Transcript Species Richness

AP Environmental Science
Community Interactions, Species Diversity
and Succession
Community Ecology
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Community = an association of interacting
species inhabiting some defined area
Community structure = includes attributes such
as the number of species and the kinds of
species comprising a community
Community function = includes attributes such
as energy flow through food webs
Are ecological communities “real” functional
units?
Yes! Frederick E. Clements
(1874 – 1945)
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Holistic community concept
(Closed community)
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Community as a superorganism
Based his conclusion on a study of
vegetation zones along altitudinal
gradients
Basis of the modern-day “Gaia
concept”
Vegetation Zones
Closed Communities
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Along an environmental gradient (e.g. temperature,
moisture) there are sometimes sharp boundaries
between species assemblages
Areas between assemblages are known as
ecotones
Best seen where there are sharp physical
boundaries between species assemblages
 Between aquatic and terrestrial areas
 Between distinct soil types
 Between north and south facing slopes
Closed Communities
No! Henry A. Gleason
(1882-1975)
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Individualistic concept
(Open community)
 The appearance of
communities are simply
due to the individual
interactions of species and
don’t reflect any innate
organization above the
species level
Open Communities
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Closer examination of the deciduous forests of eastern
North America revealed no sharp boundaries between
forest types
There were generally two gradients of forest types:
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A north-south gradient based on temperature
An east-west gradient based on moisture
Gradient analysis is a technique invented by
Robert H. Whittaker to analyze changes in (plant)
species abundance along gradients
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Gradients are variables that affect plant community structure (i.e
temperature, light & water availability, soil nutrients, etc.)
Put to rest the extreme closed community concept of Clements
Open Communities
Factors Affecting Biological
Community Structure
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Species diversity – function of the number of different species that
a community contains (species richness) and the relative
abundance of individuals of each species (species evenness).
Niche structure:
 J.H. Brown (1981) bases this on ‘capacity rules’ (how many
potential ecological niches occur) and ‘allocation rules’ (how
these ecological niches can be divided up among species).
Geographical location
 Consistent climates foster high diversity but ‘specialist species’
 Variable climates foster low diversity but ‘generalist species’
Species Richness
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Species Richness is a measure of the total
number of different species in a community
Observations regarding species richness:
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Gradual increase in diversity/richness Pole to
Equator
In a given ecosystem, species richness increases
over time.
Larger ecosystems tend to have greater species
richness.
Usually increases with ecological succession.
Why these trends?
Why These Trends?
Productivity?
 Spatial complexity/heterogeneity?
 However, an aspect of environmental
structure that is important to one group may
not be important to another group
 Therefore, you must know something about
the ecological requirements of species to
predict how environmental structure affects
their diversity.
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Species Richness
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Factors that affect species richness
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saturation
rates of colonization and extinction (MacArthur &
Wilson (1967)
• Depend on size of ecosystem and
• Distance from a source of colonists
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disturbance
severity, variability, and predictability (Slobodkin and
Sanders, 1969)
• i.e. rainforests – favorable and constant; therefore
predictable; polluted stream – severe and variable; therefore
unpredictable
Calculating Species Richness
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Generally exclude “accidental species” in the count
Does not take into account abundance patterns
among the various species
Species counts depend on the sample size or
sample area (rarely do you get every one!)
“Equilibrium model of island biogeography”
(MacArthur & Wilson, 1967)
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# of species (species richness) on an “island” is determined
by immigration and emigration of species
Larger islands and islands closer to “mainland” will have
greater species richness.
Equilibrium
Model of Island
Biogeography
Species-Area
Curves
• Species-area curve
• S = CAz
• log S = log C + z log A
• An equation for a straight line
where log c = Y intercept and z
= the slope of the line
S
Species
Richness
C
constant
A
Area
z
constant
Species Area Curves
Diversity Indices
Shannon-Weiner Index
Also known as the “Shannon Index” or
the “Shannon-Weaver Index”
 Based on Information Theory
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Measures the amount of order in a
system in bits of information
 Determines
rarity or commonness of
species
 Combines measure or species
richness with species evenness.
Calculating the
Shannon-Weiner Index
H
= -∑(pi) (ln pi)
H = Shannon-Weiner Diversity Index
 pi = proportion of total sample belong to
the ith species
 ln pi = the natural log of pi
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An Example
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Community A: 99 individuals in species 1;
individual in species 2
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“Uneven” community
• H = -[(p1)(ln p1) + (p2)(ln p2)]
• H = -[0.99 (ln 0.99) + 0.01 (ln 0.01)]
• H = -[(-0.0099) + (-0.0461)] = +0.056
Community B: 50 individuals in species 1;
50 individuals in species 2
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“Even” community
• H = -[(0.5) (ln 0.5) + (0.5) (ln 0.5)]
• H = -[(-0.3466) + (-0.3466)] = +0.693
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Comparison of Shannon-Weiner Index Values for Even and
Uneven Communities
3.5000
3.0000
2.5000
H
2.0000
1.5000
1.0000
0.5000
0.0000
0
5
10
15
# of species
H - even community
H- uneven community
20
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Pielou’s Evenness Index
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Compares the evenness component of species
diversity in a community with the maximum
amount of evenness possible given the same
species richness
Also known as Shannon’s equitability (EH)
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EH = H/Hmax = H/ln S
Hmax = ln S
Community A--> J = 0.056/0.693 = 0.081
Community B--> J = 0.693/0.693 = 1.000
EH ranges from 0-->1.0 with 1.0 indicating maximum evenness
Comparison of Pielou's Evenness Index for Even and
Uneven Communities
1.2
1
1
1
1
1
0.0434
0.0338
0.0304
0.0285
Evenness
0.8
0.6
0.4
0.2
0
#of species
5
10
# of species
EH - even community
EH - uneven community
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Use of Diversity Indices
Used in applied ecology to measure
differences in diversity between two
or more communities
 Also used to measure changes in
diversity within a community after
some environmental modification or
disturbance
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Robert MacArthur (1930-1972)
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UPenn & Princeton
Studied the diversity
of warblers in
northeastern forests
Discovered that
different warbler
species fed in
different parts of the
forest canopy
MacArthur’s Warblers
Bird Species Diversity vs.
Foliage Height Diversity
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Also showed that the
greater the Foliage
Height Diversity of
the forest the greater
the Bird Species
Diversity
Ecological Succession
Plant (and animal) communities
develop in often predictable ways
following disturbances, a process
known as succession
 Natural, gradual changes in the types
of species that live in an area
 Classified as primary or secondary
succession
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Indiana Dunes Succcession
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Henry Chandler Cowles
(1869-1939)
American botanist and
ecological pioneer
Born in Kensington,
Connecticut, he attended
Oberlin College in Ohio and
the University of Chicago
He obtained his Ph.D. in 1898
for his study of vegetation
succession on the Lake
Michigan sand dunes.
Kinds & qualitative characteristics of
disturbances that impact communities
Disturbance magnitude
Asteroid impact
Volcanism
Glaciation
Landslide
Ice Storm
Hurricane
Fire
Treefall
Disturbance frequency
Primary Succession
General characteristics
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Begins in a place without any soil
 Sides of volcanoes
 Landslides
 Flooding
 Remnants of glaciers
Starts with the arrival of living things such as
lichens that do not need soil to survive
Lichens (pioneer species) secrete weak acids
which in turn break rocks into soil
Called the Pioneer Stage
Lichens:
Symbiosis of
a fungus &
an alga
Example of a
mutualism
Primary Succession
General characteristics
Soil starts to form as lichens and the
forces of weather and erosion help break
down rocks into smaller pieces
 When lichens die, they decompose,
adding small amounts of organic matter to
the rock to make soil
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Primary Succession
General characteristics
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Simple plants like mosses and ferns can
grow in the new soil
Primary Succession
General characteristics
The simple plants die, adding more
organic material
 The soil layer thickens, and grasses,
wildflowers, and other plants begin to take
over
 Called Seral Stages
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Primary Succession
General characteristics
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These plants die, and they add more
nutrients to the soil
Shrubs and trees can survive now
Still called Seral Stages
Primary Succession
General characteristics
Insects, small birds, and mammals have
begun to move in
 What was once bare rock now supports a
variety of life
 Called a
Climax
Community
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Climax Community
A
stable group of plants and animals
that is the end result of the
succession process
 Does not always mean big trees
Grasses in prairies
 Cacti in deserts
 Mosses, sedges and lichens in the tundra
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Secondary Succession
 Begins
in a place that already has
soil and was once the home of living
organisms
 Occurs faster and has different
pioneer species than primary
succession
 Example: after forest fires
Intermediate Disturbance
Hypothesis
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J.P. Grime (1973), Henry Horn (1975) and
Joseph Connell (1978)
The Intermediate Disturbance Hypothesis is an
ecological hypothesis which proposes that
biodiversity is highest when disturbance is
neither too rare nor too frequent.
With low disturbance, competitive exclusion by
the dominant species arises.
With high disturbance, only species tolerant of
the stress can persist.
Role of Disturbance in
Succession