BIOL 4120: Principles of Ecology
Lecture 15: Community
Ecology
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
Outline (Chapter 16)
Community structure
16.1
16.2
16.3
16.4
16.5
16.6
16.7
16.8
16.9
Species Diversity
Dominance
Keystone species
Food web
Functional groups
Physical structure
Zonation
Boundaries between communities
Two contrasting views of community
16.1 Species diversity
Biological structure of a community is defined by the mix of
species, including both their number and relative abundance.
Attributes of community structure
1. Species richness: number of species that occur within the
community
2. Relative abundance: counting all individuals of each species in
a number of sample plots within a community and determining
what percentage each contributes to the total number of
individual of all species.
Table 16.1 and 16.2
3. Rank-abundance diagram: graphical way to show relative
abundance
4. Species eveness: equitable distribution of individuals among
species
Gradual slope in the rank-abundance diagram
Compared to 1st forest stand, this stand has less species. Also, two
species make up 83.5% of the total tree density.
Rank-abundance diagram
Species diversity
Attributes of community structure (cont.)
Diversity indexes: an index considering both the number and
relative abundance of species within the community.
1. Simpson’s index (D): three definitions
Definition:
Where ni is the number of individuals in species i; n is the
total number of individuals of all species
D measures the probability that two individuals randomly
selected from a sample will belong to the same species
Range of D: 0 to 1 (1 is no diversity)
Species diversity
Attributes of community structure (cont.)
Simpson’s index of diversity=1-D
Simpson’s reciprocal index =1/D
also called Simpson’s diversity index
Calculate Simpson’s Index using Tables 16.1 and 16.2
16.2 Dominance
Dominants: when a single or a few species
predominate within a community.
Measurement: Simpson’s index D
D=1 represent complete dominant (only one
species)
Dominance typically means the greatest in
number
But individual size should be considered too.
Consider both number and body size.
Dominate species are usually good competitors
(American chestnut tree before and now).
16.3 Keystone species
Keystone species: a species that has a
disproportionate impact on the community
relative to its abundance.
Keystone species could be dominant.
Remove of keystone species initiates changes
in community structure and results in
significant diversity loss.
Role in community: create or modify habitats,
or influence interactions with others.
Keystone species examples
Coral (Oculina arvbuscula) in the eastern coast
of US
This coral has complex branch and provide
shelter of more than 300 species of
invertebrates
African elephants in the
savannas of southern
Africa
Elephants are destruct
feeders, damage trees,
shrubs, but increase grass
growths.
16.4 Food webs describe species
interactions
Species interaction: predation, parasitism,
competition, mutualism
Fundamental issue (process): resource (food,
nutrient or energy acquisition)
Food web describes species interaction and is
an important part of community structure.
Food webs describe species
interactions
Food web and food chain
An abstract representation of feeding
relationships within a community.
Food chain
Grass  grasshopper  sparrow  hawk
A series of arrow, each pointing one species to
another, representing flow of food from prey
to predator.
Food webs describe species
interactions
Food web:
Involve numerical food chains meshed
together.
Hypothetical food web:
Circle: species
Links: arrows from one to
another
Basal species: feed on no
others, but are fed upon by
others
Intermediate species: feed
on others, and are fed upon
Top predator: not subjected
to predator, but prey on other
species.
Trophic levels
More in next
chapter.
A food web for a prairie grassland community in the midwestern US
16.5 Species within a community can be
classified into Functional groups
Functional groups: C3, C4, CAM
Autotroph and heterotroph
Trophic levels: groups of species that derive their
food energy in a similar way
Herbivores, carnivores, omnivores
Here subdivided each trophic level into groups
according to exploit a common resource in a
similar fashion----guilds.
Guilds: a ground of species utilizing the same
resources in similar ways
Examples: seed-eating birds, insect-eating birds
etc.
Benefit: instead study of individual species, focus
on manageable subset of the community
An example
Spider in major crops, Uetz et al. 1999
16.6 Communities have a definitive
physical structure
Communities have both biological structure
(species richness, abundance, species interactoins)
and physical structure.
Physical structure reflects:
Abiotic factors such as depth and flow of water
in aquatic environments
Biotic factors such as spatial arrangement of
organisms (size and height of tree in forests;
density and distribution of populations).
Communities are often defined by vegetation
types:
Terrestrial:
Shrubs, trees, herbs or
Deciduous, conifers, broad-leave evergreen etc
Or dominant plant growth form + physical
Forests, woodlands, grasslands
Aquatic:
Seagrass meadow, coral reefs
Physical structure
Photic zone,
aphotic zone
benthic zone
Vertical sectional view of communities from aquatic to terrestrial.
Height (ft)
Vertical distribution of birds species within the forest community on
16.7 Zonation is spatial change in
community structure
Zonation: (Arrangement or formation
in zones)
Changes in physical and biological
structure of communities as moving
across the landscape.
Reflect patterns of spatial variation in
community structure
Changes in species composition of forest stands along a topographic
gradient in Virginia
Patterns of zonation in a New England salt marsh.
Life on a sandy ocean beach along Mid-Atlantic Coast
16.8 Defining boundaries between
communities is often difficult
How to determine the community?
Do you consider the forest from hilltop
to bottomland as one or two
communities?
If two, how to separate?
Sampling and data analysis
Community similarity
Sorensen’s coefficient of community
Based on the species presence or absence
Example: s1=24, s2=10, c=9
CC=2*9/(24+10)=0.529
CC ranges from 0 to 1
Community similarity
Another one is Percent Similarity (PS)
Based on the relative abundance
PS= add the lowest percentage for each
species that the communities have in
common
PS=29.7+4.7+4.3+…+0
.4=47.2
PS ranges from 0 to 100
An example of forest zonation
Topographic distribution of forest communities in the Great Smoky?
Mountains National Park (west-facing)
F: Frazir fir; SF: spruce-fir; S: Red spruce; OCF: Oak-chestnut forest
H: Helmock forest; GB: grassy balds
HB:Heath balds
Arrangement of
community types
according to
elevation and
aspect
Large scale zonation
Distribution of
deciduous forest
communities in the
eastern United States
(Braun 1950)
16.9 Two contrasting views of the
community
Community is defined as groups of species
(populations) that occupy a given area,
interacting with each other directly or
indirectly.
How do they interact? Two groups
Organismic concept:
Individualistic (or continuum) concept:
Organismic concept:
Distribution of species is discrete (associations)
Association: a type of community with
1.
relative consistent species;
2.
a uniform general appearance;
3.
a distribution that is characteristics of a
particular habitats such as hilltop or valley
Transitional between communities are narrow, with
few species in common
Suggest a common evolutional history and similar
foundational response and tolerances for
component species
Mutualism and coevolution play important roles in
the species that make up association.
Individualistic (or continuum) concept:
Relationship among co-existing species is
the result of similarities in their
requirements and tolerances, not the
result of strong interactions or common
evolutionary history.
Gradual change in species abundance along
environmental gradient (no associations)
Transitions are gradual and difficult to
detect.
THE End