Chapter 24: History and Biogeography

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Chapter 24 History and Biogeography

• Robert E. Ricklefs
• The Economy of Nature, Fifth Edition

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History and Biogeography

• The origin and maintenance of the earths
  biodiversity is one of ecologys central issues
• has earths biodiversity been maintained at a
  steady state or has it varied through time?
• the answer to this question can help us choose
  between equilibrium and nonequilibrium viewpoints
• We can look to the fossil record for evidence of
  past changes in biological diversity
• the record over the past 600 million years yields
  useful information about the history of
  biodiversity

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Diversity has generally increased over geologic
time
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Biodiversity in the Fossil Record

• Findings vary for different groups
• diversity has remained constant in some groups,
  while increasing in others
• increasing in flowering plants, fishes, birds,
  and mammals
• diversity has also decreased dramatically in many
  groups at various times
• regional species pools have declined because of
  catastrophic events and grown because of
  biological diversification
• we must question whether ecological systems ever
  truly achieve equilibrium

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The Tropical Zone

• Can we find explanations for tropical
  biodiversity at large temporal and spatial
  scales?
• tropical conditions appeared on earth much
  earlier than colder conditions
• tropical and subtropical zones now cover much
  more area than temperate and polar regions, even
  more so in times past

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A Record of Dramatic Changes

• Both high and low latitudes experienced drastic
  fluctuations in climate and extent during the Ice
  Age of the past 2 million years
• during periods of glacial expansion
• the tropics experienced low rainfall and reduced
  temperatures
• rain forest habitats were restricted and
  fragmented

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Biological History

• The history of life reveals itself to us in
• the geochemical record of past environments
• fossil traces left by long-extinct taxa
• geographic distributions and evolutionary
  relationships of living species
• the most obvious consequences of this history is
  the nonuniform distribution of plant and animal
  forms over the earths surface
• every part of the earth has a distinctive fauna
  and flora

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Phylogenetic Effects

• Morphology, physiology, and behavior of organisms
  reflect
• conditions and resources of their present
  environments
• constraints imposed by the characteristics of
  their ancestors
• marsupials are presently most diverse in
  Australia
• this pattern is not the result of unique
  ecological properties of the continent of
  Australia, but rather of evolutionary inertia
• characteristics shared by a lineage irrespective
  of environmental factors are phylogenetic effects

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Australias unusual terrestrial organisms (a)
Eucalyptus (b) Banksia inflorescence (C ) red
kangaroo
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Phylogenetic effects influence ecology.

• Phylogenetic effects affect the structure and
  functioning of ecosystems
• would Australian ecosystems function in the same
  manner if eucalyptus were replaced by some other
  kinds of plants?
• would forests of different kinds of trees be less
  susceptible to fires?
• if so, what consequences would this have for
  ecosystem function?

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The history of life can be gauged by the
geological time scale.

• Ecologists recognize key features of the
  geological record
• earth formed 4.5 billion years ago
• life arose within the first billion years
• life remained primitive for most of earths
  history
• ancient physical environments were quite
  different from those of the present
• the early atmosphere had little oxygen and early
  microbes used anaerobic metabolism
• increased oxygen led to diversification of
  complex life forms

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The Geologic Record

• About 590 Mya, most of the modern phyla of
  invertebrates appeared in the fossil record
• these early animals began to protect themselves
  with hard shells, which make excellent fossils
• the Paleozoic era is thus the first of three
  major divisions of geologic time reflecting
  diversification of animals
• Paleozoic 590 Mya to 248 Mya
• Mesozoic 248 Mya to 65 Mya
• Cenozoic 65 Mya to present

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Hardened outer shells in the Cambrian seas
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Continental Drift

• The continents are islands of low-density rock
  floating on the denser material of the earths
  interior and carried along by convection
  currents
• the movements of the continents over time are
  called continental drift
• These movements have two important ecological
  consequences
• positions of continents, ocean basins influence
  climate
• continental drift creates and breaks barriers to
  dispersal

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Continental Drift Mesozoic to Present

• In the early Mesozoic era, 200 Mya, continents
  formed a single giant landmass called Pangaea
• By 144 Mya (beginning of the Cretaceous period)
  the northern continents (Laurasia) had separated
  from the southern continents (Gondwana)
• at this time Gondwana itself was also breaking
  apart
• By the end of the Mesozoic era (65 Mya), South
  America and Africa were widely separated, and
  many other patterns were emerging.

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Positions of the continents have changed over
geologic timeMya 200 million years before)
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Continental drift changed routes of dispersal
(units in millions of years when dispersal
routes were broken or created)
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Consequences of Continental Drift

• Details of continental drift have yet to be
  resolved, but implications for evolution of
  animals and plants are clear for example
• the distributions of the flightless ratite birds
  (such as ostriches) are the results of connection
  between the southern continents that made up
  Gondwana
• these birds are descended from a common Gondwanan
  ancestor
• splitting of a widely distributed ancestral
  population by continental drift is called
  vicariance

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Lineages of ratite birds separated by
fragmentation of Gondwana
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Biogeographic Regions

• The modern distributions of animals led Alfred
  Wallace to recognize six major biogeographic
  regions
• these correspond to landmasses isolated millions
  of years ago by continental drift
• over the course of this isolation, the animals
  and plants of these regions evolved independently
  and developed distinctive characteristics

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Major zoogeographic regions of the earth based
on distribution of animals
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Biogeographic Regions 1

• Nearctic - North America
• maintained connections to Palearctic for 100 My
• Palearctic - Eurasia
• shares many groups of plants and animals with
  Nearctic
• Ethiopian - Africa
• has a long history of isolation from the rest of
  the world

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Biogeographic Regions 2

• Australian - Australia
• has a long history of isolation from the rest of
  the world
• Oriental - Southeast Asia
• isolated from rest of tropical world, but has
  some affinities to Palearctic, where a high
  percentage of trees are derived from tropical
  forests
• Neotropical - South America
• isolated from Nearctic until about 3 Mya
  (formation of isthmus of Panama)

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Changes in Climate 1

• The distribution of heat over the surface of the
  earth depends largely on circulation of the
  oceans
• 50 to 30 Mya, polar regions were covered by
  oceans that extended to tropical regions,
  resulting in much warmer polar climates
• after this time, drifting continents curtailed
  this circulation, resulting in a cooling and
  drying trend at high latitudes
• result was greater stratification, during later
  Tertiary, of temperate and tropical biotas with
  distinctive adaptations

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Changes in Climate 2

• During the past 2 million years, gradual cooling
  of the earth gave way to violent oscillations in
  climate, the Ice Age or Pleistocene epoch
• glacial advances drove temperate species
  southward and may have restricted tropical
  species to isolated refuges with moist conditions
• migrations of forest trees in eastern North
  America have been well documented
• after the last glacial retreat beginning 18,000
  years ago, a general pattern of reforestation
  ensued

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Changes in Climate 3

• Migrations of trees in eastern North America from
  18,000 years ago to present are known from pollen
  grains deposited in bogs and lakes
• the compositions of communities shifted as
  species migrated across the landscape
• in particular, the composition of forests during
  the past 18,000 years has
• included combinations of species that do not
  occur today
• lacked combinations of species that do occur at
  present

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Catastrophes 1

• The Mesozoic era (Cretaceous period) ended with a
  catastrophic disturbance 65 Mya
• evidence points to collision of an asteroid with
  Earth that struck in shallow seas off the Yucatan
  Peninsula of Mexico
• much of earths biomass was destroyed by massive
  tidal waves, fires, and ensuing darkness and cold
  temperatures
• among the groups falling victim to this mass
  extinction were the dinosaurs
• other groups (birds, mammals) survived and may
  have taken advantage of empty niches vacated by
  extinct groups

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Catastrophes 2

• Major catastrophes have occurred at intervals of
  10 to 100 millions of years
• such events have disrupted ecosystems and changed
  the course of community development
• thousands of years may be required for
  environmental conditions to return to normal
• such events may also
• eliminate species and thus reduce diversity
• foster rapid evolutionary responses to new
  conditions
• create opportunities for development of new
  biological associations

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Convergence

• Convergence is the process whereby unrelated
  species living under similar ecological
  conditions come to resemble one another more than
  their ancestors did
• there are numerous examples of convergence
• woodpecker-like birds that fill the woodpecker
  niche in many systems lacking woodpeckers
• similarities of plants and animals of North and
  South American deserts
• similar body forms of dolphins and penguins,
  which both resemble tuna, whose swimming
  lifestyle they share

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convergence
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Convergence is often incomplete.

• Detailed study often turns up remarkable
  differences between plants and animals occupying
  superficially similar habitats
• the ancient Monte Desert of South America lacks
  the bipedal, seed-eating, water-independent
  rodents of North America (kangaroo rats) and Asia
  (gerbils)
• superficially similar lizards of Australia and
  North America differ in diet, optimal activity
  temperature, burrowing behavior, and annual cycle

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Isolation has unique evolutionary consequences.

• Many of the unique attributes of the reptile
  fauna of Australia may be related to poor soils
• most Australian soils are old, deeply weathered,
  and have few nutrients
• plants have low nutrient content and high levels
  of toxic substances
• these plants support few insects
• birds, which depend on insects, are not common
• released from bird predation, lizards have
  proliferated in ways not possible elsewhere

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Local Community Diversity

• To what extent are community attributes, such as
  diversity, convergent?
• do local processes determine numbers of
  coexisting species?
• are numbers of species in communities occupying
  similar habitats independent of the regional
  species pool?
• to what extent do regional differences in
  diversity also contribute to local diversity?

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A Test for Local Control of Diversity

• If regional processes influence local diversity,
  then local communities should sample regional
  species pools in the same proportion
• this would result in a linear relationship
  between local and regional diversity
• local control would result in saturation, beyond
  which increasing regional diversity would add
  nothing to local diversity
• Available data support the idea that communities
  are open to invasion at any level of diversity
  when more species are present in the regional
  species pool
• seen in data for fish communities in Africa and
  South America

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Processes on many scales regulate biodiversity.

• History and geographic position may influence
  diversity of an entire region and its local
  inhabitants
• interactions of species within local habitats
  make up only half of the diversity equation!
• for example, mangroves in the Indo-West Pacific
  region are far more diverse than mangroves in the
  Caribbean
• both regions have roughly equal areas of a
  similar variety of mangrove habitats

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Indo-West Pacific versus Caribbean Mangroves

• Differences in diversity in mangroves of these
  regions appear related to several factors
• plant taxa have invaded the mangrove habitat more
  frequently in the Indo-West Pacific region
• fewer lineages in the Indo-West Pacific region
  appear to have suffered extinctions
• wet conditions may have prevailed in Southeast
  Asia through much of the Tertiary, while adjacent
  terrestrial habitats in the Caribbean many have
  been dry during the latter Tertiary
• fragmentation of the Indo-West Pacific habitats
  may have isolated populations and fostered
  speciation

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Summary 1

• Life first appeared several billion years ago,
  but modern forms appeared about 590 Mya, the
  point marking the beginning of the Paleozoic era.
• More recent eras include the Mesozoic (beginning
  248 Mya) and Cenozoic (beginning 65 Mya).
• Continental drift has altered climates and
  pathways of dispersal among the continents.

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Summary 2

• Animals and plants have evolved to some extent
  independently in each of six major biogeographic
  regions.
• The climate of the earth cooled during the
  Cenozoic, leading to greater distinctions between
  tropical and temperate biotas.
• Glacial advances during the Ice Age resulted in
  shifts in distribution and extinctions of many
  species of plants and animals.

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Summary 3

• Catastrophes have punctuated the development of
  life on earth, resulting in mass extinctions and
  new opportunities for surviving lineages.
• Convergence is often observed in biota of similar
  but geographically isolated regions.
• Nonconvergence in diversities of biotas from
  similar habitats indicates the role played by
  regional species pools in determining local
  diversity.