Origins

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Origins Maintenance of Diversity
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Origins Maintenance of Diversity
What are we trying to explain? A variety of
patterns, at multiple scales, including 1.
Origin of diversity (actually S) within lineages
What are we trying to explain?
For example, why do some lineages contain more
species than others? This is a macroevolutionary
question relevant processes are speciation
extinction (therefore, including all the
processes that influence speciation extinction
rates)
Whenever the rate of speciation outpaces
extinction, diversity increases
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Origins Maintenance of Diversity
What are we trying to explain? A variety of
patterns, at multiple scales, including 2.
Origin of diversity (actually S) within a site
A site may be a very large area (e.g., a
continent), or a very small area (e.g., a 1-m2
quadrat of ground cover within a Louisiana pine
savanna) By what combination of processes did
the species occupying a site appear within the
site? How did the community assemble?
The question may also be restricted to a given
taxon, guild, etc. The relative influences of
macroevolutionary processes (e.g., speciation)
vs. ecological processes (such as colonization,
competition), may differ from case-to-case and
are likely to be scale dependent
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Origins Maintenance of Diversity
What are we trying to explain? A variety of
patterns, at multiple scales, including 2.
Origin of diversity (actually S) within a site
Appearance
Extant S
Disappearance
Cumulative S
t
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Origins Maintenance of Diversity
What are we trying to explain? A variety of
patterns, at multiple scales, including 2.
Origin of diversity (actually S) within a site
cradle effect
Extant S
Appearance
Disappearance
Cumulative S
t
For more on cradle museum effects, see
Chown Gaston (2000)
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Origins Maintenance of Diversity
What are we trying to explain? A variety of
patterns, at multiple scales, including 2.
Origin of diversity (actually S) within a site
museum effect
Appearance
Extant S
Disappearance
Cumulative S
t
For more on cradle museum effects, see
Chown Gaston (2000)
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Origins Maintenance of Diversity
Appearance (Speciation and Colonization)
Disappearance (Extinction and Extirpation)
At the largest spatial scales, speciation
extinction dominate
At the smallest spatial scales, colonization
extirpation dominate
Sregional
Cumulative S
Slocal
t
t
t1
t1
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Origins Maintenance of Diversity
What are we trying to explain? A variety of
patterns, at multiple scales, including 3.
Maintenance of diversity (S or D) within a site
Is status quo species richness maintained over
the long term? How to define the long term?
Forever? Relative to the lifespans of the
organisms?
If so, is status quo species composition
maintained?
In other words, do we need a mechanism that
counters species loss that is equilibrial for S
alone, or also for species composition?
Speciation colonization could account for
equilibrial S, but additional mechanisms would be
required for the maintenance of species
composition (species list and relative abundances)
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Origins Maintenance of Diversity
What are we trying to explain? A variety of
patterns, at multiple scales, including 4.
Origin of gradients in diversity (usually S)
Why do sites differ in diversity? At the
largest spatial scales of sampling (comparisons
of regions or continents) the answer probably
depends mostly on differences in
macroevolutionary large-scale biogeographic
processes, whereas at smaller spatial scales the
answer is likely a combination of source-pool
sizes small-scale ecological processes
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Origins Maintenance of Diversity
What are we trying to explain? A variety of
patterns, at multiple scales, including 4.
Origin of gradients in diversity (usually S)
Why do sites differ in diversity? At the
largest spatial scales of sampling (comparisons
of regions or continents) the answer probably
depends mostly on differences in
macroevolutionary large-scale biogeographic
processes, whereas at smaller spatial scales the
answer is likely a combination of source-pool
sizes small-scale ecological processes
5. Maintenance of gradients in diversity
(usually S)
Is a particular gradient maintained over the long
term? If so, what processes maintain the
gradient?
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients 1. Time Origins more time more
net speciation events
Extant S
Appearance of Species
Disappearance of Species
Cumulative S
t
t1
t2
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients Not specifically mentioned by Pianka,
but area is also important Willis (1922) used
Age and Area to explain the richness
of Indian flora Terborgh (1973) used age and
area to explain favorableness
cradle museum effects
Extant S in large area
Extant S in small area
Appearance
Disappearance
Cumulative S
t
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Origins Maintenance of Diversity
Time area provide opportunities for various
processes that could promote speciation For
example, four main hypotheses for the origins of
bird diversity in South America rely on large
areas long periods of time
1. Forest Refugia Hypothesis
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Origins Maintenance of Diversity
Time area provide opportunities for various
processes that could promote speciation For
example, four main hypotheses for the origins of
bird diversity in South America rely on large
areas long periods of time
2. Andean Uplift Hypothesis
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Origins Maintenance of Diversity
Time area provide opportunities for various
processes that could promote speciation For
example, four main hypotheses for the origins of
bird diversity in South America rely on large
areas long periods of time
3. Riverine Barrier Hypothesis
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Origins Maintenance of Diversity
Time area provide opportunities for various
processes that could promote speciation For
example, four main hypotheses for the origins of
bird diversity in South America rely on large
areas long periods of time
4. Marine Transgression Hypothesis
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients 2. Spatial Heterogeneity Origins
through cradle effect and maintenance through
competitively-expressed niche differences
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients 2. Spatial Heterogeneity Origins
through cradle effect and maintenance through
competitively-expressed niche differences
Temporal Heterogeneity was later recognized for
its potential contributions toward
maintaining diversity, especially under
circumstances in which lottery competition
the storage effect cause species to
increase more in good years than they
decrease in bad years (e.g., Chesson Warner
1981)
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients 2. Spatial Heterogeneity Origins
through cradle effect and maintenance through
competitively-expressed niche differences
Temporal Heterogeneity was later recognized for
its potential contributions toward
maintaining diversity, especially under
circumstances in which lottery competition
the storage effect cause species to
increase more in good years than they
decrease in bad years (e.g., Chesson Warner
1981).
Spatial Heterogeneity could be either
physically based or biotically based
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients 3. Competition Origins through
greater species packing (narrower niches due to
greater constancy impact of competitive
interactions) maintenance through
competitively-expressed niche differences
requires spatial heterogeneity
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients 3. Competition Origins through
greater species packing (narrower niches due to
greater constancy impact of competitive
interactions) maintenance through
competitively-expressed niche differences
requires spatial heterogeneity
4. Predation Maintenance through keystone
predators
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients 5. Climatic Stability Origins
stability was proposed as a mechanism that could
increase niche packing Maintenance through
competitively-expressed niche differences etc.
Dramatic climatic instability could
alternatively increase extinction rates (e.g.,
ice ages), and in the extreme set the Time
clock back to zero
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients 5. Climatic Stability Origins
stability was proposed as a mechanism that could
increase niche packing Maintenance through
competitively-expressed niche differences etc.
Dramatic climatic instability could
alternatively increase extinction rates (e.g.,
ice ages), and in the extreme set the Time
clock back to zero
Temporal Heterogeneity was later recognized
for its potential contributions toward
maintaining diversity, especially under
circumstances in which lottery competition
the storage effect cause species to
increase more in good years than they
decrease in bad years (e.g., Chesson Warner
1981)
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Origins Maintenance of Diversity
Pianka (1966) Six explanations for latitudinal
gradients 6. Productivity. Origins
maintenance through influence on trophic levels,
population sizes, etc. Productivity was
later championed by Wright (1983) and Currie
Paquin (1987) in the Species-Energy
Hypothesis
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Origins Maintenance of Diversity
Connell (1978) Six explanations for high rain
forest coral reef diversity Although he used a
restrictive definition of equilibrium, his
paper was a useful foil against the proliferation
of equilibrial hypotheses in light of the
empirical evidence for abundant non-equilibrial
dynamics in nature Non-equilibrium
Hypotheses 1. Intermediate Disturbance
Hypothesis
Diversity
Disturbance frequency Time since
disturbance Disturbance intensity
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Origins Maintenance of Diversity
Connell (1978) Six explanations for high rain
forest coral reef diversity Although he used a
restrictive definition of equilibrium, his
paper was a useful foil against the proliferation
of equilibrial hypotheses in light of the
empirical evidence for abundant non-equilibrial
dynamics in nature Non-equilibrium
Hypotheses 1. Intermediate Disturbance
Hypothesis 2. Competitive Equivalence or
Equal Chance Hypothesis
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Origins Maintenance of Diversity
Connell (1978) Six explanations for high rain
forest coral reef diversity Although he used a
restrictive definition of equilibrium, his
paper was a useful foil against the proliferation
of equilibrial hypotheses in light of the
empirical evidence for abundant non-equilibrial
dynamics in nature Non-equilibrium
Hypotheses 1. Intermediate Disturbance
Hypothesis 2. Competitive Equivalence or Equal
Chance Hypothesis Hubbell (1979, 2001
Hubbell Foster 1986) Explore the community-
level consequences of species equivalence
(ecological symmetry)
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Origins Maintenance of Diversity
Connell (1978) Six explanations for high rain
forest coral reef diversity Although he used a
restrictive definition of equilibrium, his
paper was a useful foil against the proliferation
of equilibrial hypotheses in light of the
empirical evidence for abundant non-equilibrial
dynamics in nature Non-equilibrium
Hypotheses 1. Intermediate Disturbance
Hypothesis 2. Competitive Equivalence or
Equal Chance Hypothesis 3. Gradual Climate
Change Hypothesis
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Origins Maintenance of Diversity
Connell (1978) Six explanations for high rain
forest coral reef diversity Although he used a
restrictive definition of equilibrium, his
paper was a useful foil against the proliferation
of equilibrial hypotheses in light of the
empirical evidence for abundant non-equilibrial
dynamics in nature Equilibrium Hypotheses 1.
Niche Differentiation Hypothesis
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Origins Maintenance of Diversity
Species-packing mechanisms that illustrate
niche-differentiation explanations for
differences in diversity between two sites
Figure from Remsen (1991)
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Origins Maintenance of Diversity
Species-packing mechanisms that illustrate
niche-differentiation explanations for
differences in diversity between two sites
Figure from Remsen (1991)
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Origins Maintenance of Diversity
Species-packing mechanisms that illustrate
niche-differentiation explanations for
differences in diversity between two sites
Figure from Remsen (1991)
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Origins Maintenance of Diversity
Connell (1978) Six explanations for high rain
forest coral reef diversity Although he used a
restrictive definition of equilibrium, his
paper was a useful foil against the proliferation
of equilibrial hypotheses in light of the
empirical evidence for abundant non-equilibrial
dynamics in nature Equilibrium Hypotheses 1.
Niche Differentiation Hypothesis 2. Circular
Networks Hypothesis. Applied to competitors, but
similar to loops in food webs (which ironically
destabilize model food webs)
Competitive hierarchy in which A gt B gt C, but C
eliminates A directly, e.g., A overshadows B, B
overshadows C, but C poisons A
A
B
C
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Origins Maintenance of Diversity
Connell (1978) Six explanations for high rain
forest coral reef diversity Although he used a
restrictive definition of equilibrium, his
paper was a useful foil against the proliferation
of equilibrial hypotheses in light of the
empirical evidence for abundant non-equilibrial
dynamics in nature Equilibrium Hypotheses 1.
Niche Differentiation Hypothesis 2.
Circular Networks Hypothesis. Applied to
competitors, but similar to loops in food webs
(which ironically destabilize model food
webs) 3. Compensatory Mortality Hypothesis.
Rare species advantage
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Origins Maintenance of Diversity
Janzens (1970) graphical representation of the
Janzen-Connell model, one potential mechanism of
compensatory mortality
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Origins Maintenance of Diversity
Tilman Pacala (1993) Several explanations for
the maintenance of diversity in plant communities
The extraordinary diversity of the terrestrial
fauna, which is much greater than that of the
marine fauna, is clearly due largely to the
diversity provided by terrestrial plants on the
whole the problem still remains, but in the new
form Why are there so many kinds of
plants? Hutchinson (1959) Homage to Santa
Rosalia
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Origins Maintenance of Diversity
According to the competitive exclusion principle,
close competitors share requirements for
limiting resources, and complete competitors
cannot coexist Hutchinson (1961) The
paradox of the plankton
So, how can there be more species than limiting
resources?
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Origins Maintenance of Diversity
Hutchinson (1959, 1961) criticized the very
restrictive models that were being used to
attempt to explain diversity The models
(Lotka-Volterra-style) assumed a spatially
temporally homogeneous habitat, populations at
equilibrium, a 2-tiered trophic structure, no
limiting physical factors, no neighborhood
effects, simple life histories
Most of these assumptions are broken in the real
world each broken assumption could result in the
maintenance of unlimited species richness!
(Tilman Pacala 1993)
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Origins Maintenance of Diversity
Tilman Pacala (1993) Several explanations for
the maintenance of diversity in plant communities
Almost any model that assumes some sort of
environmental complexity, and that assumes
allocation-based trade-offs in the abilities of
organisms to respond to their constraints, has
the potential to predict the existence of many
more species than there are limiting resources
and physical factors Suggested that the
question should shift from Why are there so
many species? To Why are there a particular
number of species, and not many, many more?
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Origins Maintenance of Diversity
Tilman Pacala (1993) Several explanations for
the maintenance of diversity in plant
communities
Observations from the real world (New England
forests) suggest to them that a variety of
life-history trade-offs maintain diversity
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Two trade-offs are especially important
1. High light growth vs. low light survivorship
1996
(High light growth rate)
Figure from Pacala et al. (1996)
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Two trade-offs are especially important
1. High light growth vs. low light survivorship
2. Competition vs. colonization
1996
(Competitive dominance)
(High light growth rate)
Figure from Pacala et al. (1996)
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Origins Maintenance of Diversity
Hubbell and Foster (1986) Several explanations
for the maintenance of diversity in plant
communities
Observations from the real world (Panamanian
forests) suggest to them that a combination of
biology, chance history determine a sites
diversity
Pioneer vs. non-pioneer guilds, defined by
trade-offs
Habitat-association guilds
Dispersal limitation competitive equivalence
within guilds
Density- frequency-dependence also contribute
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Origins Maintenance of Diversity
A major goal in diversity research is to
understand the relative contributions that
individual processes make
But this is no easy task...
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Origins Maintenance of Diversity
Example Imagine two regions sampled plots
yield a similar ratio of diversity to the larger
regional totals, suggesting that differences in
regional species-pool sizes contribute toward
determining local species richness
Region 1 100 spp. of snails
Region 2 50 spp. of snails
10 spp.
5 spp.
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Origins Maintenance of Diversity
Example Imagine two regions sampled plots
yield a similar ratio of diversity to the larger
regional totals, suggesting that differences in
regional species-pool sizes contribute toward
determining local species richness
Experiment 1 Predator-removal experiments
(indicated in pink) demonstrate an additional,
ecological, keystone predator influence on
local diversity
Region 1 100 spp. of snails
Region 2 50 spp. of snails
10 spp.
5 spp.
5 spp.
3 spp.
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Origins Maintenance of Diversity
Example Imagine two regions sampled plots
yield a similar ratio of diversity to the larger
regional totals, suggesting that differences in
regional species-pool sizes contribute toward
determining local species richness
Experiment 1 Predator-removal experiments
(indicated in pink) demonstrate an additional,
ecological, keystone predator influence on
local diversity
Experiment 2 Substrate-homogenization
experiments (blue) demonstrate an additional,
ecological, influence on local diversity, perhaps
due to competition (e.g., each species competes
for algae best on a specific substrate type)
Region 1 100 spp. of snails
Region 2 50 spp. of snails
5 spp.
3 spp.
10 spp.
5 spp.
5 spp.
3 spp.
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Origins Maintenance of Diversity
Palmer (1994) More than 100 explanations for
the origin and maintenance of diversity and
diversity gradients
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Origins Maintenance of Diversity
Ricklefs Schluter (1993) Much of the
traditional emphasis has been on species richness
and diversity per se, but what of species
composition? Ricklefs Schluter (1993) advocate
much more use of historical, biogeographic,
and systematic (including molecular) data to
reconstruct the development of species
assemblages...