AP Environmental Science Community Interactions, Species Diversity
and Succession
Community Ecology 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)
Holistic community concept (Closed community) 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
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)
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
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: 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 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
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 Species Richness is a measure of the total
number of different species in a community Observations regarding species richness:
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.
Species Richness
Factors that affect species richness saturation rates of colonization and extinction (MacArthur & Wilson
(1967)• Depend on size of ecosystem and • Distance from a source of colonists
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 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) # 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 IndicesShannon-Weiner Index
Also known as the “Shannon Index” or the “Shannon-Weaver Index”
Based on Information Theory 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
An Example
Community A: 99 individuals in species 1; 1 individual in species 2
“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
“Even” community• H = -[(0.5) (ln 0.5) + (0.5) (ln 0.5)]• H = -[(-0.3466) + (-0.3466)] = +0.693
Comparison of Shannon-Weiner Index Values for Even and Uneven Communities
0.0000
0.5000
1.0000
1.5000
2.0000
2.5000
3.0000
3.5000
0 5 10 15 20 25
# of species
H
H - even community H- uneven community
Pielou’s Evenness Index 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) 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 1 1 1
0.02850.03040.03380.04340
0.2
0.4
0.6
0.8
1
1.2
#of species 5 10 15
# of species
Ev
en
ne
ss
EH - even community EH - uneven community
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
Robert MacArthur (1930-1972) 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
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
Indiana Dunes Succcession
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 frequency
Dis
turb
ance
mag
nitu
de
TreefallIce Storm
Fire
Glaciation
Volcanism
Asteroid impact
Landslide Hurricane
Primary SuccessionGeneral characteristics
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 SuccessionGeneral 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
Primary Succession General characteristics
Simple plants like mosses and ferns can grow in the new soil
Primary SuccessionGeneral 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
Primary SuccessionGeneral characteristics
These plants die, and they add more nutrients to the soil
Shrubs and trees can survive now Still called Seral Stages
Primary SuccessionGeneral 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
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
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 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