Chapter One

Introduction

In this chapter, you will study about the meaning, scope and brief history of biogeography. In
addition, you will distinguish the different branches and subfields of biogeography as well as
problems in the study of biogeography.

Since this chapter is the introductory part of the text, understanding it enhances understanding
of the proceeding chapters. As a result, to have a thorough understanding of the whole text
basis here.
Objectives:

At the end of this chapter, the learners will be able to:

 Understand the meaning and scope of biogeography.

 Explain the brief history and historical development of biogeography.

 Identify the main problems in the study of biogeography.

1.1. The Living World

Living things are incredibly diverse. There are probably somewhere between 5 million and 50 million kinds
of animals, and microbes living on earth today. Of these, fewer than 2 million have been formally recognized
as species and described in the scientific literature. The remainders are represented by specimens in museums
waiting to be described or individuals in nature waiting to be discovered. Additional untold millions; probably
billions, of species lived at some time in the pastbut are now extinct; only a small fraction of them have been
preserved as fossils.
Nearly everywhere on earth, from the frozen wastes of Antarctica to the warm, humid rainforests of the
tropics, from cold, dark abyssal depths of the near-boiling waters of hot springs- even in rocks several
kilometers beneath the earths surface- at least some kinds of organism can be found. But on single species is
able to live in all places. In fact, most species are restricted to a small geographic area and a narrow range of
environmental conditions. The spatial patterns of global biodiversity are aconsequence of the ways in which
the limited geographic ranges of millions of species overlap and replace each other over the earth’s vast
surface.
Meaning and Scope of Biogeography
The meaning of Biogeography is different for people with different disciplines. To biologist, it is traditionally
the history and geography of animals (Zoogeography) and plants (phytogeography). Biogeography is the
science that attempts to document and understand spatial patterns of biodiversity. It is the study of distribution
of organisms, both past and present, and of related patterns of variation over the earth in the numbers and
kinds of living.

Biogeography is the study of the distribution of plants and animals over the surface of the Earth in both a
spatial and temporal context. The discipline is important because it study the Earth, its environs and its
organisms, to better manage the future to ensure its wellbeing.
Biogeography is the science that attempts to document and understand spatial patterns of biodiversity. It is
the study of distributions of organisms, both past and present, and of related patterns of variation over the
earth in the numbers and kinds of living things
Biogeography is a broad field. To be a complete biogeography, one must acquire and synthesize a tremendous
amount of information. Notice both “spatial” and “temporal” contexts are mentioned, because biogeography
is concerned with the analysis and explanation of patterns of distribution, and with the understanding of
changes in these distributions that have taken place in the past and which are taking place to day. Here one
can easily identify tow scales: spatial and temporal.
Biogeography is the science that attempts to document and understand spatial patterns of biodiversity. It is
the study of distributions of organisms, both past and present, and of related patterns of variation over the
earth in the numbers and kinds of living things. So, it is the subset of geography that deals with living things.
Central questions commonly asked in biogeography include:
 What taxa are found where?
 How are the organisms adapted to the conditions of the environment in this area?
 What environmental or biological factors exist that prevents the organism from existing in adjacent areas?

Themes of Biogeography
Hierarchy: Provides a scheme for studying living things: Example
Individuals–Populations-Communities – ecosystems.
 Populations are large groups of individuals
 Communities are assemblages of species of different populations that show a degree of cohesion and
interdependence.
  Ecosystems are the interactions between the biotic and abiotic worlds within and between these
communities. These concepts fall within the field known as Ecological Biogeography.
Human impact: The arrival and impact of humans have had a profound impact on the worlds Biogeography.
The evolution of humans is sometimes considered a catastrophe in the sense that humans manipulate the
environment by deflecting energy from other parts of the natural world into the support of one species.
Therefore, the impact of humans on ecosystems is a central theme in Biogeography.
Temporal scale: A phenomenon can be studied as it evolves over one day or over several million years. Our
expediently of nature represents just one tiny pointing time when compared to the constantly changing mosaic
of the landscape as it responds to environmental and climatic change. Biogeography can therefore never be a
static.
Spatial scale: a recurring theme in Geography. Different observers conduct their analyses across different
spatial and temporal scales. An ecosystem can, in fact be a rotting log or the entire Earth. To help our
investigations, Biogeography’s use classification schemes, such as the use of biomes, also, known as
formations. Biomes include tropical rainforest, savanna grassland, boreal forest, desert, subtropical evergreen
forest.
Discipline: When analyzing effects of change over millions of year, such as the impacts of Ice Ages and sea
level rise on ecosystems, we use an approach known as Historical Biogeography.
 The following are persistent themes in biogeography.
 Explaining the differences in numbers as well as types of species among geographic
areas.
 Reconstructing the historical development of biotas, including their origin, spread, and diversification
 Classifying geographic regions based on their biota
 Explaining geographic variation in the characteristics of individuals and populations of closely related
species, including trends in morphology, behavior, and demography.

Scope of Biogeography
Like geography, there is no clear demarcation of the scope of biogeography. Some limit its content to the
study of plant and animal geography with emphasis upon the former; others would broaden its scope to include
the study of soils and some aspects of human geography.

In simple and narrower sense, Biogeography is defined as the study of the origin, distributes adaptation and
association of plants and animals. However, the scope of biogeography extend across the fields of plant and
animal geography, with many overlaps in genetics, human geography, anthropology, and the social sciences.
All of these together form the domain of biogeography.

Biogeography deals with the geography, ecology and history of life-where it lives, how it lives there, and how
it came to live there. It has three main branches-analytical biogeography, ecological geography and historical
biogeography.

The Subfields of Biogeography

Biogeography is a recognizable subdivision of geography. The facts of plant and animal distribution and the
existence of veritable communities of both plants and animals are themselves facts of geography that help us
to differentiate the Earth’s surface and are relevant factors in human environment. Evidently, in its widest
sense, as distinct from physical geography, biogeography must include human geography. In working
practice, it is taken as concerned simply with plant and animal geography.

In broad terms, biogeography is concerned with the phenomena of the biosphere. More specifically, and
traditionally, biogeography has concerned itself with the study of the geographical aspects of plants and
animal life, especially in terms of their distributions, and two branches have long been cerography of animals.
Historical biogeography considers the influence of continental drift, global climatic change, and other large-
scale environmental factors on the long-term evolution of life.
Ecological biogeography looks at the relation between life and the environmental complex.
Analytical biogeography examines where organisms live today and they spread. It may be considered as a
division of ecological biogeography.
Here we shall talk biogeography to include some study of plant and animal geography, the world of the soil,
particular aspects of man. We shall not be concerned so much with man for this is the subject of anthropology
and ethnography, but rather with man as animate being who plays an important role in the biosphere through
his activities, which change, alter, upset and destroy the balance of nature. Man cannot be left out of the
biological reckoning, for he is the most significant agent or factor in the situation.
A biogeography may specialize and become a:
 Phytogeographer, studying plants:
 Zoogeographer, studying animals;
 Ecological geographer, studying the present distributions of plants and animals in relation to their physical
environment; and
 Historical biogeography, attempting to reconstruct the origin, dispersal and extinction of taxa and biotas.

Brief History of biogeography

 How old a subject is biogeography? Many of the greatest scientists including Charles Darwin,
Alfred Russell Wallace and Alexander Humboldt, to mention a few, could be regarded as
biogeographers even though they did not usually refer to themselves as biogeographers.
However, it is only recently that biogeography has emerged as a widely respected branch of
study. The stimulus to the emergence of biogeography as a vigorous science can be attributed
to several coincident and interesting developments.

The first stimulus to the emergence and transformation of the discipline from a largely
descriptive science closely linked to traditional taxonomy to conceptually oriented discipline
concerned with thebuilding and testing of biogeographic theory is attributed to new
developments in the area ofmathematical modeling and advances in Earth sciences. The
introduction of mathematical theory into ecology, evolution and systematic biology and
contemporary advances in the Earth sciences, especially the theory of plate tectonics and a
wealth of data from fossil records have contributed a lotin bringing major conceptual progress
in biogeography.

A second stimulus to the progress of biogeography has been the development and application
of newtechnology. As known computers have allowed the compilation and manipulation of
enormous quantities of data on distributional records and other information of organisms on
truly geographic scales. Advances in computer science have made it possible for using
simulation modeling, geographic information system techniques and very many complex
statistical methods to study distribution of organisms in a much better way than ever before.
Satellites, submersible vessels, and ground based data collection systems have provided a
wealth of new information on the environment. of the Earth, including distributional aspects
of its organisms. Such and other techniques have, thus, permitted increasingly accurate
reconstruction of the history of both the Earth and its organisms. Biogeography, being one of
the synthetic sciences, has readily adopted new technologies from many other disciplines and
has made a rapid use of the new kinds of information that they can provide. Thus, in less than
two decades, biogeography has grown from being rather unappreciated science toa vigorous
and respected science whose practitioners are using the latest conceptual advances and
technological tools to say and do important things about the Earth and its living inhabitants.

Historical development of biogeography

 Biogeography has had a long history that is inextricably woven into the development of
evolutionary biology and ecology. The problems of distribution and variation on geographic
scales were matters of primary interest to evolutionary biologists, including the distinguished
“fathers” of the field such as Lamarck, Darwin, and Wallace.

The field of ecology, a relatively young offspring of this lineage, grew out of attempt explain
biogeography patterns in terms of the influence of environmental conditions interactions
among species. The development of biogeography, evolution, and ecology is tied to the
exploration. Early European explorers and naturalists did far more than just label catalogue
their specimens. They immediately, perhaps irresistibly, took to the comparing biotas across
regions and developing explanations for the similarity differences they observed. The
comparative method served these earlynaturalists and by the eighteenth century, the study of
biogeography began to crystallize are fundamental patterns of distribution and geographic
variation. We here trace development of biogeography from the age of exploration to its
current status mature and respected science.

Many, if not all, of the themes central to modern biogeography have their origins pre-
Darwinian Period. This is not to say that biogeography has not advert tremendously in the past
few decades- only that modern biogeography’s owe debt to those before them who shared the
same fascination with, and asked the types of questions about, the geography of nat
 The Age of Exploration
It is hard for us to appreciate that roughly 250 years ago, biologists had desk and classified only one
percent of all the plant and animal species we know to biogeography was essentially founded and
rapidly accelerated by world exploration the accompanying discovery of new kinds of organisms.

Biologists and naturalists of the eighteenth century were largely driven by calling serve God. The
prevailing belief was that the mysteries of creation would be reveal these naturalist/explorers
developed more complete catalogues of the diversity of life until the mid-eighteenth century the
prevailing world view was one of stasis- the earth, its climate and its species were immutable.
However, as the early biogeography’s (then called naturalists or simply geologists) returned with their
growing wealth of specimens and accounts, two things became clear. These are:
1. Biologists needed to develop a standardized and systematic scheme to classify the rapidly
growing wealth of specimens; and
2. It was becoming increasingly obvious that there were too many species to have been
accommodated by the biblical Noah’s ark.

It was just difficult for these early biologists to explain how animals and plants now isolated and
perfectly adapted to dramatically different climates and environments, could have coexisted at the
landing site of ark before they spread to populate all regions of the globe.

One of the most ambitious and visionary of these eighteenth – century biologists was Carolus Linnaeus
(1707-1778). He developed a scheme to classify all life- the system of binomial taxonomy that we
continue to use today.
[

Like his contemporaries, he believed that the earth and its species were immutable. He realized that
the chilling was not just in explaining the number of species, but in explaining patterns of diversity
as well. The rapidly growing list of species included organisms adapted to environments ranging from
the moist tropics to deserts, forests and tundra.

Linnaeus’s solution, although perhaps immature in retrospect, was logically sound hypothesized that
life had originated or survived the biblical flood, along the slopes Mount Ararat, a high mountain near
the border of Turkey and Armenia where the ark was said to have landed. At successively
higher elevations was a series of environments ranging from deserts to alpine tundra. Linnaeus reasons
that each of these elavational zones harbored a different assemblage of species each immutable but
perfectly adapted to that environment. Once the flood receded, these species migrated down from the
mountain and spread to eventually colonize and inhabit their respective environments in different
regions of the globe.

Comte de Buffon (1707-1788) was a contemporary of Linnaeus, but his studies of living and fossil
mammals led him to a very different view of the origin and spread of life. Buffon noted twoproblems
with Linnaeus’s explanation:
1. He observed that different portions of the globe, even those with the same climatic and
environmental conditions were often inhabited by distinct kinds of plants and animals. The
tropics, in particular contained a great diversity of organisms.
2. Buffon reasoned that Linnaeus’s view of the spread of life required that species migrate across
inhospitable habitats following the flood. Species adapted to Montana forests, for example,
would have had to migrate across deserts before they could colonize deciduous and coniferous
forests to the north. These environmental barriers would have blocked theirspread.

Buffon, therefore, hypothesized that life originated not on a mountain in the temperate regions of
Eurasia, but on a landmass in the far north, in an earlier period when climatic conditions were more
equable. He speculated that this northern landmass was relatively continuous with both the New and
Old Worlds; thus when climates cooled, life forms could have migrated south to their current locations.
During this migration, the populations of the New and Old Worlds were separated and became
increasingly modified, until tropical biotas of the New and Old worlds shared few if any forms. While
Buffon’s explanation may now seem imaginary, it provided two key elements of what would become
central parts of modern biogeography theory the ideas that climates and species are mutable.

Moreover, Button’s observation that environmentally similar but isolated regions have distinct
assemblages of mammals and birds became the first principle of biogeography, known as Buffon’s
law.
We can explain a lot about the current distribution of species and of similar species by looking at
how the land masses were positioned. One possible exception for the occurrence of similar species
that occur today in locations far away from each other, such as on separate continents, is that their
ancestors once occurred very near each other when the continents were arranged differently.

From 1750 to the early 1800s, many of Button’s colleagues continued to explore the diversity of nature
and to write catalogues and general syntheses of their work. One of the most prominent
naturalist/collectors of this period was Sir Joseph Banks, who, during a 3-year voyage around the
world with Captain James cook on the Endeavor (1768-1771), collected some 3600 plant specimens,
including over 1000 species not known to science (i.e., in addition to the 6000 species described by
Linnaeus in his species plant arum).

The efforts of Banks and many other naturalist/explorers resulted in two important developments.
First, they affirmed and generalized Button’s law. Second, they developed a much more thorough
understanding and appreciation for the complexity of the natural world.
Banks and his colleagues discovered some interesting exceptions to Button’s law specifically,
cosmopolitan species. Moreover, they noted other biogeography patterns, which in their own right
would become major themes as biogeography developed.

In the latter part of the eighteenth century, Johann Reinhold Forster (1729-1798) made manyimportant
contributions to phytogeography in particular and to biogeography in general. In his account of his
circumnavigation of the globe, he presented one of the first systematic world views of biotic regions,
each defined by its distinct plant assemblages. He found that Button’s law applied to plants as well as
to mammals and birds, and to all regions of the world, not just the tropics. Forster also described the
relationship between regional floras and environmental conditions and, how animal associations
changed with those of plants.

Buffon provided some important early insights into what was to become island biogeography and
species diversity theory. He 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 (island area
and variety of habitats). Forster also noted the tendency for plant diversity to decrease from the
equator to the poles, a pattern that he attributed to latitudinal trends in surface heat on earth.

In 1792, another German botanist, Karl Willdenow (1765-1812), wrote a major synthesis of plant
geography. He not only described the floristic provinces of Europe, but offered a novel explanation
for their origin. Rather than one site of creation (or survival during the biblical flood), Willdenow
suggested that there were many sites of origination –mountains that in ancient times were separated
by global seas. Each of these mountain refuges was inhabited by a distinct assemblage of locally
created plants. As the flood receded, these plants spread downward to form the floristic regions of
the world.

Ironically, it is not Willdenw, but one of his students, Alexander von Humboldt (1769-1859) who is
generally viewed as the father of phytogeography. Humboldt was able to advance the insights of his
mentor and add many of his own from his experiences in the New World Tropics. He was a great
naturalist, and was keenly aware that fundamental laws of nature could be discovered through the
study of distribution.

After studying the works of Latreille on arthropods and Curvier on reptiles, Humboldt further
generalized Button’s law to include plants as well as most terrestrial animals. Humboldt noted that the
floristic zonation that Forster described along latitudinal gradients could also be observed at a more
local scale along lavational gradients. Following conducting many floristic surveys in the Andes,
Humboldt concluded that even within regions, plants were distributed in lavational zones, or floristic
belts, ranging from equatorial tropical equivalent at low elevations to boreal and arctic equivalents at
the summits.

Thus, Forster, Willdenw and Humboldt all observed that plant assemblages were strongly associated
with local climate. One of Humboldt’s friends and colleagues, Swiss botanist Augustin P. de Candlle
(1778-1841), added another fundamentally important insight.

Some of the key elements of ecological biogeography and modern ecology were established by the
early 1800s. Moreover, in an essay published in 1820, Candolle appears to be the first to write about
competition and the struggle for existence, a theme that would prove central to the development of
evolutionary and ecological theory.

Biogeography in the Nineteenth Century

By the early 1800s, the first three themes of biogeography were well established. Biogeographies
were studying the distinctness of regional biotas, their origin and spread, and the factors responsible
for differences in the numbers and kinds of species among local and regional biotas.
Causal expansions would have to include factors accounting for the differences as well as the
similarities among isolated regions. Explanations for Buffon’s law would also have to account for
the exceptions to it.

During the next century, the legacy of Buffoon’s work would be evidenced by a long and continuing
succession of explanations for his law, which would lead from a view of a static earth populated by
immutable, cosmopolitan species to one in which the earth, its climate, and its species were dynamic.
This view would prove essential for classifying biogeography regions based on their biota and for
reconstructing the origin, spread, and diversification of life. Other biogeographies of the nineteenth
century would focus their energies on fundamental patterns of species richness.

Cosmopolitan species are species that are not localized to a certain area. They are found in many
areas of the earth’s surface.
For the field come of age, and to develop more rigorous, testable explanations for its fundamental
patterns, it would have to await three important advances of the nineteenth century:
1. A better estimate of the age of the Earth
2. A better understanding of the dynamic nature of the continents and oceans (i.e., continental drift
and plate tectonics), and
3. A better understanding of the mechanisms involved in the spread and diversification of species-
specifically, dispersal, vicariance, extinction and evolution.
To make these advances, biogeography had to draw on new discoveries in geology and paleontology.
During the nineteenth century, Adolph Brongniart (1801-1876) and Charles Lyell (1797-1875,
regarded as the fathers of palebeotany and geology, respectively, concluded that the
earth’s climate was highly changeable. Both men used the fossil record to infer conditions of past
climates. They found that many life forms adapted to tropical climates once flourished in the now
temperate regions of northern Europe.

Lyell documented that sea levels had changed, and that the earth’s surface had been transformed by
the lifting up and eroding down of mountains. This, he argued, was the only way to account for the
existence of marine fossils on mountain summits. Lyell also provided incontrovertible evidence for
the process of extinction. Many fossil forms, once dominant and presumably perfectly adapted to
existing climatic conditions, had perished and left no further trace in the fossil record. The causal
agent, again, was inferred to be climatic variation and associated changes in sea level.

Perhaps most prominent among the nineteenth-century naturalists were four British scientists namely
Charles Darwin, Joseph Dalton Hooker, Philip Lustily Sclater, and Alfred Russel wallace. Although
many others produced important works during this period, these four British scientists are responsible
for major advances in biogeography and evolutionary biology. They all studied theworks of
Linnaeus/ Buffon, Forster, Condole, and Lyell. They shared similar experiences as naturalists and
explorers, traveling to distant archipelagoes, high mountains, and tropical and temperate regions of
the New and old Worlds.

They also shared a common goal: account for the diversity of life, including the origin, spread, and
diversifying of biota’s. The development of biogeography and evolutionary biology, or that they
developed great mutual respect and lasting friendships. They reveal their shared preoccupation with
what we now call biodiversity and their conviction that the key to understanding the natural world was
to study pattern of distribution. Darwin referred to the study of geographic distribution as “that grand
subject, that almost keystone of the laws of creation.” The patterns of variability in the Galapagos
Archipelago, where different forms of tortoises and finches inhabited different islands, suggested to
him the idea that geographic isolation facilities inherited changes within and between populations.
It is difficult to overemphasize Darwin’s contribution to the field of biogeography. He along with
Wallace provided the basis for understanding changes in the adaptations and distribution of organisms
across time as well as space. He proposed that the diversification and adaptation of biotas from natural
selection, while the spread and eventual isolation disjunction of biotas resulted frame long-distance
dispersal.

Darwin argued this latter point perhaps more passionately and more convincingly than any one in the
history of biography. His arguments were drawn not only from inferences based on the distributions
of isolated biotas, but also on ingenious “experiments” on dispersal and colonization. Through these
studies Darwin was able to show that seemingly unlikely events, such as dispersal of seeds embedded
in mud clinging to the feet of birds, were the most likely means by which land plants had colonized
oceanic islands.

Darwin’s arguments on dispersal threatened to over turn the long-held static view of biogeography. In
the mid-nineteenth century, this older view was championed by Louis Agassiz (1807-1873) a Swiss-
born naturalist. Agassiz argued that not only were species immutable and static, but so were their
distributions, with each remaining at or near its site of creation. However, as a result of Darwin’s
arguments, which were later bolstered by those of Asa Gray and Alfred Wallace, the static view was
abandoned by most biogeography’s of the nineteenth century.

But the battles to be waged by the dispersalist camp had just begun. They were soon challenged by
much more formidable adversaries-the “extension’s,” whose ranks were on less prestigious than those
of the dispersalists, and included such respected scientists as Charles Lyell, Edward Forbes,and
Joseph Hooker.

Both camps agreed that distributions were dynamic in time and space. The extension’s, however,
argued that long-distance dispersal across great and persistent barriers was too unlikely to explain
distributional dynamics and related phenomena such as cosmopolitan species and disjunct
distribution. Rather they proposed that species had spread across great but now submerged land
bridges and ancient continents.
Despite Darwin’s persistent and passionate argument’s the extension’s camp would remain a viable
and influential force throughout the latter decades of the nineteenth century. One of its greatest
proponents was Joseph Dalton Hooker. He was a remarkably ambitious plant collector. During his
expedition, Hooker studied the floras of many southern landmasses, including Australia, Tasmania,
Tierra del fuego, and many archipelagoes in temperate and sub Antarctic waters. Hooker alsotraveled,
to Africa, Syria, India, and North America, where he studied the Horn of the Rocky Mountain region
with Asa Gray.

Hooker contended that, for the most part, long – distance was an insufficient explanation for the
distributions of species. He argued that the biogeography patterns and peculiarities of the southern
floras were not consistent with Darwin’s dispersals hypothesis.

During most of its early, biogeography was dominated by the contributions of botanists, from Linnaeus
and Candolle to Forster and Hooker. It is no great mystery why zoogeography lagged behind
phytogeography. There are many more animal species than plant species, and most animals are
relatively small and difficult to collect and identify.

In addition to Darwin and Hooker, two other British zoologists made major contributions to
biogeography: Philip Lutley Sclater and Alfred Russel Wallace. Sclater is an eminent ornithologist,
who first described 1067 species, 135 genera, 2 families of birds. Wallace, who is considered the father
of zoogeography, developed many of the basic concepts and tenets of the field combining the insights
of others with his own experiences as a collector and his theory of evolution through natural selection.
Many of the concepts enunciated by Wallace were actually introduced by earlier scientists, but Wallace
rested, documented, and interpreted them in evolutionary context.
Wallace collected innumerable specimens in the East Indies, and made many natural history
observations on the biota of that region. He was the first person to analyze faunal regions based on the
distributions of multiple of terrestrial animals.

Other Contributions in the Nineteenth Century

Other scientists of the 19th Century were also looking for and interpreting important patterns in
distributional data. Rather than considering only names and numbers of species, some of these
pioneering biogeography’s began to analyze geographic variation in the characteristics of individuals
and populations. Chief among these early contributors were the generalized morph geographic rules
of C.L. Gloger (1833), C. Bergmann (1847) and J.A.Allen (1878). Glover’s rule holds that, within a
species, individuals from more humid habitats tend to be darker in color than those from drier habitats.

Along this same line of reasoning, Allen’s rule states that among endothermic species, limbs and other
extremities are shorter and more compact in individuals living in colder climates. Birds and mammals
of Polar Regions tend to be stout with short limbs.

A similar phenomenon was reported by D.S. Jordan in 1891 for exothermic (cold-blooded) tallest
fishes inhabiting marine environments. According to Jordan’s law of vertebrae, as one moves farther
from the equator, the vertebrae of tallest fishes become smaller and more numerous.

In addition to studying geographic variation in the traits of individuals, biogeography’s noted that
the demographic characteristics of populations also varied across regions. In 1859, Darwin observed
that within most genera, species, which range widely over the world, are the most diffused in their
own country, and are most numerous in individuals.
In other words, wide-ranging species tend to occur at relatively high densities. This pattern, while
not a major emphasis of early biogeography, was rediscovered and documented for a variety of taxa
during the twentieth century, the study of how population-level parameters vary along geographic
dimensions is now a central focus of an emerging discipline in biogeography termed as macro
ecology.

Some early evolutionary “rules” were described by paleontologists who searching for patterns in the
history of life and trying to interpret the fossil record. The theory of orthogenesis is example. This
theory states that the evolution of a group continues in only one direction and that this orientation is
an intrinsic property of the organism and is not controlled by natural selection.

A special type of orthogenesis was Cape’s rule, which states that the evolution of a group shows a
trend toward increased body size. Although there are many exceptions to this rule, it does seem that
certain advantages of large size have resulted in repeated increases in size in many animal lineages.
Large body size, however, seems to make species susceptible to extinction, so that large forms (such
as the dinosaurs and many now extinct groups of giant birds and mammals) die out and is replaced
by large representatives of new groups, which in turn evolve to a larger size.

Early botanists such as Candle and Humboldt were well aware that different types of vegetation
occurred at different elevations, but a zoologist, C. Hart Merriam (1894), provided one of the most
valuable insights into these broad patterns. Based on his extensive field studies in southwestern North
America, Merriam confirmed that lavational changes in vegetation type and plant species composition
are generally equivalent to the latitudinal vegetation changes found as one movestoward the
poles. He called these belts of similar vegetation life zones.
Although Merriam was not successful in generalizing his concept of life zones to animals and to other
region of North America, he correctly concluded that lavational zonation of vegetation, like latitudinal
zonation, is a response of species and communities environmental gradients of temperature and
rainfall.
 The First Half of the Twentieth Century
From 1900 through the early 1960, several major trends in research had extraordinary effects on
biogeography. Paleontology in particular deserves credit for providing new and fascinating
descriptions of faunal changes on each continent.

Numerous paleontologists (C. Ameghino, W.D. Matthew, G,G. Simpson, E.H Colbert A.S. Romer,
E.C Olson, and B. Kurten), described the origin, dispersal, radiation, and decline of land vertebrates.
They showed that new groups increase in number of species, radiate to fill new ecological roles,
expand their geographic ranges, and become dominant over and contribute to the extinction of older
forms. Thus, our present-day continental faunas have extremely long and complex histories that can
be understood only by elucidating the phylogenies of the groups involved that can be understood
only by elucidating the phylogenies of the groups involved and the history of their movements between
landmasses.

Early in the twentieth century, researchers began to investigate patterns of variation within single
species. Following the lead of Bergmann and Allen on choreographic patterns, Joseph Grinnell, Lee
R. Dice, and B. Rensch-demonstrated close relationship between geographic and ecological properties
of the environment and patterns of morphological variation with and among species.

By early 1940s, evolutionary biologists were building on Darwin’s syntheses to investigate patterns
of geographic variation and to infer the mechanisms responsible for the origin of new species. Along
list of scientists contributed to our understanding of the modes of speciation.

Ernst Mary made major contributions in the fields of systematic, evolution and historical
biogeography. Out of this work arose one unifying theme, the biological species concept, which states
that a species is definable as a group of populations that is reproductively isolated from all other such
groups. Moreover, Mary’s studies of patterns in the known as allopathic speciation enabled an
important new synthesis in evolutionary biology and biogeography.
By the middle of the twentieth century, many authors had produced new and more general syntheses
of biogeography patterns for various taxa. These included studies focusing on: vertebrates by Phillip,
marine zoogeography, vascular plants, plants, and island biogeography.

Biogeography since the 1950s

Four major developments have refreshed biogeography in the last 40 years. These are:
1. The acceptance of plate tectonics,
2. The development of new phylogenetic methods.
3. New ways of conducting research in ecological biogeography, and
4. Investigations of the mechanisms that limit distributions.

Up to the 1960s the emphasis in biogeography had been primarily an evolutionary and historical one,
emphasizing the phylogenies of groups and their means of dispersing in to and surviving in different
regions and habitats. By the late 1950s, G.E. Hutchinson began to focus attention on questions about
the processes that determine the diversity of life and the number of species that coexist in local areas
or habitats. Ecologists began to emphasis the importance of competition, predation, and mutualism
in influencing the distributions of species and their coexistence as ecological communities.

Biogeography and other Fields of study

Biogeography, as a recent field of synthetic science is still dependant on several other sciences for its
investigation. It, therefore, is related to a number of natural and social sciences with respect to the
various topics, which it studies about life. In other words, it has to draw basic knowledge and
detailed facts and figures required in the description and explanation of the distributions of living
organisms on Earth from other sciences. Among the various fields of studies to with which
biogeography has strong relationships are biology, geology, genetics evolution, climate, etc. In this
respect, it is much more related to ecology than any other field of science.

Ecology is the science of the interrelations between organisms among themselves and their
environment. It is concerned especially with biology of groups of organisms and their functional
processes on the lands, in the oceans and in fresh waters. Thus, it is interested in the study of the
structure and function of nature (it being understood that mankind is a part of nature).
What are the similarities and difference between biogeography and ecology? Both
biogeography andecology are similar in that their subject matter is the same. The basic
difference between them lies in the emphasis of the aspect of life, which they study.
Ecology confines itself strictly to the study ofthe relationships of organisms among
themselves and that to the physical environment, while biogeography places much
emphasis on the explanation of the distributional inequalities of organisms on the surface
of the Earth.

Problems in the study of biogeography

As it is true in other fields of studies, biogeography has several problems, the leading
being the following:

The main problem in the study of biogeography arises from the fact that its scope and
field of interest is not clearly demarcated. While some scholars would mainly confine its
content to the study of plant and animal geography, with emphasis upon the former,
others would like it to be broader to include the study of many aspects of physical and
human geography, soils, evolution, ecology, genetics, anthropology and many other
branches of physical and social sciences.

Another basic problem is that in many parts of the world, a full count of species and
varieties of organisms has not yet been made and the consequences of their interactions
with each other are little understood. In the absence of such basic data, it is generally
difficult to talk about clearly defined patterns of distribution of organisms at different
scales.

Besides, the techniques of measuring environmental components that affect organisms

such as heat and moisture balance of the Earth and the atmosphere are complicated and
recently introduced. Hence, our knowledge of the details of the pathways of energy and
chemical substances between organisms and the environment is imprecise and less
definitely known.