CHAPTER 5
Biogeography
Global Provincialism of How do scientists explain the abundance of marsupials in Australia and
Mammalian Distributions South America, along with their scarcity on northern continents? Why
Biogeographic Regionalization are there members of Camelidae in central Asia, North Africa, and
Faunal Regions South America? What factors led to the present distribution of pri-
mates, extending from Japan to Africa and including South, but not
Historical Biogeography North, America? Biogeography is the study of the distribution of or-
Abiotic Processes ganisms, both living and extinct, on the Earth (Lomolino et al. 2017).
Biotic Processes We have already encountered geographic considerations in our
Biogeographic Inference discussion of home ranges (the “distributions” of individual organ-
Examples isms), faunal surveys, and phylogeographic analyses of species bound
aries (Chapter 2), as well as the influence of continental drift on
Ecological Biogeography mammalian radiations (Chapter 3). The most basic datum in bioge-
Ecogeographic Patterns in ography is the species range—t he complete area of the Earth over
Mammals which individuals of a particu lar species occur. Species ranges are
Gradients in Species Diversity usually inferred from museum-specimen records, but observational
data and ecological modeling are also important for mammals. Ranges
are dynamic, changing over time b ecause of abiotic and biotic f actors.
For example, lions were once widespread throughout Africa and
southwestern Asia; t oday, they are restricted to several isolated popu-
lations scattered throughout Africa and one small population in
northwest India (Figure 5.1). A fundamental question posed by bio-
geography is, what f actors determine the range of a species? The same
question, but from a slightly different perspective, is also important:
why does a particu lar region harbor the particu lar set of species we
observe there? The answers invariably have to do with two kinds of
causal factors, history and ecology, that define major research tradi-
tions within biogeography.
Historical biogeography emphasizes the study of changes in spe-
cies ranges that have taken place over evolutionary time. It encom-
passes evolutionary and earth history, and brings information from
both to bear on biogeographic problems. One of the distribution pat-
terns most intriguing to historical biogeographers is endemism, the
restriction of a species’ range to a circumscribed area. Why, for ex-
ample, are long-beaked echidnas (Zaglossus bruijni) found only in
New Guinea? Even more striking are patterns of endemism that
80
, Chapter 5 Biogeography 81
Global Provincialism of
Mammal Distributions
BIOGEOGRAPHIC REGIONALIZATION
If one w ere to tabulate the numbers of species in major
clades of virtually any group of animals or plants that oc-
cur in different continental regions of the Earth, two pat-
terns would be readily apparent. First, different regions
harbor distinct taxonomic assemblages—that is to say,
t here is endemism on a worldwide scale. Second, t here are
dramatic differences in species richness among continen-
tal regions: some regions constitute centers of diversity
and others do not. T hese observations, together with
knowledge of phylogenet ic relationships, demonstrate the
provincialism of life on Earth, a pattern evident in the tet-
Figure 5.1 Changes in species range. The lion (Panthera rapod fossil record since the Early Mesozoic (Sidor et al.
leo) was once distributed throughout much of Africa and
southwestern Asia, including the Arabian Peninsula (dark blue
2013). Provincialism in terrestrial animal distributions led
shading). Today, lions still inhabit many areas of Africa (black Wallace (1876) to divide the world into 6 faunal regions,
shading), but their range in Asia has been reduced to a small each with a distinct assemblage of species: Palearctic, Ne-
remnant population in the Gir Forest of India (black dot with arctic, Neotropical, Ethiopian, Oriental, and Australian
arrow). Redrawn from Burton and Pearson (1987). (Figure 5.2). This was one of the first attempts at biogeo-
graphic regionalization, the estimation of boundaries be-
tween areas of endemism/centers of diversity. Darlington
characterize areas— why are so many mammal species (1957) and Simpson (1965) provided important syntheses
found only in New Guinea (Flannery 1995)? A second of descriptive information on vertebrate distributions, gen-
pattern of interest is the disjunct distribution—a gap in erally endorsing the regions recognized by Wallace. Re-
the range of related species or clades. Marsupials are now cently, Holt and colleagues (2013) refined the boundaries of
found in Australasia and South America. How does this world zoogeographic regions based on distributional data
distribution relate to the evolutionary history of marsu- and phylogenetic relationships for 21,037 species of am-
pials? Do the species on each continent represent sepa- phibians, birds, and mammals. They identified 11 major
rate monophyletic groups that are each other’s closest realms comparable to Wallace’s faunal regions, but recog-
relatives? How did these groups, which clearly have a nized distinct realms for some of Wallace’s transition zones.
single common ancestor, become separated by two For example, Holt and colleagues (2013) distinguished a
oceans? It is often the case that several groups show the Panamanian realm comprising the Central American com-
same disjunctions: monotremes, though currently en- ponent of Wallace’s Neotropics, a Saharo-A rabian realm
demic to Australasia, also have fossil representatives in across northern Africa to the western edge of the Indian
South America (Pascual et al. 1992). subcontinent, and a Sino-Japanese realm from the Tibetan
Ecological biogeography focuses on the current dis- Plateau to the China coast. They also assigned New Guinea
tributions of species and seeks to explain those distribu- to Oceania and Madagascar to its own realm. The extent to
tions in terms of community-level interactions among or- which this new regionalization will replace Wallace’s origi-
ganisms and their environment. One common line of nal remains to be seen (Kreft and Jetz 2013), but the work
inquiry has to do with species richness: why do some re- highlights the increasing interdependence of biogeography
gions of the Earth (e.g., the tropics) harbor vastly more and phylogenetics (Knapp 2013).
species than other regions (e.g., Antarctica)? What deter- As reflected in Holt and colleagues (2013), biogeogra-
mines the number and identity of species on an island? phers are intrigued by transition zones between regions, as
Because answers to such questions involve evolutionary well as what species compositions in these zones can tell us
adaptations, ecological biogeography frequently entails about the historical-ecological determinants of biodiversity.
studying the patterns of morphological, physiological, or Perhaps the most famous transition zone is that between
life-history variation among organisms in different places. Wallace’s Oriental and Australian regions, where the posi-
Until recently, historical and ecological biogeography were tion of Wallace’s Line (Figure 5.2) has stimulated over a
largely separate disciplines (Posadas et al. 2006), but prac century of studies on the mixture of faunal elements in the
tit ioners now realize that both perspectives are necessary Malay Archipelago (van Oosterzee 2006, Esselstyn et al.
to arrive at complete explanations of geographic patterns 2010). Although Wallace and subsequent authors noted a
(Morrone 2009). sharp break in faunal compositions between the islands of
Biogeography
Global Provincialism of How do scientists explain the abundance of marsupials in Australia and
Mammalian Distributions South America, along with their scarcity on northern continents? Why
Biogeographic Regionalization are there members of Camelidae in central Asia, North Africa, and
Faunal Regions South America? What factors led to the present distribution of pri-
mates, extending from Japan to Africa and including South, but not
Historical Biogeography North, America? Biogeography is the study of the distribution of or-
Abiotic Processes ganisms, both living and extinct, on the Earth (Lomolino et al. 2017).
Biotic Processes We have already encountered geographic considerations in our
Biogeographic Inference discussion of home ranges (the “distributions” of individual organ-
Examples isms), faunal surveys, and phylogeographic analyses of species bound
aries (Chapter 2), as well as the influence of continental drift on
Ecological Biogeography mammalian radiations (Chapter 3). The most basic datum in bioge-
Ecogeographic Patterns in ography is the species range—t he complete area of the Earth over
Mammals which individuals of a particu lar species occur. Species ranges are
Gradients in Species Diversity usually inferred from museum-specimen records, but observational
data and ecological modeling are also important for mammals. Ranges
are dynamic, changing over time b ecause of abiotic and biotic f actors.
For example, lions were once widespread throughout Africa and
southwestern Asia; t oday, they are restricted to several isolated popu-
lations scattered throughout Africa and one small population in
northwest India (Figure 5.1). A fundamental question posed by bio-
geography is, what f actors determine the range of a species? The same
question, but from a slightly different perspective, is also important:
why does a particu lar region harbor the particu lar set of species we
observe there? The answers invariably have to do with two kinds of
causal factors, history and ecology, that define major research tradi-
tions within biogeography.
Historical biogeography emphasizes the study of changes in spe-
cies ranges that have taken place over evolutionary time. It encom-
passes evolutionary and earth history, and brings information from
both to bear on biogeographic problems. One of the distribution pat-
terns most intriguing to historical biogeographers is endemism, the
restriction of a species’ range to a circumscribed area. Why, for ex-
ample, are long-beaked echidnas (Zaglossus bruijni) found only in
New Guinea? Even more striking are patterns of endemism that
80
, Chapter 5 Biogeography 81
Global Provincialism of
Mammal Distributions
BIOGEOGRAPHIC REGIONALIZATION
If one w ere to tabulate the numbers of species in major
clades of virtually any group of animals or plants that oc-
cur in different continental regions of the Earth, two pat-
terns would be readily apparent. First, different regions
harbor distinct taxonomic assemblages—that is to say,
t here is endemism on a worldwide scale. Second, t here are
dramatic differences in species richness among continen-
tal regions: some regions constitute centers of diversity
and others do not. T hese observations, together with
knowledge of phylogenet ic relationships, demonstrate the
provincialism of life on Earth, a pattern evident in the tet-
Figure 5.1 Changes in species range. The lion (Panthera rapod fossil record since the Early Mesozoic (Sidor et al.
leo) was once distributed throughout much of Africa and
southwestern Asia, including the Arabian Peninsula (dark blue
2013). Provincialism in terrestrial animal distributions led
shading). Today, lions still inhabit many areas of Africa (black Wallace (1876) to divide the world into 6 faunal regions,
shading), but their range in Asia has been reduced to a small each with a distinct assemblage of species: Palearctic, Ne-
remnant population in the Gir Forest of India (black dot with arctic, Neotropical, Ethiopian, Oriental, and Australian
arrow). Redrawn from Burton and Pearson (1987). (Figure 5.2). This was one of the first attempts at biogeo-
graphic regionalization, the estimation of boundaries be-
tween areas of endemism/centers of diversity. Darlington
characterize areas— why are so many mammal species (1957) and Simpson (1965) provided important syntheses
found only in New Guinea (Flannery 1995)? A second of descriptive information on vertebrate distributions, gen-
pattern of interest is the disjunct distribution—a gap in erally endorsing the regions recognized by Wallace. Re-
the range of related species or clades. Marsupials are now cently, Holt and colleagues (2013) refined the boundaries of
found in Australasia and South America. How does this world zoogeographic regions based on distributional data
distribution relate to the evolutionary history of marsu- and phylogenetic relationships for 21,037 species of am-
pials? Do the species on each continent represent sepa- phibians, birds, and mammals. They identified 11 major
rate monophyletic groups that are each other’s closest realms comparable to Wallace’s faunal regions, but recog-
relatives? How did these groups, which clearly have a nized distinct realms for some of Wallace’s transition zones.
single common ancestor, become separated by two For example, Holt and colleagues (2013) distinguished a
oceans? It is often the case that several groups show the Panamanian realm comprising the Central American com-
same disjunctions: monotremes, though currently en- ponent of Wallace’s Neotropics, a Saharo-A rabian realm
demic to Australasia, also have fossil representatives in across northern Africa to the western edge of the Indian
South America (Pascual et al. 1992). subcontinent, and a Sino-Japanese realm from the Tibetan
Ecological biogeography focuses on the current dis- Plateau to the China coast. They also assigned New Guinea
tributions of species and seeks to explain those distribu- to Oceania and Madagascar to its own realm. The extent to
tions in terms of community-level interactions among or- which this new regionalization will replace Wallace’s origi-
ganisms and their environment. One common line of nal remains to be seen (Kreft and Jetz 2013), but the work
inquiry has to do with species richness: why do some re- highlights the increasing interdependence of biogeography
gions of the Earth (e.g., the tropics) harbor vastly more and phylogenetics (Knapp 2013).
species than other regions (e.g., Antarctica)? What deter- As reflected in Holt and colleagues (2013), biogeogra-
mines the number and identity of species on an island? phers are intrigued by transition zones between regions, as
Because answers to such questions involve evolutionary well as what species compositions in these zones can tell us
adaptations, ecological biogeography frequently entails about the historical-ecological determinants of biodiversity.
studying the patterns of morphological, physiological, or Perhaps the most famous transition zone is that between
life-history variation among organisms in different places. Wallace’s Oriental and Australian regions, where the posi-
Until recently, historical and ecological biogeography were tion of Wallace’s Line (Figure 5.2) has stimulated over a
largely separate disciplines (Posadas et al. 2006), but prac century of studies on the mixture of faunal elements in the
tit ioners now realize that both perspectives are necessary Malay Archipelago (van Oosterzee 2006, Esselstyn et al.
to arrive at complete explanations of geographic patterns 2010). Although Wallace and subsequent authors noted a
(Morrone 2009). sharp break in faunal compositions between the islands of
