BIOL 3060 Lecture Notes - Lecture 9: California Oak Woodland, Spatial Scale, Species Richness
2 May 2018
School
Department
Course
Professor
The study of how the abundance, diversity and distributions of species vary due
to interactions with other organisms and variation in abiotic conditions
•
Community ecologists seek to explain the underlying mechanisms that create,
maintain, and determine the fate of biological communities
•
"an aggregate of living plants having mutual relations among themselves
and to the environment" (Oosting, 1956)
○
"a collection of plant populations found in one habitat type in one area and
integrated to a degree by competition, complementarity, and dependence"
(Grubb, 1987)
○
Community concept first applied to plants, more recently applied to animals
(most definitions only refer to plants)
•
Introduction to Community Ecology:
Dispersal constraints
1)
Environmental constraints
2)
Internal dynamics
3)
Species that assemble to form a community are determined by:
Environment or habitat where community occurs (e.g. lakes, sand-dunes, coastal
rock pools)
1)
Largest or most abundant or prominent species (e.g. pine forest, oak woodland,
grassland, Sphagumbog)
2)
How are Communities Recognized?
No fixed size
•
Can range from very small to huge expanses of grassland and forest
•
Size of community:
Ex. Trees in a rainforest are stratified into canopy, several understories, a ground level
and roots. Each level is the habitat of a distinct collection of species. Pools of water
that collect at the base of tree branches, may harbor entire unique communities.
Scale/size
•
Spatial and temporal structure
•
Species richness
•
Species diversity
•
Trophic structure
•
Succession and disturbance
•
Emergent Properties of a Community:
Ex. General structure of the mammalian gut and common members of the gut
communities
•
Communities Across Scales:
Alpha diversity -the number of species in any one community
○
Beta diversity -the change in what species you encounter as you move
from community to community in the region
○
*see slide
○
Regional diversity -γdiversity
•
Local -populations, communities & habitat patches - 1-100years
!
Landscape -between communities -100-1000 years
!
Regional -regions, countries -10,000 years
!
Continental -continents - 1-10million years
!
Global -global biodiversity & biosphere -10-100million years
!
Spatial scale -biological scale -temporal scale
○
Spatial and temporal scales of biodiversity are closely related:
•
Species Diversity Across Scales:
Latitude
!
Climate
!
Elevation
!
Influenced by:
•
Species diversity is higher in the tropics
!
Species diversity on islands of different sizes
!
Species diversity varies over big spaces:
•
Some of these patterns can be explained by biogeographical processes
•
Actually co-discover of natural selection (Darwin is given most
credit)
!
Wallace better remembered for his contributions to biogeography
!
Biogeography is the study of variation in species composition and
diversity among geographic locations
!
Discovered that the faunas on the two sides of this line were
very different
□
Fauna of the Philippines are closer to Africa (5500 km) than to
New Guinea (750 km)
□
He did not understand the mechanism
□
"Wallace's Line" -runs through Indonesia
!
Alfred Wallace (1823-1913)
•
Explains Wallace's line
!
Explains why different lineages of plants/animals in 6 regions of the
world proposed by Wallace
!
Explains divergence like that seen in flightless birds
!
Plate Tectonics: responsible for difference in large regions of globe
•
Ex. New World Bats / Palms
!
Birds
□
Mammals
□
Freshwater fishes
□
Trees
□
Epiphytes
□
Marine gastropods
□
Marine bivalves
□
Marine fishes
□
Corals
□
Insects
□
Groups that provide evidence for a latitudinal gradient in diversity:
!
The mid-domain effect: for anything that affects organisms' ranges,
more organisms will be found in the middle of the gradient (and the
equator is at the middle of the world)
!
As latitude increases (N/S), the number of species (and therefore diversity)
decreases
•
Broad Scale1)
Productivity
!
Disturbance
!
Influenced by:
•
Fine Scale2)
Patterns of Species Richness:
Differential rates of extinctiona.
Inadequate time for species to recolonizeb.
Historical Perturbations -places that have been disturbed (e.g. by glaciation) may
have fewer species due to :
1.
Speciation is faster then extinctiona.
More "evolutionary experiments" tried, and more niches filled b.
Differential rates of evolution -places with more resources or higher temperature
may have faster rates of evolution because:
2.
Climate stability -stable climate may promote specialization (and speciation)
while reducing extinctions
3.
Harshness -harsh conditions may limit species number4.
Interspecific interactions -biotic interactions may promote specialization and
coexistence and are more intense in the tropics
5.
Habitat heterogeneity -diverse habitat structure may permit finer subdivision of
resources and greater specialization
6.
Productivity/energy -greater available energy may allow for greater numbers of
species to coexist
7.
Explanations for the Latitudinal Gradient in Diversity:
Temperature decreases linearly above latitudes of ~30 degrees
•
Favorable climate + more stable climate --> bigger populations, faster evolution
and less extinction
•
Decreases with latitude
•
Increases with precipitation (to a certain point)
•
Increases with PET and AET
•
Tree-species richness:
•
Increases to plateau in birds, mammals, amphibians, and reptiles
•
Species richness vs. potential evapotranspiration
•
Figures:
Sea birds -higher number of species at larger latitudes
•
Evidence that productivity matters
•
Oceans are more productive at high latitudes
•
Overall summary of tropical diversity: many theories are not exclusive (good
evidence for many)
•
Some Exceptions:
First developed by Robert MacArthur (generally worked with birds)
•
Come out of the idea of niches, and the competitive exclusion theory
•
Heterogeneous environmental theory
•
Very disturbed environments will have low diversity (same with stable
environments)
•
Disturbances increase environmental heterogeneity to create specialized
niches
•
'K' species will do well in stable environments (strong competitors)
!
Low levels of disturbance:
•
'r' species will do well and tolerate the disturbance (inferior
competitors)
!
High levels of disturbance:
•
'K' and 'r' species can coexist in moderate disturbance frequencies -->
middle group has higher heterogenity of environmental resources
•
Intermediate disturbance hypothesis
•
Equilibrium Island Biogeography
•
Theories that stood the test of time:
One species drives other to extinction1)
Natural selection reduces the competition between the species2)
If competition for a limited resource is intense, there are two possible
outcomes:
•
A consequence of Gause's competitive exclusion principle
•
Ex. Warblers: no two species can have the same niche = competitive
exclusion principle
•
MacArthur's first result: verticle heterogenity increases species
diversity
!
The higher the canopy of forests, the more species there will be
•
Heterogeneous environments: opportunities for specialization
•
For animals, the environment that's importance is usually structure of
vegetation
•
Soil resources are variable in space, suggesting they can help explain
plant diversity
!
Different plants may use different types of soil for nutrients
•
Plants vs Animals
•
Under varying condition, competitive exclusion occurs
•
Co-existence could be maintained if shared resources are utilized at
different times, thereby minimizing interspecific competition
•
Seasonality, interannual variation in food resources, etc.
•
But disruptive processes such as predation and disturbance also can alter
the course of competition
•
With Piaster the population could be maintained
!
Remember: sea star kept a dominant competitior from getting too abundant
•
Without variation, one species persists while the other one goes
extinct
!
Therefore, variable environments can maintain species coexistence
!
Ex. Sea palm on a rocky tidal shore that was a poor competitor but could
colonize areas of high disturbance
•
Temporal variation:
•
Heterogeneous environmental theory:
Mechanism 1: Disturbance reduces species' densities, thereby weakening
competition and preventing competitive exclusion
•
Disturbances clear patches, which are first colonized by competitively-
inferior species, which have time to reproduce and send out colonists to
other newly-disturbed patches before competitively superior species arrive
and exclude them
•
Infrequent disturbances fail to clear patches fast enough to support the
competitively-inferior species, while overly-frequent disturbances wipe out
the competitively superior species faster than they can colonize
•
Mechanism 2: Diversity may peak at intermediate disturbance levels because of a
trade-off between competitive ability and colonizing ability
•
Streams
•
Prairie dog disturbance
•
Diversity of marine algae and invertebrates
•
Normal Distributions:
•
It has been refuted on both empirical and theoretical grounds
•
Theoretically, the three major mechanisms thought to produce humped
diversity-disturbance relationships are logically invalid and do not actually
predict what they are thought to predict
•
*see slides
!
At low abundance, there are higher growth rates than other
species
□
As it experiences more competition (total abundance
increases), it decreases
□
Red species = "r strategist"
!
Blue species = "K strategist"
!
Low abundance: "r strategist" outcompetes the blue species (but both
still have positive r, so they are both growing)
!
At a higher abundance, r strategists are declining more rapidly
than K strategists
□
Mid-High abundance: "K strategist" outcompetes r and will
eventually result in competitive exclusion
!
Therefore, K strategists will always win under these conditions
unless there are some density independent event that causes
mortality and kicks the system back to where the r strategist was
more competitive
!
Fox concluded that disturbance is more likely to influence diversity
and co-existence through these population regulations rather than
creating new niches
!
Per capita growth rate ('r') vs. total abundance
•
"The intermediate disturbance hypothesis should be abandoned"
•
Many of the criticisms are misguided
•
IDH holds in nature under specific conditions thus remains a conceptually
useful model
•
Depends on spatial scales of observations
•
Preservation of biodiversity depends on both encouraging and limiting
disturbances within ecological thresholds
•
"Anthropogenic disturbances are key to maintaining the biodiversity of
grasslands"
•
Intermediate Disturbance Hypothesis:
"islands only produce a greater or less number of species as their
circumference is more or less extensive"
•
Therefore, small islands harbor fewer species than large islands
•
After exploring the island of the southern pacific he observed:
•
John Reinhold Forster (1729-1798) -served as a botanist with Captain Cook
Arrhenius (1921) -species and area
•
Censused the plants in 240 one meter squared plots in northern Michigan
•
He found that there was 27 species in total with an average of 4 species per
quadrate
•
Gleason (1922) -on the relation between species and area
•
Species richness increases with area, but the rate of increase slows for the
larger islands (richness vs. area shows a plateau)
•
Taking the log of each variable demonstrates a linear relationship
•
Island Area:
•
S = number of species
!
A = plot area
!
'c' = intercept
!
If z=1, then slope is linear (scale invariant)
□
If z<1, then slope decreases with increasing area
□
'z' -sensitivity of slope to change in area
!
--> Arrhenius equation: S=cAz
•
*as z decreases, the number of species per square kilometer also decreases
•
*as c increases, the number of species per square kilometer also increases
•
Preston (1962) -canonical distribution of commonness and rarity
•
Robert MacArthur -to do science is to look for repeated patterns in nature
•
Sampling progressively larger areas and devoting more time to data
collection translates into an increased sampling effort (Rosenzweig)
•
Sampling artifacts -some species are missed during sampling efforts1.
Ecological -a larger area is likely to be more environmentally
heterogeneous, thus containing additional species that differ in their niches
2.
General causes of species-area curves:
•
Species-Area Relationships
Include greater diversity of habitats (including habitats for species with large
ranges or requirements)
•
Provide more refugia against host of extinction forces (including disturbances)
•
Support higher population levels (more resources/energy) leading to lower
extinction rate
•
Intercept a greater diversity of immigrants
•
Provide the internal geographic isolation necessary for in situ speciation
•
Ecological Hypotheses: larger islands have higher richness because they…
Goal was to develop a general theory of biogeography based upon the dynamics
of its fundamental processes (immigration, extinction, evolution and ecological
interactions)
•
Major guiding question: can we predict the number of species that would exist on
a newly created island?
•
Insular environment = area of habitat suitable for a specific ecosystem,
surrounded by an expanse of unsuitable habitat
○
Ex. Lake and bog islands, fragmented habitat islands, mountain islands,
vacant urban lots
○
May be referred to as insular biogeography
•
Equilibrium number: rate of immigration = extinction (at a specific number of
species)
•
Large island: immigration > extinction
○
Small island: extinction > immigration
○
*see slide
○
Large island will receive more random dispersion of organisms
○
Effects of island area:
•
Immigration is greater to closer islands
○
Extinction is greater in far islands
○
Close islands will receive more random dispersion of organisms
○
The dispersal probability with island distance varies among organisms
(birds > mammals)
○
Effects of island distance:
•
MacArthur & Wilson's "Theory of Equilibrium Island Biogeography"
Diamond's rules: heavily influenced by MacArthurs warbler studies
•
Connor and Simberloff tested whether species patterns could be explained by
null models (no competition)
•
Hubbell's Unified Neutral Theory of Biodiversity: the abundance and diversity of
a species in a community is determined mainly by random dispersal, speciation
and extinction
•
Assembly Rules:
Considering all combinations that could be found for a group of related species,
only certain ones exist in nature
1.
Those permissible combinations resist invaders that would transform them to
forbidden combinations
2.
"Checkboard rule" -some pairs of species never coexist either by themselves or
as part of a larger combination (ex. Small macropygia cuckoo-doves)
3.
Diamond (1975) -proposed that the distribution of bird species over a group of islands
near New Guinea was the result of assembly rules that resulted from interspecific
competition
Predators cannot invade a habitat where there are no prey
•
Abiotic conditions limit species ranges
•
Species that arrive first can potentially exclude later-arriving species
•
Obvious examples of assembly rules:
Connor & Simberloff 1979: we show that every assembly rule is either
tautological, trivial or a pattern expected via species distributed at random
•
A stochastic view of niche theory: C&S built null niche models to test whether
species distributions could be explained without competition
•
Main complaint of C&S: even if a pattern can be shown regarding community
assemblage, it does not mean that there was competition (can't assume cause and
effect)
•
Debate: Is there a stochastic element to niche models?
The neutral theory of species diversity starts with the key assumption that all
individuals in a community of trophically simliiar species are ecologically
identical
•
Random death, speciation, extinction and dispersal from the meta-community
can lead to highly diverse communities that have similar species abundance
patterns to what we observe in real communities
•
Community assembly as result of neutral processes?
Niche based assembly -dictated by local environment filters and
competitive exclusion (Diamond)
1.
Neutral/null model assembly -species are assumed to be ecological
equivalent, or processes are stochastic (Hubbell, C&S)
2.
History based assembly -starting conditions and historical patterns of
speciation matter, perhaps more than local processes (Ricklefs)
3.
Past decade of work marrying phylogenetic analyses with community ecology
has added a third general type of community assemblage
•
The earlier emphasis on competitive exclusion led to the assumption that
evolutionary processes are not relevant on the time scale of ecological processes
•
Rebirth of Assembly Rules with Phylogenetics:
Environmental Filtering -low phylogenetic diversity
The paradox of phenotypic similarity: environmental filtering will select for
species with similar traits in the same environment; ecological similarity may
prevent closely related species from sharing environments because closely
related species should also experience strong competitive interactions due to
ecological similarity
•
Biotic Filtering -high phylogenetic diversity
Assembly rules have been attracted to ecologists but the debate about various
models has not be well resolved (difficult to test for casual mechanisms based on
species presence or abundance data)
•
A phylogenetic approach has provided additional insight into the mechanisms
leading to co-occurrence and can detect potential effects of competition, a
cornerstone of traditional assembly theory
•
Also, allows us to incorporate evolutionary process occuring at larger spatial and
temporal scales
•
Conclusions about Assembly Rules:
Filtering of the regional species pool to produce a local community involves past
geographic and evolutionary filters, environmental filters, biotic filters…
*see slide(s)
How do ecological communities come to be the way the are?
○
What are the constraints on community membership?
○
Is membership structured or random?
○
Key Questions:
•
Diamond's rules: interspecific competition leads to assembly (priority)
rules
○
Connor and Simberloff -hold on Diamond, we can see species patterns
with null models (no competition)
○
Hubbell proposes the Unified Neutral Theory of Biodiversity: the
abundance and diversity of species in a community is determinrd mainly
by random dispersal, speciation, and extinction
○
Key Ideas:
•
Assembly Rules (March 22)
Considering all combinations that could be found for a group of related spp. Only
cerain ones exist in nature.
1.
Those permissible combinations resist invaders that would transform them to
forbidden combinations
2.
"Checkboard rule" -some pairs of species never coexist either by themselves or
as part of a larger combinations
3.
Diamond's (1975) Assembly Rules:
Predators cannot invade habitat where there are no prey
•
Abiotic conditions limit species ranges
•
Species that arrive first can potentially exclude later-arriving species
•
Also,
Connor and Simberloff 1979: we show that every assembly rule is either
tautological, trivial, or a pattern expected via species distribution at random
•
A stochastic view of niche theory. C&S built null models to test whether species
distributions could be explained without competition
•
Even if a pattern can be shown regarding community assemblage, it does not
mean that it was the result of competition
•
Is community assembly stochastic?
In 2001, Stephan Hubbell proposed that differences between species are "neutral"
or irrelevant to their success
•
Opposing views are based on niche models, where individuals of different species
differ from each other in ways that influences their fitness.
Community assemblage is outcome of neutral processes (no species are better
than others)
•
Local --> regional through random death and dispersal
•
Is community assembly neutral? --> Unified Theory of Biodiversity
Niche-based assembly: dictated by local environmental filters and
competitive exclusion (Diamond)
1)
Neutral/null model assembly: species are assumed to be ecological
equivalent, or processes are stochastic (Hubbell, Connor&Simberloff)
2)
History based assembly: starting conditions and historical patterns of
speciation matter, perhaps more than local processes (Ricklefs)
3)
The past decade of work marrying phylogenetic analyses with community
ecology has added a third general type of community assemblage
•
The earlier emphasis on competitive exclusion led to the assumption that
evolutionary processes are not relevant on the time scale of ecological processes
•
The rebirth of assembly rules with phylogenetics:
On the other hand, ecological similarity may prevent closely related
species from sharing environments because closely related species should
also experience strong competitive interactions due to ecological similarity
○
Environmental filtering will select for species with similar traits in the same
environment
•
Environmental filtering -low phylogenetic diversity
○
Biotic filtering -high phylogenetic diversity
○
Ex.
•
The paradox of phenotypic similarity:
Likely because it is very difficult to test for casual mechanisms based on
species presence or abundance data
○
Assembly rules have been attractive to ecologists for a very long time, but the
debate about various models has not been well resolved
•
A phylogenetic approach has provided additional insight into the mechanisms
leading to co-occurrence and can detect potential effects of competition, a
cornerstone of traditional assembly theory
•
Also allows us to incorporate evolutionary processes occurring at larger spatial
and temporal scales
•
Conclusions about Assembly Rules:
Evolutionary processes, physiological constraints and historical events influence
regional species pool
•
Past evolution and history:
Influenced by habitat selection and dispersal ability
•
Who arrives in a particular location:
Interspecific interactions influence species composition of local community
•
Interactions among species:
*filtering of the regional species pool to produce a local community involves past
geographic/evolutionary filters, environmental filters and biotic filters
Most ecologists simply accept the regional species pool as given, and ignore its
importance
•
Ricklefs has argued forcefully that such myopia is unwise
•
When local and regional species diversity are equal (slope=1), all species
within a region will be found in all communities within region
○
When local diversity values are lower than regional diversity, but still
increase with them proportionally (slope<1), regional processes dominate
over local processes
○
Local vs. Regional Diversity:
•
Understanding the nature of the regional species pool requires a knowledge of this
evolutionary history of the biota (plate tectonics…etc)
Community Ecology
Wednesday,+ March+1,+2017
9:28+AM
The study of how the abundance, diversity and distributions of species vary due
to interactions with other organisms and variation in abiotic conditions
•
Community ecologists seek to explain the underlying mechanisms that create,
maintain, and determine the fate of biological communities
•
"an aggregate of living plants having mutual relations among themselves
and to the environment" (Oosting, 1956)
○
"a collection of plant populations found in one habitat type in one area and
integrated to a degree by competition, complementarity, and dependence"
(Grubb, 1987)
○
Community concept first applied to plants, more recently applied to animals
(most definitions only refer to plants)
•
Introduction to Community Ecology:
Dispersal constraints1)
Environmental constraints2)
Internal dynamics3)
Species that assemble to form a community are determined by:
Environment or habitat where community occurs (e.g. lakes, sand-dunes, coastal
rock pools)
1)
Largest or most abundant or prominent species (e.g. pine forest, oak woodland,
grassland, Sphagum bog)
2)
How are Communities Recognized?
No fixed size
•
Can range from very small to huge expanses of grassland and forest
•
Size of community:
Ex. Trees in a rainforest are stratified into canopy, several understories, a ground level
and roots. Each level is the habitat of a distinct collection of species. Pools of water
that collect at the base of tree branches, may harbor entire unique communities.
Scale/size
•
Spatial and temporal structure
•
Species richness
•
Species diversity
•
Trophic structure
•
Succession and disturbance
•
Emergent Properties of a Community:
Ex. General structure of the mammalian gut and common members of the gut
communities
•
Communities Across Scales:
Alpha diversity -the number of species in any one community
○
Beta diversity -the change in what species you encounter as you move
from community to community in the region
○
*see slide
○
Regional diversity -γdiversity
•
Local -populations, communities & habitat patches - 1-100years
!
Landscape -between communities -100-1000 years
!
Regional -regions, countries -10,000 years
!
Continental -continents - 1-10million years
!
Global -global biodiversity & biosphere -10-100million years
!
Spatial scale -biological scale -temporal scale
○
Spatial and temporal scales of biodiversity are closely related:
•
Species Diversity Across Scales:
Latitude
!
Climate
!
Elevation
!
Influenced by:
•
Species diversity is higher in the tropics
!
Species diversity on islands of different sizes
!
Species diversity varies over big spaces:
•
Some of these patterns can be explained by biogeographical processes
•
Actually co-discover of natural selection (Darwin is given most
credit)
!
Wallace better remembered for his contributions to biogeography
!
Biogeography is the study of variation in species composition and
diversity among geographic locations
!
Discovered that the faunas on the two sides of this line were
very different
□
Fauna of the Philippines are closer to Africa (5500 km) than to
New Guinea (750 km)
□
He did not understand the mechanism
□
"Wallace's Line" -runs through Indonesia
!
Alfred Wallace (1823-1913)
•
Explains Wallace's line
!
Explains why different lineages of plants/animals in 6 regions of the
world proposed by Wallace
!
Explains divergence like that seen in flightless birds
!
Plate Tectonics: responsible for difference in large regions of globe
•
Ex. New World Bats / Palms
!
Birds
□
Mammals
□
Freshwater fishes
□
Trees
□
Epiphytes
□
Marine gastropods
□
Marine bivalves
□
Marine fishes
□
Corals
□
Insects
□
Groups that provide evidence for a latitudinal gradient in diversity:
!
The mid-domain effect: for anything that affects organisms' ranges,
more organisms will be found in the middle of the gradient (and the
equator is at the middle of the world)
!
As latitude increases (N/S), the number of species (and therefore diversity)
decreases
•
Broad Scale
1)
Productivity
!
Disturbance
!
Influenced by:
•
Fine Scale2)
Patterns of Species Richness:
Differential rates of extinctiona.
Inadequate time for species to recolonizeb.
Historical Perturbations -places that have been disturbed (e.g. by glaciation) may
have fewer species due to :
1.
Speciation is faster then extinctiona.
More "evolutionary experiments" tried, and more niches filled b.
Differential rates of evolution -places with more resources or higher temperature
may have faster rates of evolution because:
2.
Climate stability -stable climate may promote specialization (and speciation)
while reducing extinctions
3.
Harshness -harsh conditions may limit species number4.
Interspecific interactions -biotic interactions may promote specialization and
coexistence and are more intense in the tropics
5.
Habitat heterogeneity -diverse habitat structure may permit finer subdivision of
resources and greater specialization
6.
Productivity/energy -greater available energy may allow for greater numbers of
species to coexist
7.
Explanations for the Latitudinal Gradient in Diversity:
Temperature decreases linearly above latitudes of ~30 degrees
•
Favorable climate + more stable climate --> bigger populations, faster evolution
and less extinction
•
Decreases with latitude
•
Increases with precipitation (to a certain point)
•
Increases with PET and AET
•
Tree-species richness:
•
Increases to plateau in birds, mammals, amphibians, and reptiles
•
Species richness vs. potential evapotranspiration
•
Figures:
Sea birds -higher number of species at larger latitudes
•
Evidence that productivity matters
•
Oceans are more productive at high latitudes
•
Overall summary of tropical diversity: many theories are not exclusive (good
evidence for many)
•
Some Exceptions:
First developed by Robert MacArthur (generally worked with birds)
•
Come out of the idea of niches, and the competitive exclusion theory
•
Heterogeneous environmental theory
•
Very disturbed environments will have low diversity (same with stable
environments)
•
Disturbances increase environmental heterogeneity to create specialized
niches
•
'K' species will do well in stable environments (strong competitors)
!
Low levels of disturbance:
•
'r' species will do well and tolerate the disturbance (inferior
competitors)
!
High levels of disturbance:
•
'K' and 'r' species can coexist in moderate disturbance frequencies -->
middle group has higher heterogenity of environmental resources
•
Intermediate disturbance hypothesis
•
Equilibrium Island Biogeography
•
Theories that stood the test of time:
One species drives other to extinction1)
Natural selection reduces the competition between the species2)
If competition for a limited resource is intense, there are two possible
outcomes:
•
A consequence of Gause's competitive exclusion principle
•
Ex. Warblers: no two species can have the same niche = competitive
exclusion principle
•
MacArthur's first result: verticle heterogenity increases species
diversity
!
The higher the canopy of forests, the more species there will be
•
Heterogeneous environments: opportunities for specialization
•
For animals, the environment that's importance is usually structure of
vegetation
•
Soil resources are variable in space, suggesting they can help explain
plant diversity
!
Different plants may use different types of soil for nutrients
•
Plants vs Animals
•
Under varying condition, competitive exclusion occurs
•
Co-existence could be maintained if shared resources are utilized at
different times, thereby minimizing interspecific competition
•
Seasonality, interannual variation in food resources, etc.
•
But disruptive processes such as predation and disturbance also can alter
the course of competition
•
With Piaster the population could be maintained
!
Remember: sea star kept a dominant competitior from getting too abundant
•
Without variation, one species persists while the other one goes
extinct
!
Therefore, variable environments can maintain species coexistence
!
Ex. Sea palm on a rocky tidal shore that was a poor competitor but could
colonize areas of high disturbance
•
Temporal variation:
•
Heterogeneous environmental theory:
Mechanism 1: Disturbance reduces species' densities, thereby weakening
competition and preventing competitive exclusion
•
Disturbances clear patches, which are first colonized by competitively-
inferior species, which have time to reproduce and send out colonists to
other newly-disturbed patches before competitively superior species arrive
and exclude them
•
Infrequent disturbances fail to clear patches fast enough to support the
competitively-inferior species, while overly-frequent disturbances wipe out
the competitively superior species faster than they can colonize
•
Mechanism 2: Diversity may peak at intermediate disturbance levels because of a
trade-off between competitive ability and colonizing ability
•
Streams
•
Prairie dog disturbance
•
Diversity of marine algae and invertebrates
•
Normal Distributions:
•
It has been refuted on both empirical and theoretical grounds
•
Theoretically, the three major mechanisms thought to produce humped
diversity-disturbance relationships are logically invalid and do not actually
predict what they are thought to predict
•
*see slides
!
At low abundance, there are higher growth rates than other
species
□
As it experiences more competition (total abundance
increases), it decreases
□
Red species = "r strategist"
!
Blue species = "K strategist"
!
Low abundance: "r strategist" outcompetes the blue species (but both
still have positive r, so they are both growing)
!
At a higher abundance, r strategists are declining more rapidly
than K strategists
□
Mid-High abundance: "K strategist" outcompetes r and will
eventually result in competitive exclusion
!
Therefore, K strategists will always win under these conditions
unless there are some density independent event that causes
mortality and kicks the system back to where the r strategist was
more competitive
!
Fox concluded that disturbance is more likely to influence diversity
and co-existence through these population regulations rather than
creating new niches
!
Per capita growth rate ('r') vs. total abundance
•
"The intermediate disturbance hypothesis should be abandoned"
•
Many of the criticisms are misguided
•
IDH holds in nature under specific conditions thus remains a conceptually
useful model
•
Depends on spatial scales of observations
•
Preservation of biodiversity depends on both encouraging and limiting
disturbances within ecological thresholds
•
"Anthropogenic disturbances are key to maintaining the biodiversity of
grasslands"
•
Intermediate Disturbance Hypothesis:
"islands only produce a greater or less number of species as their
circumference is more or less extensive"
•
Therefore, small islands harbor fewer species than large islands
•
After exploring the island of the southern pacific he observed:
•
John Reinhold Forster (1729-1798) -served as a botanist with Captain Cook
Arrhenius (1921) -species and area
•
Censused the plants in 240 one meter squared plots in northern Michigan
•
He found that there was 27 species in total with an average of 4 species per
quadrate
•
Gleason (1922) -on the relation between species and area
•
Species richness increases with area, but the rate of increase slows for the
larger islands (richness vs. area shows a plateau)
•
Taking the log of each variable demonstrates a linear relationship
•
Island Area:
•
S = number of species
!
A = plot area
!
'c' = intercept
!
If z=1, then slope is linear (scale invariant)
□
If z<1, then slope decreases with increasing area
□
'z' -sensitivity of slope to change in area
!
--> Arrhenius equation: S=cAz
•
*as z decreases, the number of species per square kilometer also decreases
•
*as c increases, the number of species per square kilometer also increases
•
Preston (1962) -canonical distribution of commonness and rarity
•
Robert MacArthur -to do science is to look for repeated patterns in nature
•
Sampling progressively larger areas and devoting more time to data
collection translates into an increased sampling effort (Rosenzweig)
•
Sampling artifacts -some species are missed during sampling efforts1.
Ecological -a larger area is likely to be more environmentally
heterogeneous, thus containing additional species that differ in their niches
2.
General causes of species-area curves:
•
Species-Area Relationships
Include greater diversity of habitats (including habitats for species with large
ranges or requirements)
•
Provide more refugia against host of extinction forces (including disturbances)
•
Support higher population levels (more resources/energy) leading to lower
extinction rate
•
Intercept a greater diversity of immigrants
•
Provide the internal geographic isolation necessary for in situ speciation
•
Ecological Hypotheses: larger islands have higher richness because they…
Goal was to develop a general theory of biogeography based upon the dynamics
of its fundamental processes (immigration, extinction, evolution and ecological
interactions)
•
Major guiding question: can we predict the number of species that would exist on
a newly created island?
•
Insular environment = area of habitat suitable for a specific ecosystem,
surrounded by an expanse of unsuitable habitat
○
Ex. Lake and bog islands, fragmented habitat islands, mountain islands,
vacant urban lots
○
May be referred to as insular biogeography
•
Equilibrium number: rate of immigration = extinction (at a specific number of
species)
•
Large island: immigration > extinction
○
Small island: extinction > immigration
○
*see slide
○
Large island will receive more random dispersion of organisms
○
Effects of island area:
•
Immigration is greater to closer islands
○
Extinction is greater in far islands
○
Close islands will receive more random dispersion of organisms
○
The dispersal probability with island distance varies among organisms
(birds > mammals)
○
Effects of island distance:
•
MacArthur & Wilson's "Theory of Equilibrium Island Biogeography"
Diamond's rules: heavily influenced by MacArthurs warbler studies
•
Connor and Simberloff tested whether species patterns could be explained by
null models (no competition)
•
Hubbell's Unified Neutral Theory of Biodiversity: the abundance and diversity of
a species in a community is determined mainly by random dispersal, speciation
and extinction
•
Assembly Rules:
Considering all combinations that could be found for a group of related species,
only certain ones exist in nature
1.
Those permissible combinations resist invaders that would transform them to
forbidden combinations
2.
"Checkboard rule" -some pairs of species never coexist either by themselves or
as part of a larger combination (ex. Small macropygia cuckoo-doves)
3.
Diamond (1975) -proposed that the distribution of bird species over a group of islands
near New Guinea was the result of assembly rules that resulted from interspecific
competition
Predators cannot invade a habitat where there are no prey
•
Abiotic conditions limit species ranges
•
Species that arrive first can potentially exclude later-arriving species
•
Obvious examples of assembly rules:
Connor & Simberloff 1979: we show that every assembly rule is either
tautological, trivial or a pattern expected via species distributed at random
•
A stochastic view of niche theory: C&S built null niche models to test whether
species distributions could be explained without competition
•
Main complaint of C&S: even if a pattern can be shown regarding community
assemblage, it does not mean that there was competition (can't assume cause and
effect)
•
Debate: Is there a stochastic element to niche models?
The neutral theory of species diversity starts with the key assumption that all
individuals in a community of trophically simliiar species are ecologically
identical
•
Random death, speciation, extinction and dispersal from the meta-community
can lead to highly diverse communities that have similar species abundance
patterns to what we observe in real communities
•
Community assembly as result of neutral processes?
Niche based assembly -dictated by local environment filters and
competitive exclusion (Diamond)
1.
Neutral/null model assembly -species are assumed to be ecological
equivalent, or processes are stochastic (Hubbell, C&S)
2.
History based assembly -starting conditions and historical patterns of
speciation matter, perhaps more than local processes (Ricklefs)
3.
Past decade of work marrying phylogenetic analyses with community ecology
has added a third general type of community assemblage
•
The earlier emphasis on competitive exclusion led to the assumption that
evolutionary processes are not relevant on the time scale of ecological processes
•
Rebirth of Assembly Rules with Phylogenetics:
Environmental Filtering -low phylogenetic diversity
The paradox of phenotypic similarity: environmental filtering will select for
species with similar traits in the same environment; ecological similarity may
prevent closely related species from sharing environments because closely
related species should also experience strong competitive interactions due to
ecological similarity
•
Biotic Filtering -high phylogenetic diversity
Assembly rules have been attracted to ecologists but the debate about various
models has not be well resolved (difficult to test for casual mechanisms based on
species presence or abundance data)
•
A phylogenetic approach has provided additional insight into the mechanisms
leading to co-occurrence and can detect potential effects of competition, a
cornerstone of traditional assembly theory
•
Also, allows us to incorporate evolutionary process occuring at larger spatial and
temporal scales
•
Conclusions about Assembly Rules:
Filtering of the regional species pool to produce a local community involves past
geographic and evolutionary filters, environmental filters, biotic filters…
*see slide(s)
How do ecological communities come to be the way the are?
○
What are the constraints on community membership?
○
Is membership structured or random?
○
Key Questions:
•
Diamond's rules: interspecific competition leads to assembly (priority)
rules
○
Connor and Simberloff -hold on Diamond, we can see species patterns
with null models (no competition)
○
Hubbell proposes the Unified Neutral Theory of Biodiversity: the
abundance and diversity of species in a community is determinrd mainly
by random dispersal, speciation, and extinction
○
Key Ideas:
•
Assembly Rules (March 22)
Considering all combinations that could be found for a group of related spp. Only
cerain ones exist in nature.
1.
Those permissible combinations resist invaders that would transform them to
forbidden combinations
2.
"Checkboard rule" -some pairs of species never coexist either by themselves or
as part of a larger combinations
3.
Diamond's (1975) Assembly Rules:
Predators cannot invade habitat where there are no prey
•
Abiotic conditions limit species ranges
•
Species that arrive first can potentially exclude later-arriving species
•
Also,
Connor and Simberloff 1979: we show that every assembly rule is either
tautological, trivial, or a pattern expected via species distribution at random
•
A stochastic view of niche theory. C&S built null models to test whether species
distributions could be explained without competition
•
Even if a pattern can be shown regarding community assemblage, it does not
mean that it was the result of competition
•
Is community assembly stochastic?
In 2001, Stephan Hubbell proposed that differences between species are "neutral"
or irrelevant to their success
•
Opposing views are based on niche models, where individuals of different species
differ from each other in ways that influences their fitness.
Community assemblage is outcome of neutral processes (no species are better
than others)
•
Local --> regional through random death and dispersal
•
Is community assembly neutral? --> Unified Theory of Biodiversity
Niche-based assembly: dictated by local environmental filters and
competitive exclusion (Diamond)
1)
Neutral/null model assembly: species are assumed to be ecological
equivalent, or processes are stochastic (Hubbell, Connor&Simberloff)
2)
History based assembly: starting conditions and historical patterns of
speciation matter, perhaps more than local processes (Ricklefs)
3)
The past decade of work marrying phylogenetic analyses with community
ecology has added a third general type of community assemblage
•
The earlier emphasis on competitive exclusion led to the assumption that
evolutionary processes are not relevant on the time scale of ecological processes
•
The rebirth of assembly rules with phylogenetics:
On the other hand, ecological similarity may prevent closely related
species from sharing environments because closely related species should
also experience strong competitive interactions due to ecological similarity
○
Environmental filtering will select for species with similar traits in the same
environment
•
Environmental filtering -low phylogenetic diversity
○
Biotic filtering -high phylogenetic diversity
○
Ex.
•
The paradox of phenotypic similarity:
Likely because it is very difficult to test for casual mechanisms based on
species presence or abundance data
○
Assembly rules have been attractive to ecologists for a very long time, but the
debate about various models has not been well resolved
•
A phylogenetic approach has provided additional insight into the mechanisms
leading to co-occurrence and can detect potential effects of competition, a
cornerstone of traditional assembly theory
•
Also allows us to incorporate evolutionary processes occurring at larger spatial
and temporal scales
•
Conclusions about Assembly Rules:
Evolutionary processes, physiological constraints and historical events influence
regional species pool
•
Past evolution and history:
Influenced by habitat selection and dispersal ability
•
Who arrives in a particular location:
Interspecific interactions influence species composition of local community
•
Interactions among species:
*filtering of the regional species pool to produce a local community involves past
geographic/evolutionary filters, environmental filters and biotic filters
Most ecologists simply accept the regional species pool as given, and ignore its
importance
•
Ricklefs has argued forcefully that such myopia is unwise
•
When local and regional species diversity are equal (slope=1), all species
within a region will be found in all communities within region
○
When local diversity values are lower than regional diversity, but still
increase with them proportionally (slope<1), regional processes dominate
over local processes
○
Local vs. Regional Diversity:
•
Understanding the nature of the regional species pool requires a knowledge of this
evolutionary history of the biota (plate tectonics…etc)
Community Ecology
Wednesday,+ March+1,+2017 9:28+AM
The study of how the abundance, diversity and distributions of species vary due
to interactions with other organisms and variation in abiotic conditions
•
Community ecologists seek to explain the underlying mechanisms that create,
maintain, and determine the fate of biological communities
•
"an aggregate of living plants having mutual relations among themselves
and to the environment" (Oosting, 1956)
○
"a collection of plant populations found in one habitat type in one area and
integrated to a degree by competition, complementarity, and dependence"
(Grubb, 1987)
○
Community concept first applied to plants, more recently applied to animals
(most definitions only refer to plants)
•
Introduction to Community Ecology:
Dispersal constraints1)
Environmental constraints2)
Internal dynamics3)
Species that assemble to form a community are determined by:
Environment or habitat where community occurs (e.g. lakes, sand-dunes, coastal
rock pools)
1)
Largest or most abundant or prominent species (e.g. pine forest, oak woodland,
grassland, Sphagum bog)
2)
How are Communities Recognized?
No fixed size
•
Can range from very small to huge expanses of grassland and forest
•
Size of community:
Ex. Trees in a rainforest are stratified into canopy, several understories, a ground level
and roots. Each level is the habitat of a distinct collection of species. Pools of water
that collect at the base of tree branches, may harbor entire unique communities.
Scale/size
•
Spatial and temporal structure
•
Species richness
•
Species diversity
•
Trophic structure
•
Succession and disturbance
•
Emergent Properties of a Community:
Ex. General structure of the mammalian gut and common members of the gut
communities
•
Communities Across Scales:
Alpha diversity -the number of species in any one community
○
Beta diversity -the change in what species you encounter as you move
from community to community in the region
○
*see slide
○
Regional diversity -γdiversity
•
Local -populations, communities & habitat patches - 1-100years
!
Landscape -between communities -100-1000 years
!
Regional -regions, countries -10,000 years
!
Continental -continents - 1-10million years
!
Global -global biodiversity & biosphere -10-100million years
!
Spatial scale -biological scale -temporal scale
○
Spatial and temporal scales of biodiversity are closely related:
•
Species Diversity Across Scales:
Latitude
!
Climate
!
Elevation
!
Influenced by:
•
Species diversity is higher in the tropics
!
Species diversity on islands of different sizes
!
Species diversity varies over big spaces:
•
Some of these patterns can be explained by biogeographical processes
•
Actually co-discover of natural selection (Darwin is given most
credit)
!
Wallace better remembered for his contributions to biogeography
!
Biogeography is the study of variation in species composition and
diversity among geographic locations
!
Discovered that the faunas on the two sides of this line were
very different
□
Fauna of the Philippines are closer to Africa (5500 km) than to
New Guinea (750 km)
□
He did not understand the mechanism
□
"Wallace's Line" -runs through Indonesia
!
Alfred Wallace (1823-1913)
•
Explains Wallace's line
!
Explains why different lineages of plants/animals in 6 regions of the
world proposed by Wallace
!
Explains divergence like that seen in flightless birds
!
Plate Tectonics: responsible for difference in large regions of globe
•
Ex. New World Bats / Palms
!
Birds
□
Mammals
□
Freshwater fishes
□
Trees
□
Epiphytes
□
Marine gastropods
□
Marine bivalves
□
Marine fishes
□
Corals
□
Insects
□
Groups that provide evidence for a latitudinal gradient in diversity:
!
The mid-domain effect: for anything that affects organisms' ranges,
more organisms will be found in the middle of the gradient (and the
equator is at the middle of the world)
!
As latitude increases (N/S), the number of species (and therefore diversity)
decreases
•
Broad Scale1)
Productivity
!
Disturbance
!
Influenced by:
•
Fine Scale
2)
Patterns of Species Richness:
Differential rates of extinction
a.
Inadequate time for species to recolonize
b.
Historical Perturbations -places that have been disturbed (e.g. by glaciation) may
have fewer species due to :
1.
Speciation is faster then extinction
a.
More "evolutionary experiments" tried, and more niches filled
b.
Differential rates of evolution -places with more resources or higher temperature
may have faster rates of evolution because:
2.
Climate stability -stable climate may promote specialization (and speciation)
while reducing extinctions
3.
Harshness -harsh conditions may limit species number4.
Interspecific interactions -biotic interactions may promote specialization and
coexistence and are more intense in the tropics
5.
Habitat heterogeneity -diverse habitat structure may permit finer subdivision of
resources and greater specialization
6.
Productivity/energy -greater available energy may allow for greater numbers of
species to coexist
7.
Explanations for the Latitudinal Gradient in Diversity:
Temperature decreases linearly above latitudes of ~30 degrees
•
Favorable climate + more stable climate --> bigger populations, faster evolution
and less extinction
•
Decreases with latitude
•
Increases with precipitation (to a certain point)
•
Increases with PET and AET
•
Tree-species richness:
•
Increases to plateau in birds, mammals, amphibians, and reptiles
•
Species richness vs. potential evapotranspiration
•
Figures:
Sea birds -higher number of species at larger latitudes
•
Evidence that productivity matters
•
Oceans are more productive at high latitudes
•
Overall summary of tropical diversity: many theories are not exclusive (good
evidence for many)
•
Some Exceptions:
First developed by Robert MacArthur (generally worked with birds)
•
Come out of the idea of niches, and the competitive exclusion theory
•
Heterogeneous environmental theory
•
Very disturbed environments will have low diversity (same with stable
environments)
•
Disturbances increase environmental heterogeneity to create specialized
niches
•
'K' species will do well in stable environments (strong competitors)
!
Low levels of disturbance:
•
'r' species will do well and tolerate the disturbance (inferior
competitors)
!
High levels of disturbance:
•
'K' and 'r' species can coexist in moderate disturbance frequencies -->
middle group has higher heterogenity of environmental resources
•
Intermediate disturbance hypothesis
•
Equilibrium Island Biogeography
•
Theories that stood the test of time:
One species drives other to extinction1)
Natural selection reduces the competition between the species2)
If competition for a limited resource is intense, there are two possible
outcomes:
•
A consequence of Gause's competitive exclusion principle
•
Ex. Warblers: no two species can have the same niche = competitive
exclusion principle
•
MacArthur's first result: verticle heterogenity increases species
diversity
!
The higher the canopy of forests, the more species there will be
•
Heterogeneous environments: opportunities for specialization
•
For animals, the environment that's importance is usually structure of
vegetation
•
Soil resources are variable in space, suggesting they can help explain
plant diversity
!
Different plants may use different types of soil for nutrients
•
Plants vs Animals
•
Under varying condition, competitive exclusion occurs
•
Co-existence could be maintained if shared resources are utilized at
different times, thereby minimizing interspecific competition
•
Seasonality, interannual variation in food resources, etc.
•
But disruptive processes such as predation and disturbance also can alter
the course of competition
•
With Piaster the population could be maintained
!
Remember: sea star kept a dominant competitior from getting too abundant
•
Without variation, one species persists while the other one goes
extinct
!
Therefore, variable environments can maintain species coexistence
!
Ex. Sea palm on a rocky tidal shore that was a poor competitor but could
colonize areas of high disturbance
•
Temporal variation:
•
Heterogeneous environmental theory:
Mechanism 1: Disturbance reduces species' densities, thereby weakening
competition and preventing competitive exclusion
•
Disturbances clear patches, which are first colonized by competitively-
inferior species, which have time to reproduce and send out colonists to
other newly-disturbed patches before competitively superior species arrive
and exclude them
•
Infrequent disturbances fail to clear patches fast enough to support the
competitively-inferior species, while overly-frequent disturbances wipe out
the competitively superior species faster than they can colonize
•
Mechanism 2: Diversity may peak at intermediate disturbance levels because of a
trade-off between competitive ability and colonizing ability
•
Streams
•
Prairie dog disturbance
•
Diversity of marine algae and invertebrates
•
Normal Distributions:
•
It has been refuted on both empirical and theoretical grounds
•
Theoretically, the three major mechanisms thought to produce humped
diversity-disturbance relationships are logically invalid and do not actually
predict what they are thought to predict
•
*see slides
!
At low abundance, there are higher growth rates than other
species
□
As it experiences more competition (total abundance
increases), it decreases
□
Red species = "r strategist"
!
Blue species = "K strategist"
!
Low abundance: "r strategist" outcompetes the blue species (but both
still have positive r, so they are both growing)
!
At a higher abundance, r strategists are declining more rapidly
than K strategists
□
Mid-High abundance: "K strategist" outcompetes r and will
eventually result in competitive exclusion
!
Therefore, K strategists will always win under these conditions
unless there are some density independent event that causes
mortality and kicks the system back to where the r strategist was
more competitive
!
Fox concluded that disturbance is more likely to influence diversity
and co-existence through these population regulations rather than
creating new niches
!
Per capita growth rate ('r') vs. total abundance
•
"The intermediate disturbance hypothesis should be abandoned"
•
Many of the criticisms are misguided
•
IDH holds in nature under specific conditions thus remains a conceptually
useful model
•
Depends on spatial scales of observations
•
Preservation of biodiversity depends on both encouraging and limiting
disturbances within ecological thresholds
•
"Anthropogenic disturbances are key to maintaining the biodiversity of
grasslands"
•
Intermediate Disturbance Hypothesis:
"islands only produce a greater or less number of species as their
circumference is more or less extensive"
•
Therefore, small islands harbor fewer species than large islands
•
After exploring the island of the southern pacific he observed:
•
John Reinhold Forster (1729-1798) -served as a botanist with Captain Cook
Arrhenius (1921) -species and area
•
Censused the plants in 240 one meter squared plots in northern Michigan
•
He found that there was 27 species in total with an average of 4 species per
quadrate
•
Gleason (1922) -on the relation between species and area
•
Species richness increases with area, but the rate of increase slows for the
larger islands (richness vs. area shows a plateau)
•
Taking the log of each variable demonstrates a linear relationship
•
Island Area:
•
S = number of species
!
A = plot area
!
'c' = intercept
!
If z=1, then slope is linear (scale invariant)
□
If z<1, then slope decreases with increasing area
□
'z' -sensitivity of slope to change in area
!
--> Arrhenius equation: S=cAz
•
*as z decreases, the number of species per square kilometer also decreases
•
*as c increases, the number of species per square kilometer also increases
•
Preston (1962) -canonical distribution of commonness and rarity
•
Robert MacArthur -to do science is to look for repeated patterns in nature
•
Sampling progressively larger areas and devoting more time to data
collection translates into an increased sampling effort (Rosenzweig)
•
Sampling artifacts -some species are missed during sampling efforts1.
Ecological -a larger area is likely to be more environmentally
heterogeneous, thus containing additional species that differ in their niches
2.
General causes of species-area curves:
•
Species-Area Relationships
Include greater diversity of habitats (including habitats for species with large
ranges or requirements)
•
Provide more refugia against host of extinction forces (including disturbances)
•
Support higher population levels (more resources/energy) leading to lower
extinction rate
•
Intercept a greater diversity of immigrants
•
Provide the internal geographic isolation necessary for in situ speciation
•
Ecological Hypotheses: larger islands have higher richness because they…
Goal was to develop a general theory of biogeography based upon the dynamics
of its fundamental processes (immigration, extinction, evolution and ecological
interactions)
•
Major guiding question: can we predict the number of species that would exist on
a newly created island?
•
Insular environment = area of habitat suitable for a specific ecosystem,
surrounded by an expanse of unsuitable habitat
○
Ex. Lake and bog islands, fragmented habitat islands, mountain islands,
vacant urban lots
○
May be referred to as insular biogeography
•
Equilibrium number: rate of immigration = extinction (at a specific number of
species)
•
Large island: immigration > extinction
○
Small island: extinction > immigration
○
*see slide
○
Large island will receive more random dispersion of organisms
○
Effects of island area:
•
Immigration is greater to closer islands
○
Extinction is greater in far islands
○
Close islands will receive more random dispersion of organisms
○
The dispersal probability with island distance varies among organisms
(birds > mammals)
○
Effects of island distance:
•
MacArthur & Wilson's "Theory of Equilibrium Island Biogeography"
Diamond's rules: heavily influenced by MacArthurs warbler studies
•
Connor and Simberloff tested whether species patterns could be explained by
null models (no competition)
•
Hubbell's Unified Neutral Theory of Biodiversity: the abundance and diversity of
a species in a community is determined mainly by random dispersal, speciation
and extinction
•
Assembly Rules:
Considering all combinations that could be found for a group of related species,
only certain ones exist in nature
1.
Those permissible combinations resist invaders that would transform them to
forbidden combinations
2.
"Checkboard rule" -some pairs of species never coexist either by themselves or
as part of a larger combination (ex. Small macropygia cuckoo-doves)
3.
Diamond (1975) -proposed that the distribution of bird species over a group of islands
near New Guinea was the result of assembly rules that resulted from interspecific
competition
Predators cannot invade a habitat where there are no prey
•
Abiotic conditions limit species ranges
•
Species that arrive first can potentially exclude later-arriving species
•
Obvious examples of assembly rules:
Connor & Simberloff 1979: we show that every assembly rule is either
tautological, trivial or a pattern expected via species distributed at random
•
A stochastic view of niche theory: C&S built null niche models to test whether
species distributions could be explained without competition
•
Main complaint of C&S: even if a pattern can be shown regarding community
assemblage, it does not mean that there was competition (can't assume cause and
effect)
•
Debate: Is there a stochastic element to niche models?
The neutral theory of species diversity starts with the key assumption that all
individuals in a community of trophically simliiar species are ecologically
identical
•
Random death, speciation, extinction and dispersal from the meta-community
can lead to highly diverse communities that have similar species abundance
patterns to what we observe in real communities
•
Community assembly as result of neutral processes?
Niche based assembly -dictated by local environment filters and
competitive exclusion (Diamond)
1.
Neutral/null model assembly -species are assumed to be ecological
equivalent, or processes are stochastic (Hubbell, C&S)
2.
History based assembly -starting conditions and historical patterns of
speciation matter, perhaps more than local processes (Ricklefs)
3.
Past decade of work marrying phylogenetic analyses with community ecology
has added a third general type of community assemblage
•
The earlier emphasis on competitive exclusion led to the assumption that
evolutionary processes are not relevant on the time scale of ecological processes
•
Rebirth of Assembly Rules with Phylogenetics:
Environmental Filtering -low phylogenetic diversity
The paradox of phenotypic similarity: environmental filtering will select for
species with similar traits in the same environment; ecological similarity may
prevent closely related species from sharing environments because closely
related species should also experience strong competitive interactions due to
ecological similarity
•
Biotic Filtering -high phylogenetic diversity
Assembly rules have been attracted to ecologists but the debate about various
models has not be well resolved (difficult to test for casual mechanisms based on
species presence or abundance data)
•
A phylogenetic approach has provided additional insight into the mechanisms
leading to co-occurrence and can detect potential effects of competition, a
cornerstone of traditional assembly theory
•
Also, allows us to incorporate evolutionary process occuring at larger spatial and
temporal scales
•
Conclusions about Assembly Rules:
Filtering of the regional species pool to produce a local community involves past
geographic and evolutionary filters, environmental filters, biotic filters…
*see slide(s)
How do ecological communities come to be the way the are?
○
What are the constraints on community membership?
○
Is membership structured or random?
○
Key Questions:
•
Diamond's rules: interspecific competition leads to assembly (priority)
rules
○
Connor and Simberloff -hold on Diamond, we can see species patterns
with null models (no competition)
○
Hubbell proposes the Unified Neutral Theory of Biodiversity: the
abundance and diversity of species in a community is determinrd mainly
by random dispersal, speciation, and extinction
○
Key Ideas:
•
Assembly Rules (March 22)
Considering all combinations that could be found for a group of related spp. Only
cerain ones exist in nature.
1.
Those permissible combinations resist invaders that would transform them to
forbidden combinations
2.
"Checkboard rule" -some pairs of species never coexist either by themselves or
as part of a larger combinations
3.
Diamond's (1975) Assembly Rules:
Predators cannot invade habitat where there are no prey
•
Abiotic conditions limit species ranges
•
Species that arrive first can potentially exclude later-arriving species
•
Also,
Connor and Simberloff 1979: we show that every assembly rule is either
tautological, trivial, or a pattern expected via species distribution at random
•
A stochastic view of niche theory. C&S built null models to test whether species
distributions could be explained without competition
•
Even if a pattern can be shown regarding community assemblage, it does not
mean that it was the result of competition
•
Is community assembly stochastic?
In 2001, Stephan Hubbell proposed that differences between species are "neutral"
or irrelevant to their success
•
Opposing views are based on niche models, where individuals of different species
differ from each other in ways that influences their fitness.
Community assemblage is outcome of neutral processes (no species are better
than others)
•
Local --> regional through random death and dispersal
•
Is community assembly neutral? --> Unified Theory of Biodiversity
Niche-based assembly: dictated by local environmental filters and
competitive exclusion (Diamond)
1)
Neutral/null model assembly: species are assumed to be ecological
equivalent, or processes are stochastic (Hubbell, Connor&Simberloff)
2)
History based assembly: starting conditions and historical patterns of
speciation matter, perhaps more than local processes (Ricklefs)
3)
The past decade of work marrying phylogenetic analyses with community
ecology has added a third general type of community assemblage
•
The earlier emphasis on competitive exclusion led to the assumption that
evolutionary processes are not relevant on the time scale of ecological processes
•
The rebirth of assembly rules with phylogenetics:
On the other hand, ecological similarity may prevent closely related
species from sharing environments because closely related species should
also experience strong competitive interactions due to ecological similarity
○
Environmental filtering will select for species with similar traits in the same
environment
•
Environmental filtering -low phylogenetic diversity
○
Biotic filtering -high phylogenetic diversity
○
Ex.
•
The paradox of phenotypic similarity:
Likely because it is very difficult to test for casual mechanisms based on
species presence or abundance data
○
Assembly rules have been attractive to ecologists for a very long time, but the
debate about various models has not been well resolved
•
A phylogenetic approach has provided additional insight into the mechanisms
leading to co-occurrence and can detect potential effects of competition, a
cornerstone of traditional assembly theory
•
Also allows us to incorporate evolutionary processes occurring at larger spatial
and temporal scales
•
Conclusions about Assembly Rules:
Evolutionary processes, physiological constraints and historical events influence
regional species pool
•
Past evolution and history:
Influenced by habitat selection and dispersal ability
•
Who arrives in a particular location:
Interspecific interactions influence species composition of local community
•
Interactions among species:
*filtering of the regional species pool to produce a local community involves past
geographic/evolutionary filters, environmental filters and biotic filters
Most ecologists simply accept the regional species pool as given, and ignore its
importance
•
Ricklefs has argued forcefully that such myopia is unwise
•
When local and regional species diversity are equal (slope=1), all species
within a region will be found in all communities within region
○
When local diversity values are lower than regional diversity, but still
increase with them proportionally (slope<1), regional processes dominate
over local processes
○
Local vs. Regional Diversity:
•
Understanding the nature of the regional species pool requires a knowledge of this
evolutionary history of the biota (plate tectonics…etc)
Community Ecology
Wednesday,+ March+1,+2017 9:28+AM
