Transcript Chapter 24: History and Biogeography

Chapter 24: History and
Biogeography
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
(c) 2001 by W. H. Freeman and
Company
History and Biogeography
The origin and maintenance of the earth’s
biodiversity is one of ecology’s central issues:
has earth’s 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
(c) 2001 by W. H. Freeman and
Company
Diversity has generally increased over geologic time
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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
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 earth’s surface:
every part of the earth has a distinctive fauna and
flora
(c) 2001 by W. H. Freeman and
Company
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 (c)
factors
effects
2001 byare
W. H.phylogenetic
Freeman and
Company
Australia’s unusual terrestrial organisms: (a) Eucalyptus; (b)
Banksia inflorescence; (C ) red kangaroo
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?
(c) 2001 by W. H. Freeman and
Company
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 earth’s 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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
Hardened outer shells in the Cambrian seas
Continental Drift
The continents are islands of low-density rock
floating on the denser material of the earth’s
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
(c) 2001 by W. H. Freeman and
Company
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.
(c) 2001 by W. H. Freeman and
Company
Positions of the continents have changed over geologic time
Mya: 200 million years before)
Continental drift changed routes of dispersal
(units in millions of years – when dispersal routes were
broken or created)
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
(c) 2001 by W. H. Freeman and
Company
Lineages of ratite birds separated by fragmentation of
Gondwana
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
(c) 2001 by W. H. Freeman and
Company
Major zoogeographic regions of the earth –
based on distribution of animals
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
(c) 2001 by W. H. Freeman and
Company
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)
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
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 earth’s 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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
convergence
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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
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?
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
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
(c) 2001 by W. H. Freeman and
Company
speciation
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.
(c) 2001 by W. H. Freeman and
Company
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.
(c) 2001 by W. H. Freeman and
Company
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.
(c) 2001 by W. H. Freeman and
Company