Download Community and Ecosystem Ecology - Moodle

Community Ecology
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Community concepts
Niche concepts
Emergent properties of communities
Species richness, evenness, diversity
Species interactions
Consequences of competition
Factors that influence biodiversity
Succession
Community Definition
• All populations within a defined
geographic area
• Most people only study a small
subset of the community they study,
often focusing on a small number of
obviously interacting species
Community concepts
• Clements viewed species
in a community as
interdependent
superorganism with
distinct ecotones
• Gleason viewed species
as responding
idiosyncratically to
environmental gradients
• Most research supports
Gleason’s individualistic
model
Niche concepts
• An organism’s role in the environment
– E.g., The “woodpecker niche”
• G. Evelyn Hutchinson’s
N-dimensional hypervolume
– Many one-dimensional niche axes combined
A single niche axis
e.g., pH, temperature, or soil moisture
Figure 55.2a
Number of seeds
consumed
Another axis might include food eaten.
Seed size
Multiple axes
can then be
combined…
In two
dimensions…
…or three … or more!
Emergent Properties of Communities
• Species interactions, e.g., Competition,
predation
• Species richness - how many species are
present?
• Evenness - relative abundances of
commonest to rarest species
• Evenness and richness often combined
into an index of species diversity
Species
Evenness and
Diversity
• Top: diversity is
high because
evenness is high
• Bottom: diversity
is lower because
one species
dominates
Indices of Biodiversity
• Simpson’s Index: D = 1/∑pi2
– pi= proportion of sample composed of ith species
– Result is on an arithmetic scale (Dmax=S)
• Shannon’s Index H´ = ∑piln(pi)
– Result is on a logarithmic scale [Hmax= ln(S)]
– Take eH to get back to units of number of spp
• Evenness: ED = D/Dmax , EH´ = H´/Hmax
– A measure of how equally abundant species are
Interpretation fuzzy – something
like “what you might encounter
in a small sample”
Problem with interpretation: there can be
many ways to arrive at similar index value
Community 1
Community 2
Community 3
6
1
5
1
A
B
C
Species
D
E
F
Species richness: 6
Evenness: 0.75
Species diversity
(Shannon index): 1.350
More species
with lower
evenness
=lower index
1.794
Fewer species
with greater
evenness
1.610 = higher index
Relative abundances of species
Sample any
community
and you will
learn that
most
species are
rare.
And a few
abundant
species
account for
most
individuals
in sample
Rank-abundance curve
Few common
Many rare
Most abundant
Species Rank
Least abundant
Species Interactions
• Competition -- Each species harms (has
a negative effect on the per capita vital
rates of) the other (-/-)
• Predation -- One species benefits, while
the other is harmed (+/-)
• Mutualism -- Both species benefit (+/+)
• Commensalism -- One species benefits,
other is unaffected (+/0)
• Can modify exponential or logistic
equation to model these interactions
“Red Queens”
• Important difference between adaptation
to the abiotic and the biotic environment
• Abiotic environment does not respond to
an organism’s adaptation to it (e.g., polar
bear and cold)
• Organisms, however, can adapt to
changes in other organisms they interact
with (e.g., cheetah and gazelle, host and
pathogen, males and females)
Competition: What happens when
two species niches overlap?
Resource use (e.g., seed size eaten)
Competitive
Exclusion
• Species reared alone
tend to follow logistic
growth pattern
• But when reared
together, one
displaces the other
through interspecific
competition for shared
limiting resources.
Number of individuals
(a) Observation: asymmetric competition
Paramecium
aurelia
Paramecium
caudatum
Time (days)
(b) Explanation: competitive exclusion due to complete
niche overlap
Number of individuals
using resource
Figure 55.3
Species 1: Strong competitor
Species 2:
Weak competitor,
driven to extinction
Niche (range of resources used)
But what if niches overlap less?
Resource use (e.g., seed size eaten)
What if niches overlap less?
• Joe Connell (Tepper’s undergrad ecology
prof!)
• Observed young Chthamalus barnacles
(an arthropod – remember?) settle
throughout intertidal
• But adults only found in upper intertidal
Figure 55.5-2
Why is the distribution of adult
Chthamalus restricted to the upper intertidal zone?
Adult Chthamalus are outcompeted in the lower intertidal zone.
Adult Chthamalus do not thrive in the physical conditions
of the lower intertidal zone.
Chthamalus in
upper intertidal zone
Mean tide level
Semibalanus
in lower
intertidal
zone
1. Transplant rocks
containing young
Chthamalus to lower
intertidal zone.
2. Let Semibalanus
colonize the rocks.
3. Remove Semibalanus
from half of each rock.
Monitor survival of
Chthamalus on both
sides.
Chthamalus

Semibalanus
Chthamalus
Chthamalus will survive better in the absence of Semibalanus.
Percent survival
of Chthamalus
Chthamalus survival will be low and
the same in the presence or absence of Semibalanus.
Competitor
present
Competitor
absent
Chthamalus do not occur in the lower intertidal zone
because they are outcompeted by Semibalanus.
Fundamental Niche: Niche a species COULD use, based on abiotic environment
Realized Niche: Niche a species DOES use in the presence of other species
Number of individuals
using resource
Figure 55.4
Species 1
(strong competitor)
Species 2
(weak competitor)
Fundamental
niche
Realized
niche
Niche (range of resources used)
Alternatively, species may coexist, but
evolve to differ
in resource use
• Character
displacement – a
shift in phenotype of
two species when
they coexist
(compared to regions
of allopatry)
• Competition results in
divergent selection
Evidence for Character Displacement
(Peter & Rosemary
Grant)
• Galapagos Finches
• Islands where 
sympatric
• Islands where
allopatric
{
Resource
partitioning: the
ghost of
competition
past?
• Robert
MacArthur −
Congeneric
warblers forage
in different
parts of trees
Gradients in species richness
• Given problem with diversity indices, most
unequivocal measure of diversity is
richness
• Why do we find many species in some
places and few species in others?
• Still not well understood, but some general
patterns and mechanisms
Latitude
• In general, more
species in the
tropics
• Is this because
environment is more
benign?
• Primary causal
factor must be
abiotic
© 2011 Pearson Education, Inc.
Equilibrium Theory of Island
Biogeography
• Robert MacArthur and E. O. Wilson
• Number of species on islands reflects a
balance between colonization (immigration)
and extinction
Both colonization and extinction vary with
• Island size
• Distance from source of colonists
The Distance Effect
• Richness tends to
be low on islands
and other isolated
regions
• As distance from
the mainland
increases, richness
decreases
Species-area curves
S=cAz
Or:
log S = log c+z log A
Habitat
diversity
• This example
begs the question
(why is there
more complex
vegetation?)
• But similar pattern
with abiotic
(topographic or
climatic) diversity
Intermediate Disturbance Hypothesis
(Joe Connell-again!)
• Moderate intensity or frequency of
disturbance results in maximum richness
Intermediate disturbance
mechanism related to
succesion
• Succession = change in species
composition at a site over time
• Members of late successional
stages eventually displace
members of earlier stages until
“climax” is achieved
• Primary succession – on bare rock
• Secondary succession – on
existing soil
Succession and intermediate
disturbance
• So when disturbance is moderate in
frequency or intensity, either:
• Creates mosaic of different successional
stages (different ages since disturbance)
• Or
• Maintains an intermediate successional
stage, in which pioneers not all gone, midsuccessional species established, and
climax species arriving
Prescribed burns: disturbance as a
management tool
• Tallgrass prairie, savanna, some coniferous
forests persist because of fire
• Fires from lightning common in dry
seasons, but humans have intentionally
burned for millennia
• European settlers historically prevented fire
• Fires release nutrients, remove shade,
stimulate release and germination of seeds
• Question: how are animals impacted by
fire?
Community Ecology
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Community concepts
Niche concepts
Emergent properties of communities
Species interactions
Consequences of competition
Factors that influence biodiversity
Succession