Download 6_comm ecology overview

Community Ecology:
Community Ecology
I. Community structure & properties
II. Community assembly
Populations and species do not exist in a
vacuum…
III. Factors that influence community structure
Definitions
•  species interact!
I. Community structure & properties
1. Diversity
Community: a group of interacting species that
occupy the same area
2. Growth Form & Structure
3. Dominance
(Which species do you ignore?)
4. Trophic structure
5. Stability
Assemblage: a group of taxonomically related species
that occupy the same area
low evenness
A B C D E F
species
relative abundance
relative abundance
1. Diversity
•  richness = number of species
•  evenness = relative abundance of species
high evenness
1. Diversity
Metrics of Diversity
1) Species richness: number of species in a community
2) Species composition: identity of species that constitute a
community
3) Species diversity: species richness and relative
abundance (evenness)
Shannon-Weiner index of diversity:
A B C D E F
species
H' = -Σ pi (ln pi)
Where pi is the proportion of individuals in the community that are
species i
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Scales of species diversity
1. Diversity
1) Alpha (α): within habitat diversity
Illustration of Shannon-Weiner index species diversity
100
H'= 0.87
E = 0.63
75
No. of
indiv.s
50
100
50
25
25
0
0
A
B
C
H'= 1.39
E = 1.00
75
D
100
2) Beta (β): between habitat diversity
3) Gamma (γ): total diversity (α + β)
H'= 1.10
E = 0.79
75
50
25
0
A
B
C
A
D
B
C
D
Evenness can be calculated as:
Species
Rocky Intertidal
habitat
A
x
Sandy Beach
habitat
B
x
C
x
x
D
x
x
E
x
F
x
G
x
Bay habitat
x
H
E = H'/(ln S)
x
I
where, S is species richness
Max evenness = 1; Min evenness = 0
1. Diversity
x
α diversity
4
5
3
β diversity
rocky vs. sandy: 5
sandy vs. bay: 6
bay vs. rocky: 7
9
γ diversity
2. Growth form & structure
Questions about diversity:
•  What creates and maintains diversity?
•  What causes loss or gain of diversity?
•  How does diversity influence influence ecosystem
function?
•  Is there a particular form that characterizes the
community?
(e.g., massive corals vs. branching corals)
How does altering the growth form & structure affect the
members of the community?
consumer
(secondary carnivore)
4. Trophic structure
3. Dominance
•  Is there one or a few species that control the
community?
(e.g., Pisaster sea star in the intertidal)
•  who eats whom?
Producers
Herbivores
Carnivores (primary)
Consumers
Decomposers Carnivores (secondary)
consumer (primary carnivore)
What happens
when a dominant
is removed?
consumer (herbivore)
producer
What happens if the abundance of
one trophic level changes?
(trophic cascades?)
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5. Stability
4. Trophic structure
•  food webs
competition
reef shark
piscivore guild
herbivore guild
striped parrot
Sargassum
queen trigger
Diadema
Sp. A
Sp. B
Sp. C
time
Tripneustes stoplight parrot
Lobophora
Dictyota
resilience
total
abundance
predation
barracuda
resistance
abundance
- guilds
- competition
- predation
total
time
Halimeda
5. Stability
•  Are communities stable?
–  are they resistant to change?
II. Community Assembly
•  random collections or integrated, co-evolved units?
•  What attributes of communities contribute to stability?
(e.g., their diversity?)
Goal of community ecology
To understand how communities are assembled and
how they function
Are there rules that determine how
communities are assembled?
abundance
–  do resilient communities follow a fixed pattern of
succession?
abundance
–  are they resilient after a perturbation?
distance or
environmental gradient
distance or
environmental gradient
III. Factors that influence community structure
•  abiotic (e.g., temperature, salinity, O2, ocean currents)
•  biotic (e.g., competition, predation, disease)
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Five fundamental types of species interactions:
Niche: how a species fits into a community
Effect on species
A
B
–  biotic and abiotic requirements and tolerances
Competition
–  niches do not exist without a species to define them
Predation/
Parasitism
Mutualism
Commensalism
Amensalism
Concept of the Niche
3) Two types of niche
1) Best known definition of niche is Hutchinson s (e.g., 1957)
a)  role organism plays in environment
b)  role can be determined by measuring all of
an organism s activities and requirements
2) Examples:
high
2-factors
3-factors
Height in
intertidal
low
low
Wave exposure
high
er
ov e
t c ga
en oal
c
r r
Pe ac
m
3) By extension… niche defined as an N-dimensional
hyperspace
a)  fundamental: niche space determined by physical factors
( potential )
and resource requirements. Manifest in the
absence of other organisms.
b) realized: niche space determined by combined physical
( actual ) and biological factors. Realized in presence of
other organisms.
fundamental
realized
•  fundamental niche always bigger (or at least as large) - biological
interactions can (usually do) limit realized niche
(encompasses all effects and requirements of a species)
More about a few key biological interactions:
•  Competition
Competition: the common use of a resource that is in
limited supply
1) Occurs within and between species
•  Predation
a) Intraspecific - among individuals of the same species
source of density dependence discussed last time
•  Mutualism
b) Interspecific - among individuals of two or more species
2) Two types of competition
a) Interference
b) Exploitative
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3) Competitive exclusion principle:
2) Two types of competition
(Gause s Law, 1934)
a) Interference - direct competition
A
Two identical species cannot coexist on the same
limited resource
B
aggression, territoriality (e.g., in fishes,
birds, limpets)
a) Species occupying the same niche cannot coexist.
b) Exploitation - indirect competition
b) The more similar the species (the greater the niche
overlap ), the greater the likelihood of competitive
exclusion, leading to local extinction of one species.
— compete through a resource (R)
(e.g., sessile spp. -- space)
B
A
barnacles
R
mussels
c) Over evolutionary time scales leads to resource
partitioning .
space
5) Spatial patterns caused by competition
4) Resource partitioning
number
of
individuals
a) non-overlapping spatial (or temporal) distribution
adaptation
A B C DE
(evolution)
resource gradient*
A
* e.g.,
species packing
B C D E
B
rock
llow
& ye
k
c
fish
bla
rock
her
gop
resource gradient
fish
resource gradient
5) Spatial patterns caused by competition
6) Competitive release
a) Change in distribution (or some other response such
as growth) when separate and together
a) negative (inverse) relationship in abundance
i) gradient in density
AA A
B A
B AB
AB A
B B
BBA
sympatry (together)
tidal
height
s
acle
barn
sels
mus
absence of mussels
barn
ii) patchy / clumped
Abundance
sp. B
s
acle
barn
sels
mus
tidal
height
reef
depth
- food size
- depth
- height in intertidal
Abundance
sp. A
number
of
individuals
A
A AA
A AA
B
B B
B BB
A A A
A A A
acle
s
absence of barns
sels
mus
Could test for competitive release
observationally or experimentally.
Which is better?
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7) Competitive symmetry
8) Responses to competition
a) relative competitive strength
a) Individual responses:
b) superior vs. inferior (or) dominant vs. subordinate
Symmetrical
A = B
A
B
A > B
A
B
A < B
A
B
• 
• 
• 
Behavioral (feeding rates, foraging distribution)
Physiological (growth rate, reproductive rate)
Morphological (body size, biomass)
b) Population responses:
• 
• 
• 
Asymmetrical
Abundance (density)
Distribution (zonation)
Demographic rates (population growth)
How would you assess this?
Predation
Consumption of one organism (prey) by another (predator),
which by definition, occurs between organisms on different
trophic levels (vs. competition: within same trophic level)
diagrammatically:
food web
A
Herbivore
Primary producer
(plant / alga)
•  death of individuals
B
C
Indirect effects : influence of predator on variable
other than death or mortality
A
C
B
D
E
3) Effects on prey (individual and population):
Individual responses:
behavioral (feeding rates, foraging distribution)
physiological (growth rate, reproductive rate)
• 
behavioral (feeding rates, foraging distribution)
• 
physiological (growth rate, reproductive rate)
• 
morphological (body size, biomass)
F
4) Complex interactions (with other processes)
E.g., competition:
e.g., predator that specializes on barnacles and is restricted to the
mid and lower intertidal
morphological (body size, biomass)
With predators
Without predators
… and you can get completely or partially eaten
Population responses:
• 
• 
• 
• 
Direct effects : direct losses (removal of individuals)
(mortality rate of population)
food chain
Predator
• 
• 
• 
• 
2) Effects on prey (direct and indirect):
abundance, density
tidal
height
barn
s
acle
sels
mus
s
acle
barn
sels
mus
distribution (habitat use)
structure (e.g., size, age, sex ratio, genetic, spatial)
dynamics and persistence (regulation)
In absence of predator, barnacle out-competes mussels and expands
distribution down into the mid intertidal
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4) More complex predation interactions:
Apparent competition
Trophic cascades
A linear food chain
Effect on species
A B
Strong top-down effects that
produce downward rippling
effects through a food chain.
C
C
Where A and B are prey
but A is favored and C is a common predator.
Presence of either prey increases overall
predation rates, leading to negative indirect
effect on one another.
A
B
C
Trophic cascade
Where, A is primary producer,
B is an herbivore, and C is a predator.
Higher tropic level predators
indirectly affect plant biomass via
their impacts on herbivore
populations.
Predator
Herbivore
B
Effect of species on adjacent trophic level
has net positive indirect effect on next trophic
level.
Plants
A
Oksanen/Fretwell Model:
Productivity and Food Chain Length
Apex
predators
•  Depending on productivity of community, food chains can
have fewer or more than three trophic levels.
Herbivores
•  As primary productivity increases, trophic levels will be
sequentially added.
•  Food chains that have an odd number of trophic levels
should be filled with lush vegetation, because herbivores
are kept in check by predators.
Plants
•  Food chains that have an even number of trophic levels
should have low plant abundance because plants are
herbivore limited.
Abiotic
Resources
Mutualism / commensalism
Oksanen/Fretwell Model
secondary
Carnivores
Occurs within or between trophic levels, more often
between trophic levels
a) mutualisms
obligate – required for each others existence (e.g., pollinators)
facultative – not required (e.g., cleaner fish and parasitized hosts)
Biomass
Carnivores
Herbivores
b) commensalism: (also called facilitation)
Plants
Abundance
sp. A
Abundance
sp. B
Low
Environmental Productivity
High
mutualism
(symmetrical)
A = B
A
B
commensalism
(asymmetrical)
A < B
A
B
What tests could you do to distinguish
mutualism from commensalism?
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