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OUR Ecological Footprint 1.
8…
Fall 2008
IB Workshop Series
sponsored by IB academic advisors
Study Abroad for IB Majors
Thursday, October 30
4:00-5:00PM
135 Burrill Hall
Learn about Study Abroad opportunities of
special interest to IB students.
Ch 21,23,24 Community Ecology
Species Abundance + Diversity
Objectives
• Species relative abundance
• Species diversity
• Measures to quantify and compare SD
• Local SD affected by
•
Biotic Interactions
•
Disturbance
• Species-area relationship
•
Species in communities vary in relative
abundance. Are most species are rare or
common?
Figure 1
What is the likelihood of sampling a rare
species? A common species? How
accurate are the data for rare species?
Species abundance (dominance diversity)
curves…Which community has 1) ?
2) ?
Log scale…
Figure 2
Most
Least
Rank order of abundance
How can community structure be quantified
and compared?
***
Which variables can be used to describe the
species diversity of a local community?
Which community is more diverse?
• Species richness
• Species relative
abundance
Figure 3
Measures of community structure
• Species richness: # of species BUT
species differ in abundance and thus in
role
• Species diversity: weight species by their
relative abundance
• Shannon-Wiener index: H' = -  pi ln pi
Calculate Species Diversity:
Species No. Ind. pi
pi2
1
5
.25
.0625
2
4
.20
.0400
3
3
.15
.0225
4
4
.20
.0400
5
4
.20
.0400
Total (N) 20
1.00 ∑=.205
•H' = -∑ pi ln pi = 1.5965
ln pi
pi ln pi
-1.386
-.3465
-1.609
-.3218
-1.897
-.2846
-1.609
-.3218
-1.609
-.3218
∑= -1.5965
Comparisons of diversity indices among
communities.
C1
C2
C3
C4
C5
•Which community is most diverse?
•What factors increase species diversity?
• more species.
• less difference in relative abundance
among species.
Figure 4
Multiple scales of species diversity
•
•
•
•
•
Local
Regional
Latitudinal
Continental
Global
Factors Affecting SD at Local Scale:
•
•
•
•
•
Abiotic factors
Biological interactions
Dispersal limitation
Human introduction
Chance
QUESTION: Abiotic factors + Diversity
•
•
•
1)
2)
3)
4)
A 100-yr experiment tested the effect of fertilizer
on species diversity (H’) in a grassland.
RESULTS: H’ of unfertilized remained steady.
H’ of fertilized decreases through time.
Summarize the major result of the study.
What 2 components of a community does the
Shannon-Wiener Index (H’) incorporate?
What combination of these components yields
the greatest value of H’?
Explain the results in terms of competition and
niche theory.
Figure 5.
Biotic Hypotheses to explain variation in
species richness
1 Heterogeneity in space and time
e.g. (Vegetation and food complexity)
2 Herbivore and pathogen pressure
3 Competition/niches
4 Disturbance
5 Equilibrium models
1 Heterogeneity in space and time
hypothesis
• Relates to niche arguments (see below)
Bird richness increases with greater
structural complexity.
Species richness increases as a stream
becomes larger and has more habitat and
food diversity.
2) Pest pressure (herbivores + pathogens)
hypothesis for maintaining tree species
richness
Figure 6.
Distance-dependent (and/or densitydependent) mortality is consistent with the
pest pressure hypothesis.
3 Competition hypothesis:
• High richness --> less competitive
exclusion?--> more species
• Why? By what means?
Niche metrics
How can more species be added to a
community?
• Increase
total niche
space
• Increase
niche
overlap
• Decrease
niche
breadth
Figure 8
4 Competition hypothesis, cont.:
• High richness --> less competitive exclusion?
• Why? By what means?
•
greater specialization (narrower niche)
•
greater resource availability (greater
niche space); less niche overlap
•
reduced resource demand (smaller
populations)
• Greater niche space from greater number of
niche axes and length of each axis?
• relates to hypothesis of
heterogeneity in space/time
Does increase in niche diversity --> increase
in species richness? As s.r. increases, so
does morphological diversity.
Populations in regions with few
species show ecological release (and
larger realized niches).
• Realized niche is always smaller than
fundamental niche,
• but with ecological release --->
larger realized niches
4 Intermediate Disturbance Hypothesis
• Richness peaks at intermediate levels
Too low disturbance -->
competitive exclusion
Too high disturbance -->
limited number of species adapted
5 Equilibrium hypothesis
• Richness reaches an equilibrium when
factors removing species = factors adding
species.
•  more additions (e.g. speciation) or and/or
fewer deletions (e.g. extinctions) = greater
species richness.
Multiple scales of species diversity
•
•
•
•
•
•
Local
Local Affected by Regional
Regional
Latitudinal
Continental
Global
Local diversity as f (regional diversity).
Figure 9
Many factors influence regional and
local species richness.
Figure 10
Local communities contain a subset of
the regional species pool.
•
***What determines whether a species can
be a member of a given community?
1 Adaptations of species to environmental
conditions (habitat selection)
2 Persistence in face of competitors,
predators, and parasites
Local communities are assembled from
the regional species pool.
• Species sorting = processes that determine
local community composition.
Experimentally-composed communities show
species sorting. What caused the sorting?
Regional:
# species
available
Fertility:
low Local:
high realized
#
Environmental filters eliminate species that
can’t tolerate conditions---> species sorting
H1:Species sorting should be greatest where
regional species pool is largest.
• When species pool is smaller, competition
should be relaxed--->
•
ecological release = species expand into
habitats normally filled by other species and
increase in population density
• Ecological release provides evidence for
hypothesis of local interactions controlling
species diversity.
•
(e.g. competition for resources structures
communities and limits # species)
Species richness (# species) has both local
and regional components.
•  (alpha) = local # species in small area of
homogeneous habitat
•  (beta) = # species turnover between
habitats
•  (gamma) = (landscape) regional: total #
species in all habitats within a barrier-free
geographic area
Above species richness measures
determined by ecology and regional
pool
•  (delta) = available pool of species
within dispersal distance (up to
continental scale)
• determined evolutionarily
What is the relationship between species #
and area? What scales are used?
log
log
Figure 11
Species - area relationship:
• S = c Az
• S = # of species
• A = area
• c and z = fitted constants
• log S = log c + z log A = linear
***Why do larger areas have more species?
• in part because…
• larger areas give larger samples
but also…
• greater habitat heterogeneity (sample
more types of habitats)
• larger islands---> bigger target for
immigrants
• larger populations --->
– greater genetic diversity
– broader distributions over habitats
– numbers large enough to prevent stochastic
extinction
What contributes to these species-area
relationships?.
Figure 12
OBJECTIVES
• Species Diversity at larger scales
• Equilibrium theory + Island Biog. Theory
• Regional effects on local SD
• Regional SD
• Latitudinal SD
• Continental/Global SD