introduction and characteristics of biological community

I. Basic Patterns and
Elementary Processes
Population ecology focuses on patterns and
processes of single species groups of individuals
But…
Any individual or population does not exist in isolation
but exists as an integrated component within a
complex ecological whole.

A. Community Ecology Defined
One level in the hierarchical levels of
organization in Ecology.

Most definitions include idea of collection of species
found in a particular place.
Whitaker (1975) – Assemblage of populations that live
in environment and interact, forming a distinctive
living system with its own composition, structure,
environmental relations, development and function.

Ricklefs (1990) associations of animals and plants
that are spatially delimited and that are dominated
by one or more prominent species or by physical
characteristics.
Does not emphasize interactions

Elton (1927) closely knit society or community similar
to our own.
Emphasizes roles of species

Community: the all inclusive entirety of the biotic
elements of an ecosystem: an interactive assemblage
of species occurring together within a particular
geographic area, a set of species whose ecological
function and dynamics are in some way
interdependent.

Not a flat mosaic of co-occurring species but as a
dynamic interactive system of interdependent
populations

Parameters that define it
• Composition – the spp present and their relative
abundance
• Nature and Form of Relationship – direction, relative
strength, and impact of those relationships
• Dynamics – its flux in both time and space

Community Classification
1. Physically defined community – defined by discrete
habitat boundaries
Includes assemblages of species in particular
habitat or place
Lakes, ponds, rotting fruit etc.
Biomes also example

Community Classification
2. Taxonomically defined (Taxocene) - Organisms of
a particular taxon occurring together in one place
(e.g., “plant community”)

Examples would include the Beech and Maple forests
of the northeastern United States, or the Tallgrass
prairie of the Great Plains.
In both cases the predominance of one or two species
defines the community. In some cases the species
that defines the community may also play an
important role in defining its physical structure.

3. Statistically defined communities. A set of
species whose abundances are significantly
correlated, positively or negatively over space or
time.
The approach makes use of overall patterns in the
identity and abundance of species to quantify
similarities and differences among communities.

Provides a unbiased interpretation of a community.
Method is called ordination. Outside the scope of
this class.

4. Interactively defined community – consists of those
subsets of species in a particular place or habitat
whose interactions significantly influence their
abundances.
Only some, and perhaps none of the species in a
physically defined community may constitute an
interactively defined community.

Hairston (1981) used this approach to point out that
only a small subset of species of salamanders found
in the Mts of North Carolina could be shown to
interact and affect one another’s abundances.
7 spp found there but only 2 most abundant affected
each other’s abundances.

So…
Key point is that the assignment of membership in a
community, whether based on taxonomy or physical
definition does not guarantee that species will really
interact.

Most communities are extraordinarily complex.
• difficult to assemble a complete list of species for a
given area.
• often focus attention on conspicuous and readily
identifiable sets of species (ecologically or
taxonomically similar)

• Guild- a collection of species that use resources in
similar ways.
– Granivore guilds in deserts of SW consists of
birds, rodents and insects that consume seeds as
their primary source of food.

• Trophic Level – provides a way to recognize
subsets of spp within a community that acquire
energy in similar ways.
1° producers-----1°consumers (herbivores) -----2° consumers (carnivores).
At the ends of chains are decomposers (detritivores).

Depicted in a
linear path the
actual trophic
relations are
more complex
and called a food
web.

B. Community Properties
• How does one describe a community?
• How can you compare two different communities?
Ecologists have various descriptors to condense and
summarize information about the #, identity, and
relative abundance of a species, but no single # or
index exists that can provide a complete description
of a community.

1. Species richness
Robert May – 1 single number that goes a long way
toward characterizing a biological community is
simply the total # of spp present.

Species richness = the number of species per unit
area of ground (difficult number to obtain)
a. don’t have complete taxonomic information about
many of the groups of organisms found in even the
most studied communities
b. still problem of deciding whether we had searched
long and hard enough to say that all spp in that
place had been found.

One way to determine if enough sampling has been
done is to plot cumulative # of species by sampling
effort.
Beyond a certain amount of spp vs. effort
curve should reach asymptote. Reasonable estimate
of the # species present.

There is increasing evidence that species richness is
not just merely a nice way to compare and contrast
communities.
Spp richness is related to the important functional
attributes of communities.
Tilman and Downing 1994. Nature 367:363-365
McGrady-Steed et al. 1997. Nature 390:162-165

Recent experimental work indicates
• primary production
• resistance to natural disturbance
• resistance to invasion
all increase as species richness increases

2. Diversity
a. Definition
Diversity = Informal definition = variety;
Formal definition = the way in which the number of
organisms (or their biomass) is allocated among the
species present,
incorporates species richness and evenness

wrote about diversity in
amazon rainforest.
Intuitively that region has
high diversity.
There are few individuals
and many species =
High spp diversity

Probability of intraspecific encounter
Cattail Marsh =
High
Rainforest =
Low

Probability of interspecific encounter
Cattail Marsh =
Low
Rainforest =
High

So…
the number of species and relative abundance of
those species are components of diversity

b. Kinds of Diversity
Point = the diversity of micro-habitat or a sample
taken or from within a homogenous habitat
α = Diversity within a single community or habitat; 1-
single place

β = The degree of change in diversity along a transect
or between habitats; measure of the turnover of spp
between two places
δ = Diversity of a larger unit such as an island or
landscape
ε = Diversity of large biogeographic areas

Scales of diversity
• Alpha – number of
species in a given plot or
area
a
c
d
a
b
c c
a
a
a
a
a b
b
b

• Beta – species turnover
across an environmental
gradient

• Gamma – regional
species richness
• Global – total number
of species of different
taxa in the whole
world. About 1.65
million identified.
Estimates range up to
about 30 million
species.

Biodiversity
(≠Species diversity)
Biodiversity
• Species level: sp diversity
• < Sp level: genetic diversity
• > Sp level: community diversity,
ecosystem diversity,……etc.

How to Measure Species Diversity
c. Diversity Indices
Many ways to measure diversity (incorporating
evenness and species number). At least 60
formulas have been published.

Species Diversity
Two factors define species diversity:
• Species Richness (S):
– Number of species in the community.
• Species Evenness:
– Relative abundance of species.

S = species richness, number of species
ni = number of individuals of spi
N = total number of individuals
Pi = relative abundance = ni/N
i. Shannon-Weiner – this formula gives weight to
both species richness and evenness
∑ Pi lnPi = H´

Index for Species diversity
-- Shannon-Wiener Index
• Shannon-Wiener Index
=Shannon-Weaver diversity index
(=Shannon diversity index), (H)
s
H’ = -Σpi logepi
i=l
H’ = Value of Shannon-Wiener diversity index.
Pi = proportion of the ith species.
Loge= natural logarithm of pi.
S = number of species in the community.

Shannon-Wiener
• Gives weight to both species richness and evenness

ii. Simpson’s – this formula gives most weight to
evenness and little weight to species richness.
1-∑ Pi
2= D

Index for Species diversity
-- Simpson index
• Origin: D=Σ Pi
2
• Modifications:

Simpson’s Index
• Expressed as – probability that 2 species chosen at
random from community will be different species.
• Gives most of the weight to evenness and little to
species richness

Evenness
• Defined as – the extent to which a group of species
is uniform in number
• Most communities consist of a few species that are
rare and most species are moderately abundant.

• Measurements have been devised to quantify the
unequal representation of the community against a
hypothetical community (where all species are
equally common).

Sp evenness/ Equitability index
--from Shannon

Similarity Index
• How similar are 2 different communities?
• Jaccard’s Coefficient

• 1959, G. Evelyn Hutchingson’s paper,
“Homage to Santa Rosalia or Why are
there so many kinds of animals?”
Æ what controls the no. & abundance of
species?

Equilibrium
1. Environmental complexity (habitat
heterogeneity)
2. Trophic interactions
Non-equilibrium
1. Intermediate disturbance
Factors affecting diversity

1. Environmental Complexity
• In general, species diversity increases with
environmental complexity or heterogeneity.
• MacArthur found warbler diversity increased
as vegetation stature increased.
– Measured env. complexity as foliage height.
• Many studies have shown positive
relationship between env. complexity and
species diversity.

introduction and characteristics of biological community

Diversity of Algae and Plants
• Hutchinson: “The paradox of the plankton”
– Phytoplankton communities present an apparent
paradox because they live in relatively simple
environments and compete for the same
nutrients, yet many species coexist without
competitive exclusion.
• Env. Complexity in the time domain may
account for significant portion of the species
diversity. Temperature, oxygen and nutrient
availability vary considerably over the year.

Diversity of Algae and Plants
• Algal niches appear to be defined by their
nutrient requirements.
– Tilman found coexistence of freshwater diatoms
depended upon ratio of silicate and phosphate.
• Found conditions allowing coexistence.
–Diatoms held different trophic niches.
»Thus different diatoms would dominate
different areas.

Niches, Heterogeneity and Diversity of
Tropical Forests
• Jordan concluded tropical forest diversity
organized in two ways:
– Large number of species live within most tropical
forest communities.
– Large number of plant communities in a given
area, each with a distinctive species
composition.

Above floodplain
1-2 m above stream

Non-equilibrium - Intermediate disturbance
hypothesis
What is a disturbance?
Disturbance – any relatively discrete event in time
that disrupts an ecosystem, community, or
population structure and changes resources,
substrate availability, or the physical environment.
IMPORTANT – Consider spatial and temporal scale

Non-equilibrium - Intermediate disturbance
hypothesis
Connell proposed that disturbance is a prevalent
feature of nature and significantly influences the
diversity of communities.
High diversity consequence of continually changing
conditions, not of competitive accomodation at
equilibrium.

Highest species richness generally found
in areas with low nutrient availability

C. Interspecific Interactions
Interactions between pairs of spp can be
categorized by assessing the net effect of
populations on each other

1. Competition
-
-

2. Predation
+
-

3. Mutualism
+
+

4. Amensalism
-
0

4. Amensalism
can be considered one-sided competition.
1 species has a negative effect on another but other
has no detectable effect on the 1st
In most experimental settings it is unclear whether
absence of reciprocal effect is real or just not
observed

5. Commensalism
+
0

5. Commensalism
can be considered one-sided mutualism.
1 species has a positive effect on another but other
has no detectable effect on the 1st

• Communities are more complex than pairs of
interactions;
For example, the idea of apparent competition
involves three interacting species giving the
appearance that competition is occurring.
• 2 prey species – A and B
• 1 predator – 1

1
A B
Neither prey species competes, but more predators
will persist when both are present.
Net result = predation more intense on both prey
when they co-occur (each prey has an indirect
negative affect on the other)

Community Ecology
I. Basic Patterns and Elementary Processes
D. Community Patterns

Goals of Community Ecology
• Finding patterns, laws and generalizations
– applicable to diverse systems
– convey understanding about those systems
• Ability to predict community properties and
processes under certain conditions

Communities
• Properties and
Patterns
–# species
–relative
abundance
–morphology
–trophic links
–succession
• Processes
–disturbances
–trophic interactions
–competition
–mutualism
–indirect effects

Community patterns arise from a hierarchy of
processes.
Many factors determine the composition of species
within a given area with no single factor providing
the explanation.

Composition of a regional species pool of potential
community members sets the upper limit on the
species composition of a new community
developing in a new place.

Membership in a regional pool is constrained by
-physiological tolerances
-historical factors
-evolutionary processes

Habitat selection and dispersal abilities then sift and
filter the species from a regional pool
set the identity of those species available to colonize a
given community.
These factors make community nonrandom subsets of
the regional pool of species.

• Interspecific interactions (or there of) influence the
subsequent success or failure of a species that
actually arrives at the community.

II. Competition
Is it omnipresent and omnipotent?

A. Definition – mutually negative
interaction between two or more
species
II. Competition

• reduced abundance
• decreased fitness
• component of fitness
ƒ body mass
ƒ growth rate
ƒ fecundity
ƒ survivorship

B. Experimental vs. Observational Data
Observational Studies – search for
patterns produced by interspecific
competition in natural communities
without manipulating the abundances
of competitors

Experimental – observe how species
respond to direct manipulation of
potential competitors

Debate as to value of experiments vs.
observational studies.
Experimental manipulations are more
direct, provides very strong inference

Observational studies have definite
value, many questions can’t be
answered via experiments.

Where the
species
overlap –
Boreal
Chickadee
at higher
elevations

E. alpinus
E. speciosus
E. amoenus
E. minimus

Have overlapping food
requirements
Least Chipmunk can occupy full
range of habitats
If Yellow pine absent Least
moves into vacated open pine
woods

If Least is absent, Yellow pine
doesn’t invade into sagebrush
zone.
Least can tolerate, hot dry
conditions of the lower
elevations (others can’t)

These studies have established a
correlation but not a cause.

To establish causation need
manipulation
• Replication
• Randomization
• Independence
• Control

Observational studies can be made more
compelling by determining if patterns differ from
those expected by chance –
Null Models

C. Ecological vs. Evolutionary Time Scale
Ecological:
• Question is how a community functions
now
• How do contemporary processes act to
maintain the observed community
structure?

Evolutionary:
• Question is the history of how a
community came to its present state
over evolutionary time
• How do species evolve in response to
selection due to community processes?

Ecological vs. Evolutionary
Questions
• Ecological studies more easily conducted
• Evolutionary studies rely less on direct experiment
and more on comparative, observational, and
theoretical models
• Evolutionary questions imply ecological questions
• Ecological questions do not necessarily imply
evolutionary questions.

D. Mechanisms of Interspecific
Competition
1. Traditional Classification
Exploitation – competition in which 2 or
more organisms consume the same
limited resource

Interference – competition in which
one species prevents resource use by
another, active inhibition

competitor #1
competitor #2
resource
- -
+
+
Resource
competition
Resource
competition
Interference
competition
Interference
competition
competitor #1
competitor #2
-
-

2. Schoener’s Classification
Difficulty in fitting all types of competition
into either exploitation or interference

a.Consumption – one species inhibits
another by consuming shared
resource
b.Preemptive – sessile organisms,
results when a physical resource is
occupied and thus made unavailable

c.Overgrowth – one organism grows
over another, with or without physical
contact
d.Chemical – chemical growth inhibitors
or toxins produced by some species,
kill or inhibit others growth (Black
Walnut, antibiotics, tadpoles).

e. Territorial – results from aggressive
behavioral exclusion of organisms from
specific units of space
f. Encounter – results when nonterritorial
encounters between foraging individuals
result in negative effects on one or both
of the interacting individuals.

E. Models of competition
- yield important predictions about
conditions promoting coexistence or
exclusion
- useful when lab or field studies are
impractical
- used to generate new hypotheses

1. Classical Model
Lotka-Volterra Competition Model
Derived independently
Model describes competition between
organisms for food or space
Based on logistic growth curve

Logistic growth:
dN / dt = r N [ K - N /K ]
ƒ r = intrinsic rate of
increase
ƒ K = carrying
capacity
N
t
K

2 species interact, each is affecting
population growth of the other

Need a term to express species 2 in
terms of species 1 and vice versa
sp 2
sp 1
sp 1
sp 1
sp 1
=

α , β = competition factors
This can be defined by N1 = α *N2

Lotka-Volterra Competition
ƒα = competition coefficient
ƒ effect of species 1 on species 2
ƒβ = competition coefficient
ƒ effect of species 2 on species 1
ƒequivalence of N1 and N2

N1 N2 r1 r2 K1 K2
[ K1 - N1 - α N2 ]
dN1 / dt = r1 N1 ⎯⎯⎯⎯⎯⎯⎯
K1
[ K2 - N2 - β N1 ]
dN2 / dt = r2 N2 ⎯⎯⎯⎯⎯⎯⎯
K2

Competition coefficients:
Competition coefficients:
ƒ β > 1 → impact of sp. 2 on sp. 1 greater
than the impact of sp. 1 on itself
ƒ β < 1 → impact of sp. 2 on sp. 1 less
than the impact of sp. 1 on itself
ƒ β = 1 → impact of sp. 2 on sp. 1 equals
the impact of sp. 1 on itself
ƒ NOTE: Impact of a species on itself =1.0

K1/ a = carrying capacity of species 1
expressed in species 2 equivalents
K2/β = carrying capacity of species 2
expressed in species 1 equivalents

Zero growth isocline
for sp. 1
for sp. 1
N2
N1
0 K1
K1 / α
dN1 /N1 dt > 0
dN1 / N1 dt < 0
Zero Growth
Isocline (ZGI)
dN1/N1dt = 0

for sp. 2
for sp. 2
N2
N1
0 K2/β
K2
dN2 /N2 dt > 0
dN2 /N2 dt < 0
Zero Growth
Isocline (ZGI)
dN2 /N2 dt = 0

Two Isoclines on same graph
Two Isoclines on same graph
ƒ May or may not cross
ƒ Indicates whether two competitors can
coexist
ƒ For equilibrium coexistence, both must
have
ƒ Ni > 0
ƒ dNi / Ni dt = 0

4 Possible Outcomes
• species 1 wins
• species 2 wins
• stable coexistence
• unstable coexistence

Lotka-Volterra Zero Growth Isoclines
N2
N1
0 K2/β
K2
K1/α
K1
c
d
e
ƒ K1 / α > K2
ƒ K1 > K2 / β
ƒ Region c
dN1/N1dt>0 &
dN2/N2dt>0
ƒ Region d
dN1/N1dt>0 &
dN2/N2dt<0
ƒ Region e
dN1/N1dt<0 &
dN2/N2dt<0
Species 1 “wins”

ƒ K2 > K1 / α
ƒ K2 / β > K1
ƒ Region c
dN1/N1dt>0 &
dN2/N2dt>0
ƒ Region d
dN1/N1dt<0 &
dN2/N2dt>0
ƒ Region e
dN1/N1dt<0 &
dN2/N2dt<0
N2
N1
0
K2/β
K2
K1/α
K1
c
d
e
Species 2 “wins”

N2
ƒ K1 / α > K2
ƒ K2 / $ > K1
ƒ Region c both
species increase
ƒ Regions d & f
one species
decreases & one
species increases
ƒ Region e both
species decrease
N1
0
K2/$
K2
K1
c d
e
K1/α
f
Stable coexistence

Stable Competitive Equilibrium
• Competitive Coexistence
• Suppose K1 ≅ K2. What values of α and $ lead to
coexistence?
α < 1.0 (small) and $ < 1.0 (small)
• effect of each species on dN/Ndt of the other is
less than effect of each species on its own dN/Ndt
• Intraspecific competition more intense than
interspecific competition

N1
0
K2/$
K2
K1
c
d
e
• K2 > K1 / α2
• K1 > K2 / α1
• Region c both
species increase
• Regions d & f
one species
decreases & one
species increases
• Region e both
species decrease
K1/α
N2
f
Unstable two
species equilibrium

Unstable Competitive Equilibrium
• Exactly at equilibrium point, both species survive
• Anywhere else, either one or the other “wins”
• Stable equilibria at:
– (N1 = K1 & N2 = 0)
– (N2 = K2 & N1 = 0)
• Which equilibrium depends on initial numbers
– Relatively more N1 and species 1 “wins”
– Relatively more N2 and species 2 “wins”

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