non-island areas that act like islands due to habitat fragmentation, such as nature preserves and national parks. Spatial Distributions of Organisms Modern biogeography explores spatial patterns in the geographic variation of individuals and populations, including genetic, physiological, and morphological variations. Organisms are often studied in biomes, consisting of distinct flora and fauna related to 7
climate, soil, and geological factors. Biogeographers analyze species ecological niches (both fundamental and realized), defined as the total requirements for resources and physical conditions. The foundation of biogeographic distribution patterns follows the first rule of geography, namely that closer equates to more similar (referred to as spatial autocorrelation ). When relating species niche to geographic distributions, key interactions and adaptations to consider include: (1) stress, with regards to climate, predation, and availability of symbiosis (close association between species), (2) competition, both between and within species which exclude some species from their fundamental niches, and (3) disturbance, which occurs less predictably and causes a greater change in the environment than stress. Biogeography seeks to answer why species distribution patterns as a response to historical and ecological limiting factors, dispersal mechanisms, and human influences. Vicariance Biogeography Geographic distributions may be separated into disjunct populations by historic events that create barriers, called vicariant events. Disjunct distribution occurs when two or more closely related taxa live today in widely separated areas. Barriers which split a continuous distribution into disjunctions are created by many processes, including changing in the distribution of land via by continental or tectonic shifts, volcanism and mountain-building, shifts in river patterns, glacial cycles, climate change, and human alterations to landscapes. Barriers may be physical (like a mountain), physiological (such as fresh versus salt water blocking for aquatic organisms), and/or ecological-behavioral (for example predators). For example, one vicariance splitting taxons ranges occurred from historical climate change during glacial periods, resulting in the drying of tropical 8
rainforest in South America into smaller fragments of rainforest refugia surrounded by grassland. Disjunct distribution from vicariant events are supports by fossil evidence and by present species can t be explained by natural dispersal. For example, Nothofagus (Southern Beech) trees occur today in such widely separated regions as southern South America and New Zealand, which can be explained by historic plate movement but not dispersal. In another example, disjunct populations of southern migrations were created during glacial ice advancements in North America, with isolated populations cut off as ice retreats (partly because soil was removed by glaciers). Organisms from temporary or fluctuating environments (such as seasonally variations in temperature) typically are less separated by barriers. For instance, crossing mountain ranges are less of a barrier to species from temperate climates (adapted to cold winters) compare to species from tropical climates. Geographic isolation from vicariant events leads to reproductive isolation. Even if the disjunct population in time reunites, the geographic isolation may have already resulted in the groups no longer being able to interbreeding (especially in animal species). In other words, when barriers geographically isolate populations over time, a different evolutionary lineage might occur with new species created (referred to as allopatric speciation). Geographic isolation from vicariant events leads to reproductive isolation because different geographic regions have different selective pressures, such as temperature, rainfall, predators and/or competitors. In addition, even if the environments separated by a barrier are not very different, the populations may differentiate because different genetic combinations and mutations occur by chance. 9
Dispersal Biogeography Disjunct biogeographic distributions may also be created by dispersal events. Dispersal can be defined as movement away from one s point of origin. Possible results of dispersal include: extending within current range by colonizing new habitat in range; or (2) colonizing distant location across a major physical barrier of unfavorable habitat. Individuals may move great distances through unsuitable habitats by traveling through corridors, flying over hostile environments, being blown or floating through sweepstake events (such as a hurricane). Dispersal agents typically are wind, water, rafting, or animals. Dispersal biogeography studies distribution patterns of organisms, emphasizing dispersal capabilities as well as ecological properties of species to evaluate origins of taxa in a biota. Dispersal mechanisms affect rate of species movement across landscape, efficiency species colonize new areas, and successfully establishments. Modes of dispersal vary from jump dispersal (such as the movement of gypsy moth to North America) to diffusion (such as gypsy moth spread within North America). Jump dispersal includes traveling over long distances across inhospitable habitat, in other words long distance dispersal mostly by organisms that can fly or swim. Jump dispersal events are rare, cover large distance, and are considered "surprising" events. These long distance dispersals can explain large discontinuous distributions of some organisms, as well as taxonomic similarity of distant biotas and populations. Most animal and all plant jump dispersals are passive, although occasionally some animals have active long distance dispersal. Those individuals who succeed at jump dispersal have the ability to: (1) travel long distances; (2) withstand unfavorable conditions during passage; and (3) establish viable population on arrival. 10
Pathways of dispersal can be broken into the categories of corridors, filters, and sweepstakes. Corridors are routes that permit spread of taxa through continuous favorable habitat with relatively little risk. These routes may in turn serve to link larger areas of habitat. Famous corridors include past land connections created by sea level changes, such as the Bering land bridge connecting Asia and North America. In contrast, a net or filter is a route that contains patches of suitable habitat interspersed between larger areas of unsuitable habitat. Filters can act as a barrier for some taxa, blocking or slowing passage of some organisms. A classic example is the Isthmus of Panama filtering the dispersal between North and South America starting around 3.5 million years ago. The sweepstakes pathway of dispersal refers to chance dispersal across a major barrier, in other words a long shot for dispersal which usually involves accidents, low probability, and unusual means of travel. For instance, sweepstakes dispersal includes birds caught in hurricanes, seeds traveling in upper atmosphere winds, and animals on floating on driftwood (called Noah s Ark if there is an assemblage of organisms deposited in mass). Human Impacts of Distributions In addition to barriers and dispersal events, biogeography also explores species distribution patterns impacted by humans, whether in limiting or expanding ranges. Humans decrease other species ranges through habitat destruction, hunting and commercial exploitation, polluting environments, deliberately or accidentally introducing competing non-native species, change historic disturbance regimes, and causing the loss of ecological partners. Species most likely to have their geographic ranges limited by humans include: species who have relatively few offsprings who are nurtured for a long 11
time ( K species), specialists who are selective about their niche conditions, economically valuable species, naturally rare species, species with naturally restricted ranges, species sensitive to pollutants, and species in competition with humans for habitat or resources. People also increase species geographic ranges, either by introducing species to a new geographic region or by expanding ranges by favoring some species that can adapt to human landscapes (weeds, agricultural species, species adapted to urban areas). Introduction of species by people into a new area may be deliberate or accidental. Although most introduced species fail to establish viable population, those who succeed typically have harmful long-term effects, competing with native species or transmitting diseases. Conclusion Biogeographers seek to develop theories to explain past, present, and changing future composition of plants and animals distribution patterns in order to better understand the processes of how organisms interact with our planet. Biogeography is both interdisciplinary within other subfields of geography as well as other disciplines, such as ecology, geology, and biology. An integrative approach helps biogeographers provide a holistic understanding of the diversity of life, and make recommendations for the conservation of biological diversity. See also in Encyclopedia of Geography: Chapters on individual Biomes, Biota, Darwinism, Ecosystems, Island Biogeography, and Landscape Ecology Joy Nystrom Mast 12
Further Readings Cox, C.B., & Moore, P.D., (2005). Biogeography: An Ecological and Evolutionary Approach. Malden, MA: Blackwell. Lomolino, M.L., & Heaney, L.R. (Eds) (2004). Frontiers of Biogeography. Sunderland, MA: J.H. Sinauer. Lomolino, M.V., Riddle, B., and Brown, J.H. (Eds.) (2006). Biogeography (3 rd Edition). Sunderland, MA: J.H. Sinauer. Lomolino, M.L., Sax, D.F., & Brown, J.H., (Eds) (2004). Foundations of Biogeography: Classic Papers with Commentaries. Chicago, IL: University of Chicago Press. MacArthur, R.H., & Wilson, E.O. (reprinted 2001). The Theory of Island Biogeography. Princeton, NJ: Princeton University Press. MacDonald, G., (2003). Biogeography: Introduction to Space, Time, and Life. New York, NY: John Wiley & Sons. Quammen, D. (1997). The Song of the Dodo: Island Biogeography in an Age of Extinction. New York, NY: Simon and Schuster. 13