ZYU3301 Session 1: Introduction to Biogeography PDF

ZYU3301-Unit I Session 1: Introduction to biogeography Session 1 Introduction to biogeography Contents Introduction 1.1 What is biogeography? p 2...

ZYU3301-Unit I Session 1: Introduction to biogeography Session 1 Introduction to biogeography Contents Introduction 1.1 What is biogeography? p 2 1.2 History of biogeography, p 4 Summary, p 10 Learning Outcomes, p 10 Introduction We humans are not alone on this planet Earth. We share it with millions of other life forms. Of these, less than 2 million forms have so far been identified. Many more, possibly 10-30 million or more kinds of animals, plants, and microbes on Earth, are unidentified or even yet to be discovered. In addition, probably billions of species have now become extinct. All these facts tell us the world we live in is incredibly diverse. This diversity of organisms is enriched by the varied habitats they live in. There are species living in terrestrial, aquatic, and aerial habitats in the tropics, wetlands, the poles, etc. But a single species will not be able to live everywhere. In fact, most species are restricted to small geographic areas and narrow environmental conditions. As a result, different species exhibit different patterns of occurrence on earth. For instance, elephants live only in Asia and Africa and not in America or Europe; giraffes are confined to Africa, kangaroos live only in Australia. Even within our small island, some species of animals and plants are confined to certain regions, such as the Sinharaja forest. We are sure, even as a child, you knew these facts. Have you ever wondered how and why the biological diversity varies over the surface of the earth in this way? This course attempts to provide answers to this question and many others, in terms of geographical, ecological and historical backgrounds. In this introductory session, we will first focus on what is biogeography? There you will understand that much of biogeography involves studying the geographic ranges of species and the processes that underlie the variations in distribution patterns. In the second part of the session we examine the history of biogeography by focusing on many of the great scientists that contributed to the development of the field of biogeography. 1 Copyright © 2009, The Open University of Sri Lanka  Unit I 1.1 What is biogeography? Conventionally, biogeography has incorporated the study of distributions of organisms, both present and past. Hence, traditionally, biogeography is defined as the study of organisms in space and time and the processes that underlie their distribution patterns. Modern biogeography however, includes studies of all patterns of geographic variation in nature, from genes to entire communities and ecosystems. Thus, in modern biogeography, all elements of biological diversity that vary across geographic gradients are studied. Therefore, biogeography is now defined as the study of spatial and temporal patterns of biological diversity and the processes that underlie these patterns. Themes central to biogeography In simple traditional terms, the task of biogeographers is to find out how the distributional patterns of biological diversity vary over the surface of the earth (both present and past) and to provide explanations for these variations. Biogeographers study distribution patterns by comparing geographic ranges and other characteristics of different or the same kinds of organisms living in different regions under the same or different environmental conditions, from observing differences in the species diversity and composition of communities over the surface of earth, etc. Once these patterns are established, they go on to examine which processes have produced these different distributions. Ecological, historical, as well as geographical factors may cause these variations in distributions. To investigate these various possibilities, biogeographers examine how organisms are adapted to conditions of life in a particular area. They also look at why an organism does not exist in adjacent areas and also collect evidences on what biological and environmental factors or barriers (both past and present) have prevented the further spread of the organism, etc. Thus, over the years, biogeographers have been attempting to examine more or less the same central questions described above. As a result, we can identify four general themes central to biogeography: Classification of geographic regions based on all species of animals, plants and microbes living in a particular area; in other words classify based on the biotas. Classifying biogeographic regions in the terrestrial ecosystem based on the vascular plants or vertebrates present would be a good example Reconstruction of the historical development of biotas, including their origin, spread, and diversification. This is to obtain important background information on ecological and evolutionary characteristics of biotas 2  ZYU3301-Unit I Session 1: Introduction to biogeography Providing explanations for the patterns of species diversity; that is, explanations for differences in types and numbers of species among geographic areas and along geographic gradients (such as area, latitude, elevation, and depth). Providing explanations for geographic variation in the characteristics of individuals and population of closely related species, including trends in morphology, behaviour, and demography. The above four themes form the conceptual framework of the field. Specialisations within the biogeography field From the different aspects covered in the above themes, it is clear that biogeography is a vast field, with several specialisations. Some of the common ones are listed below: A common specialisation is taxonomic: zoogeography is the study of animals; phytogeography is the study of plants. The biogeographers who attempt to account for distribution and geographic variation in diversity in terms of interaction between organisms and their physical and biotic environments are specialised in ecological biogeography. Those who attempt to reconstruct the origin, dispersal, and extinction of taxa are specialised in historical biogeography. They consider the influence of continental drift, global climatic changes, and other large-scale historical climatic factors on the evolution and distribution of life. In this course, our aim is not to make you a specialist in any one of the above areas, but to provide you a broad exposure to the subject of biogeography. Largely we will focus on both animals and plants, but the emphasis will be on animals. We will attempt to provide answers for questions such as: Where do different kinds of organisms live? What enables organisms to live where it does? Why are particular species confined to particular range and what prevents them from inhabiting other areas? What role does geographic variation in the biotic (the living component) and abiotic (physical/non-living component) factors play in limiting the distribution of a species, how have historical events shaped a species distribution, etc. In this Unit, you will learn about the basic geographical patterns displayed by individuals and populations. You will also learn about the ecological processes that affect the present and past distributions of organisms; for instance, you will learn how physical and biotic environments determine distributions. In the second Unit, we will describe the fundamental processes in biogeography, namely, dispersal, extinction, and speciation (evolution), through which biotas respond to spatial and temporal changes of the environment to give rise to the characteristic biogeographic patterns of distribution. 3 Copyright © 2009, The Open University of Sri Lanka  Unit I In Unit II, we will also examine the historical climatic and geological factors, such as continental drifts and glaciations of the past that have influenced geographical distributions of organisms. The third Unit deals with biogeography of islands and humans. ACTIVITY 1.1 1. Define biogeography. 2. List the themes central to the study of biogeography. 1.2 History of biogeography Aristotle (4th century BC) was one of earliest philosophers to study distribution patterns of life on earth. However, it was only in the 18 th century that a serious study of biogeography began. Thus, many of the basic themes central to modern biogeography have their origin in the 18th and the 19th centuries. In the first two sections on biogeography in the 18th and in the 19th century, you will learn about the major developments that occurred during these periods. In the final section on biogeography in the present era you will come to know how the acceptance of the theory of continental drifts/plate tectonics and other advances that occurred in the 20th century provided new evidences, mechanisms, and tools to explore long-standing distribution patterns as the field progressed. Biogeography in the 18th century Biogeography essentially started during the “age of exploration” (late 17th and 18th century) when naturalists made collections of organisms during their many explorations around the world. With the extensive and diverse collections of life, they began noticing trends in species distributions. As a result, several naturalists proposed hypotheses to account for the diversity and distribution of organisms in the 18th century. Of these, some of the most important naturalists that made a significant contribution to the study of biogeography and their key observations are listed below: Carolus Linnaeus (1707-1778), like most of the others in this century, thought the earth, its climate and its species to be unchanging. He was largely driven by religion. He hypothesised that terrestrial plants and animals were originally placed along the slopes of a mountain near the equator (Mount Ararat near the present day border of Turkey and Armenia) after Noah’s Flood (biblical flood), and then from this point spread to suitable environments across the globe. 4  ZYU3301-Unit I Session 1: Introduction to biogeography Comte de Buffon (1707-1788) had a different view of the origin and spread of life. He hypothesised that species originated in the Arctic when the climate was more hospitable (not so cold) there; then when the environment cooled, spread into the Americas (new world) and southward, changing as they colonised climatically and ecologically different regions. He speculated that tropical biotas of the new and old worlds shared few forms because of these modifications that occurred during the migration from one area to another. He provided two key contributions that greatly influenced the development of the study of biogeography during the 18th century. His most important contribution to biogeography was his observation that different parts of the globe, even those areas with similar environmental and climatic conditions were often inhabited by different kinds of mammals and birds. This became the first principle of biogeography known as the Buffon’s law, which states that “climatologically similar, but geographically separate regions of the world have distinct biotic assemblages”. Secondly, he observed that the climate and its species were dynamic, or they were subject to change. Johann Forster (1729 – 1798) made several important contributions: He presented one of the first systematic views of the biotic regions of the world defined by their distinct plant assemblages. He observed that Buffon’s law applied not only to birds and mammals but to plants as well and to all regions of the world (not just only to the tropics). He described a floristic zonation along latitudinal gradients. That is, he observed a relationship between regional floras and climatic conditions and noted a higher species diversity of plants near the equator when compared to the poles. He explained this pattern through latitudinal variations in surface heat on the earth. He also noted that island communities had fewer plant species than those on the mainland, and that the number of species on islands increased with available resources and its size. These observations later led to the island biogeography (session 13) and species diversity theories. Alexander von Humboldt (1769-1859) is generally viewed as the father of phytogeography. Humboldt further generalised Buffon’s law to include plants as well as most terrestrial animals. Like Forster who observed a zonation based on latitude, Humboldt observed that plants were distributed in elevational zones based on local climate. Augustina de Candolle (1778 – 1841) observed that not only are organisms influenced by climate, but they compete for resources as well. For instance, in an island, species number is very strongly 5 Copyright © 2009, The Open University of Sri Lanka  Unit I influenced by island area. In addition, other factors such as the age of the island, volcanism, climate, and isolation also influence diversity of flora. ACTIVITY 1.2 1. What were the major biogeographic developments that resulted in the 18th century? 2. Who were the naturalists responsible for these developments? What were their contributions to biogeography? Biogeography in the 19th century As you learnt in the previous section, by the late 18th and early 19th century biogeographers had made great progress in describing fundamental biogeographic patterns. However, explanations for what caused these patterns were proposed only later in the 19th century. For instance, throughout the 19th century biogeographers attempted to explain Buffon’s law and the exceptions to this law by examining the factors that caused the differences and similarities among isolated regions. Explanations for the fundamental patterns of biogeography were only possible after the following advances of the 19th century. Several scientists in this period played a key role in achieving these advances in biogeography. A better estimate of the age of the Earth – Early biogeographers had estimated the age of the earth to be only a few thousand years. However, Charles Lyell (1797-1875), who is considered as the father of geology, used the fossil record to conclude that surface of the earth and its biota were dynamic. Moreover, his observations led to the realisation that earth must be much older than just a few thousand years. A better understanding of the mechanisms involved in the spread and diversification of species – especially dispersal, vicariance, extinction and evolution. Four British scientists, Charles Darwin, Joseph Hooker, Philip Sclater and Alfred Russell Wallace played a major role in studying patterns of geographic distribution and the advancement of biogeography in the 19th century. Charles Darwin (1809 - 1882): During Darwin’s voyage in the HMS Beagle, he studied the geology, native plants and animals, indigenous people, etc., around the world. On his return, he developed the theory of evolution by natural selection. This theory is closely connected to all aspects of biogeography and his contributions to this field are massive. Darwin provided the theoretical basis for understanding changes in the adaptations and distributions of organisms across time and space. He proposed that diversification and adaptation of biotas resulted from natural selection while the spread and eventual isolation and disjunction (a process that gives rise to widely separated populations of taxa) of biotas resulted from long distance dispersal. Because of Darwin’s observations, most 6  ZYU3301-Unit I Session 1: Introduction to biogeography biogeographers abandoned the view that distributions of species remained unchanged. Joseph Hooker (1817-1911): Hooker also agreed that distributions were dynamic. However, he proposed that emergence and submergence of ancient and undiscovered continents and land bridges were likely to account for the spread and separation of closely related species’ (as opposed to the long distance dispersal explanation of Darwin). However, by the end of 19th century, this hypothesis was abandoned, as geological evidence for these land bridges never materialised. He also developed and applied many of the principles of what we now call “vicariance biogeography”. He studied island biotas to gain insights into biogeographic processes and confirmed Forster’s observations that isolated floras tend to be different from those on the mainland. For instance, he noted that as the isolation of the island increased, the number of plant species decreased, while the distinctness increased. He also observed that the lands with the greatest diversity of environmental conditions tend to have the most diverse floras. Philip Sclater (1829 - 1913) – His contributions had a major impact on zoogeographic patterns. He attempted to develop a biogeographic system that reflected the origin and development of distinctive biotas. In doing so, he proposed a scheme that divided the earth into biogeographic regions, based on the distribution of passerine birds. This formed the basis for the system of six biogeographic regions, we continue to use today (session5). Alfred Russell Wallace (1823-1913) – He is considered the father of zoogeography. He was the first person to analyse faunal regions based on the distributions of multiple groups of terrestrial animals. Wallace developed a detailed and accurate map of the earth’s biogeographic regions based on mammals (African, Australian, Nearctic, Neotropical, Oriental, and Palearctic). He also formalised many of the basic concepts of the field that are used even today. Before the mid-18th century, the earth, its climate and its species were thought to be static or unchanging. In addition, the species distributions were thought to remain near its site of origin. However, by the late 18 th and in the 19th century, the efforts of many naturalists had resulted in several major developments. First, Buffon gave biogeography its first principle. The others following him confirmed and generalised Buffon’s law and also noted other biogeographic patterns. Therefore, much progress was achieved in describing the fundamental biogeographic patterns in the 18th century. Secondly, the explanations proposed during the 19th century for Buffon’s law and other fundamental patterns, led to a dynamic view of the earth, its climate, its species and their distributions. In addition to the above, in the 19th century, several name rules were proposed. For instance, Bergman rule (C. Bergmann, 1847), states that 7 Copyright © 2009, The Open University of Sri Lanka  Unit I warm blooded animals in colder climates are larger. The Allen rule (J.A. Allen, 1878) states that warm blooded animals in colder climates have shorter limbs and appendages. ACTIVITY 1.3 1. What were the major biogeographic developments that resulted in the 19th century? 2. Who were the naturalists responsible for these developments? What were their contributions to biogeography? Biogeography in the present era Several major trends related to biogeography developed in the early to mid 20th century. We briefly describe these trends below: Biogeographers of the early 20th century became increasingly more specialised focusing on particular disciplines and sub- disciplines (including taxonomy, evolutionary biology and ecology), often restricting themselves to particular taxa and time periods. By the middle of the 20th century many investigators including George Simpson (1902-1984) and Philip Darlington (1904-1983) and Ernest Mayr accepted and formalised the centres of origin and dispersal model, first proposed by Darwin, to explain current biogeographic patterns of various taxa. Biogeographers also began to investigate patterns of variation within single species. They investigated the patterns of geographic variation within and among single species and the mechanisms responsible for the origin of new species. These investigations, especially by Ernest Mayr, gave rise to the biological species concept. In the latter part, especially since the 1960s, the field of biogeography saw a major revitalisation, mainly due to the following four major developments: The acceptance of the theory of plate tectonics and continental drift - Until the 1960’s most biogeographers considered the earth’s crust to be fixed and without lateral movement. In 1912 Alfred Wegener first proposed the theory of continental drift (session 10) to put forward the idea that continental plates moved laterally. However, it was only in the late 1960’s, that this theory was generally accepted, when geological evidence for the process could not be ignored any further. Once the theory of plate tectonics/continental drifts was accepted, biogeographers had to rethink their explanations for many distributional patterns. They had to accept that changes in relative sizes and positions of landmasses and oceans had resulted in changes in biogeographic patterns. The development of new phylogenetic methods that allowed the reconstruction of historical diversification of lineages and quantification of evolutionary relationships among species – Willi Hennig developed a method on phylogenetic systematics called cladistics in the 1950s. This method allowed phylogenetic classifications that trace the history and relationships of taxa. 8  ZYU3301-Unit I Session 1: Introduction to biogeography Thus, vastly improving our understanding of how biotas are and have been related. The renewed interest in using information on disjunct geographic distributions of related species to evaluate past connections – From early times biogeographers attempted to use information on geographic distributions of related species to reconstruct the histories of continents and other landmasses. In this respect, disjunct species (widely separated species) have been very useful to biogeographers because they reveal past land or water connections or long-distance dispersal between two regions. The interest in the study of disjunct species was renewed in part due to writings of L Croizat (1958) on this subject. This renewed interest to evaluate past connections has led to the testing and revising of many of the earlier biogeographic reconstructions. Investigations of mechanisms that limit species distributions – Up until 1950s, the emphasis in biogeography had primarily been evolutionary and historical. Since the 1960s, ecological biogeography came into prominence. Biogeographers such as G.E. Hutchinson investigated the importance of interspecific interactions in influencing the distributions and co-existence of species. R. MacArthur and E.O. Wilson proposed the theory of island biogeography (1963, 1967), which focused on the species diversity of island biota (session 13). In addition, other biogeographers emphasised the importance of abiotic environment in limiting the distributions of individuals and populations and in turn determining the diversity of species in different regions. Our ability to explore and analyses these biogeographic patterns from local to global scales have been possible because of advances in computer technology and related techniques including satellite imagery, geographic information systems and spatial statistics. ACTIVITY 1.4 1. Following is a list of naturalists that made a significant contribution to the study of biogeography between 18th - 20th centuries. Match these naturalists with one or more of the statements (a) to (o) to highlight their key contribution(s). i) Linnaeus; ii) Comte de Buffon; iii) Johann Forster; iv) Alexander von Humboldt; v) Charles Darwin; vi) Phillip Sclater; vii) Alfred Wallace; viii) A. L. Wegener; ix) L. Croizat; x) R. H. MacArthur and E. O. Wilson a. Plants and animals spread from Mount Ararat ………………………. b. Plants and animals spread from the Arctic ………………………. c. Distant regions with similar climate have different animal species ………………………. d. Climate and species are changeable ………………………. e. Global biotic regions for plants ………………………. f. Higher species diversity in tropics ………………………. g. Species diversity correlated with island size ………………………. h. Plant vegetation types strongly correlated with local climate ………………………. 9 Copyright © 2009, The Open University of Sri Lanka  Unit I i. Theoretical framework for biotic patterns in space and time ………………………. j. Biotic regions for birds ………………………. k. Originator of Zoogeography ………………………. l. Theory of Continental Drift ………………………. m. Disjunction of multiple species due to the growth of barriers ………………………. n. Biogeographic regions based on mammals ……………………………… o. Island Biogeography ………………………. 2. List the major trends related to biogeography that developed in the early part of the 20th century. 3. Four major developments revitalised biogeography since the 1950s. What were these developments? Summary Biogeography is the study of spatial and temporal patterns of biological diversity and the processes that underlie these distribution patterns. Themes central to biogeography involve examining the variation in distributional patterns of biological diversity over the surface of the earth and providing explanations for these variations. Many great scientists contributed to the development of biogeography by attempting to understand the origin, spread, and diversification of biotas. In the mid-1700s Buffon gave biogeography its first principle, while others tested its generality and broadened the field to explore a diversity of other patterns and their causal forces. As the field progressed, theories of natural selection and continental drift provided new mechanisms to explore long-standing patterns. Also, new comparative and experimental methods, phylogenetic analyses and advances in computer science and statistics provided invaluable tools. By the 1950s biogeography had come of age as a respected science. Learning Outcomes After studying this session, you should be able to: Define the term biogeography. Outline the major themes that form the conceptual framework of the field. Identify the main specialisations within biogeography. Identify the key scientists, their observations, and major developments that contributed to the advancement of biogeography in the 18th and 19th centuries. Explain the major developments that revitalised biogeography in the 20th century 10