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Origins & Maintenance of Diversity
“What puts the brakes on demons and prevents their
success causing the destruction of diversity?”
J. Silvertown (2005), pg. 12 from “Demons in Eden”
Origins & Maintenance of Diversity
What are
What
are we
we trying
tryingtotoexplain?
explain?
A variety of patterns, at multiple scales, including:
1. Origin of “diversity” (actually S) within lineages
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
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
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
Disappearance
Extant
S
Cumulative S
Appearance
t
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
Disappearance
Extant
S
Cumulative S
Appearance
“cradle
effect”
t
For more on “cradle” & “museum” effects, see Chown & Gaston (2000)
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
Disappearance
Cumulative S
Appearance
“museum
effect”
Extant
S
t
For more on “cradle” & “museum” effects, see Chown & Gaston (2000)
Origins & Maintenance of Diversity
Appearance (Speciation & Colonization)
Disappearance (Extinction & Extirpation)
Cumulative S
At the largest spatial scales,
speciation & extinction
dominate
At the smallest spatial scales,
colonization & extirpation
dominate
Sregional
Slocal
t
t1
t
t1
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)
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
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?
Origins & Maintenance of Diversity
Pianka (1966) – Six explanations for latitudinal gradients
1. Time – Origins; more time = more net speciation events
Disappearance
of Species
Extant
S
Cumulative S
Appearance
of Species
t
t1
t2
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”
Disappearance
Cumulative S
Appearance
“cradle & museum
effects”
t
Extant
S in
small
area
Extant
S in
large
area
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
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
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
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
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
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)
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
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
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
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
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)
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
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:
Diversity
1. Intermediate Disturbance Hypothesis
Disturbance frequency
Time since disturbance
Disturbance intensity
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
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 communitylevel consequences of species equivalence (“ecological symmetry”)
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
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
Origins & Maintenance of Diversity
Species-packing mechanisms
that illustrate “nichedifferentiation” explanations
for differences in diversity
between two sites
Figure from Remsen (1991)
Origins & Maintenance of Diversity
Species-packing mechanisms
that illustrate “nichedifferentiation” explanations
for differences in diversity
between two sites
Figure from Remsen (1991)
Origins & Maintenance of Diversity
Species-packing mechanisms
that illustrate “nichedifferentiation” explanations
for differences in diversity
between two sites
Figure from Remsen (1991)
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 > B > C, but
C eliminates A directly, e.g., A overshadows B,
B overshadows C, but C poisons A
A
C
B
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
Origins & Maintenance of Diversity
Janzen’s (1970) graphical representation of the Janzen-Connell model,
one potential mechanism of “compensatory mortality”
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”
Origins & Maintenance of Diversity
According to the competitive exclusion principle,
“close competitors” share requirements for limiting resources,
and “complete competitors cannot coexist”
So, how can there be more species than limiting resources?
Hutchinson (1961) “The paradox of the plankton”
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)
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?”
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…
Two trade-offs are especially important:
1. High light growth vs. low light survivorship
Figure from Pacala et al. (1996)
(Competitive “dominance”)
Two trade-offs are especially important:
1. High light growth vs. low light survivorship
2. “Competition” vs. “colonization”
Figure from Pacala et al. (1996)
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 site’s diversity…
Pioneer vs. non-pioneer “guilds”, defined by trade-offs
Habitat-association “guilds”
Dispersal limitation & competitive equivalence within “guilds”
Density- & frequency-dependence also contribute
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...
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
10 spp.
Region 2
50 spp. of snails
5 spp.
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
10 spp. 5 spp.
Region 2
50 spp. of snails
5 spp.
3 spp.
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
5 spp.
10 spp. 5 spp.
Region 2
50 spp. of snails
3 spp.
5 spp.
3 spp.
Origins & Maintenance of Diversity
Palmer (1994) – More than 100 explanations for the origin and
maintenance of diversity and diversity gradients
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...”