Chapter 7
Community Ecology
Core Case Study:
Why Should We Care about the
American Alligator?
 Hunters
wiped out
population to the
point of near
extinction.
 Alligators have
important ecological
role.
Figure 7-1
Core Case Study:
Why Should We Care about the
American Alligator?
 Dig

deep depressions (gator holes).
Hold water during dry spells, serve as refuges
for aquatic life.
 Build


nesting mounds.
provide nesting and feeding sites for birds.
Keeps areas of open water free of vegetation.
 Alligators

are a keystone species:
Help maintain the structure and function of the
communities where it is found.
COMMUNITY STRUCTURE AND
SPECIES DIVERSITY
 Biological
communities differ in their structure
and physical appearance.
Figure 7-2
Tropical Coniferous Deciduous Thorn
rain forest
forest
forest
forest
Thorn
scrub
Tall-grassShort-grass Desert
scrub
prairie
prairie
Fig. 7-2, p. 144
Species Diversity and Niche
Structure: Different Species Playing
Different Roles
 Biological
communities differ in the types and
numbers of species they contain and the
ecological roles those species play.
Species Diversity and Niche
 Species

diversity: combination of:
species richness - the number of
different species it contains
 species
evenness - abundance of
individuals within each of those species
Species Diversity and Niche Structure
 Niche
structure: how many potential
ecological niches occur, how they resemble
or differ, and how the species occupying
different niches interact.
 Geographic
location: species diversity is
highest in the tropics and declines as we
move from the equator toward the poles.
Species Diversity on Islands
 species
equilibrium model
 at
some point the rates of immigration and
extinction should reach an equilibrium based
on:


Island size
Distance to nearest mainland
TYPES OF SPECIES
 Native
 Nonnative
 Indicator
 Keystone
 Foundation
species
Play different ecological roles in communities.
TYPES OF SPECIES


Native: those that normally live and thrive
in a particular community.
Nonnative species: those that migrate,
deliberately or accidentally introduced into
a community.
Invasive species
Nonnative (Introduced)
 They
displace native species
 They lower biodiversity
 The can adapt very quickly to local habitats
 They contribute to habitat fragmentation
 They can reproduce very quickly
Importation of Species
 Ex.
The Chinese chestnut had a fungus
that spread & virtually eliminated the
American chestnut.
 Kudzu
Keystone Species: Major Players
 Keystone
species help determine the types
and numbers of other species in a
community thereby helping to sustain it.
Figures 7-4 and 7-5
Foundation Species:
Other Major Players
 Expansion
of keystone species category.
 Foundation species can create and enhance
habitats that can benefit other species in a
community.

Elephants push over, break, or uproot trees,
creating forest openings promoting grass growth
for other species to utilize.
Indicator Species:
Biological Smoke Alarms
 Species
that serve as early warnings of
damage to a community or an ecosystem.

Presence or absence of trout species because
they are sensitive to temperature and oxygen
levels.
Indicator species:
Why are Amphibians Vanishing?
 Frogs
serve as indicator species because
different parts of their life cycles can be easily
disturbed.
Figure 7-3
Adult frog
(3 years)
Sperm
Young frog
Tadpole develops
into frog
Sexual
Reproduction
Eggs
Tadpole
Fertilized egg
Egg hatches
development Organ formation
Fig. 7-3, p. 147
Case Study:
Why are Amphibians Vanishing?
 Habitat
loss and fragmentation.
 Prolonged drought.
 Pollution.
 Increases in ultraviolet radiation.
 Parasites.
 Viral and Fungal diseases.
 Overhunting.
 Natural immigration or deliberate introduction
of nonnative predators and competitors.
SPECIES INTERACTIONS:
COMPETITION AND PREDATION
 Species
can interact through competition,
predation, parasitism, mutualism, and
commensalism.
 Some species evolve adaptations that
allow them to reduce or avoid competition
for resources with other species (resource
partitioning).
Resource Partitioning
 Each
species minimizes
competition with the others
for food by spending at
least half its feeding time
in a distinct portion of the
spruce tree and by
consuming somewhat
different insect species.
Figure 7-7
The Fundamental Niche

The fundamental niche of an
organism is described by the full
range of environmental
conditions (biological and
physical) under which the
organism can exist.

The realized niche of the
organism is the niche that is
actually occupied. It is narrower
than the fundamental niche.
This contraction of the
realized niche is a result of
pressure from, and
interactions with, other
organisms.
Niche Specialization
 Niches
become
separated to
avoid competition
for resources.
Figure 7-6
Number of individuals
Number of individuals
Species 1 Species 2
Region
of
niche overlap
Resource use
Species 1
Resource use
Species 2
Fig. 7-6, p. 150
SPECIES INTERACTIONS:
COMPETITION AND PREDATION
 Species
called predators feed on other
species called prey.
 Organisms use their senses their senses to
locate objects and prey and to attract
pollinators and mates.
 Some predators are fast enough to catch their
prey, some hide and lie in wait, and some
inject chemicals to paralyze their prey.
PREDATION
 Some
prey escape
their predators or
have outer
protection, some
are camouflaged,
and some use
chemicals to repel
predators.
Figure 7-8
camouflage
(a) Span worm
Fig. 7-8a, p. 153
camouflage
(b) Wandering leaf insect
Fig. 7-8b, p. 153
Chemical warfare
(c) Bombardier beetle
Fig. 7-8c, p. 153
Chemical warfare
& warning coloration
(d) Foul-tasting monarch butterfly
Fig. 7-8d, p. 153
Chemical warfare
& warning coloration
(e) Poison dart frog
Fig. 7-8e, p. 153
mimicry
(f) Viceroy butterfly mimics
monarch butterfly
Fig. 7-8f, p. 153
Deceptive looks
(g) Hind wings of Io moth
resemble eyes of a much
larger animal.
Fig. 7-8g, p. 153
Deceptive
behavior
(h) When touched, snake
caterpillar changes shape
to look like head of snake.
Fig. 7-8h, p. 153
SPECIES INTERACTIONS:
PARASITISM, MUTUALISM, AND
COMMENSALIM
 Parasitism
occurs when one species feeds
on part of another organism.
 In mutualism, two species interact in a way
that benefits both.
 Commensalism is an interaction that benefits
one species but has little, if any, effect on the
other species.
Parasites: Sponging Off of Others
 Although
parasites can harm their hosts, they
can promote community biodiversity.



Some parasites live in host (micororganisms,
tapeworms).
Some parasites live outside host (fleas, ticks,
mistletoe plants, sea lampreys).
Some have little contact with host (dump-nesting
birds like cowbirds, some duck species)
Mutualism: Win-Win Relationship
 Two
species
can interact in
ways that
benefit both of
them.
Figure 7-9
(a) Oxpeckers and black rhinoceros
Fig. 7-9a, p. 154
(b) Clownfish and sea anemone
Fig. 7-9b, p. 154
(c) Mycorrhizal fungi on juniper seedlings
in normal soil
Fig. 7-9c, p. 154
(d) Lack of mycorrhizal fungi on juniper seedlings
in sterilized soil
Fig. 7-9d, p. 154
Commensalism: Using without Harming
 Some
species
interact in a way
that helps one
species but has
little or no effect
on the other.
Figure 7-10
ECOLOGICAL SUCCESSION:
COMMUNITIES IN TRANSITION
 New
environmental conditions allow one
group of species in a community to replace
other groups.
 Ecological succession: the gradual change
in species composition of a given area


Primary succession: the gradual establishment
of biotic communities in lifeless areas where
there is no soil or sediment.
Secondary succession: series of communities
develop in places containing soil or sediment.
Succession
 The
process where plants & animals of
a particular area are replaced by other
more complex species over time.
Primary Succession: Starting from Scratch
Primary succession begins with an essentially lifeless
are where there is no soil (bare rock). Soil formation
begins with lichens or moss.
Primary Succession
 Primary
begins with a lifeless area where
there is no soil (ex. bare rock). Soil
formation begins with lichens or moss.
Primary Succession
‣
Primary succession refers to
colonization of a region where
there is no pre-existing
community. Examples include:
Newly emerged coral atolls,
volcanic islands
Newly formed glacial
moraines
Islands where the previous
community has been
extinguished by a volcanic
eruption
Hawaii: Local plants are able to rapidly
recolonize barren areas
Primary Succession
‣ A classical sequence of colonization
begins with lichens, mosses, and
liverworts, progresses to ferns,
grasses, shrubs, and culminates in a
climax community of mature forest.
In reality, this scenario is rare. Unless
there is the formation of a new island or
volcanic eruption.
Mosses and
liverworts
Bare rock
and lichens
Grasses and
herbaceous
plants
Shrubs and
fast growing
trees
Mature, slow
growing trees
Secondary Succession:
Starting Over with Some Help
 Secondary
succession
begins in an
area where
the natural
community
has been
disturbed.
Figure 7-12
Fig. 7-12, p. 157
Secondary
Succession

Secondary succession occurs
where an existing community
has been cleared by a
disturbance that does not
involve complete soil loss.
Cyclone

Such disturbance events
include cyclone damage,
forest fires, hillside slips and
clear-cutting.
Clear-cutting reduces
ecosystem services
provided to humans like
oxygen, food, and removal
of carbon
Forest fire
Secondary Succession

Because there is still soil present,
the ecosystem recovery tends to
be more rapid than primary
succession, although the time
scale depends on the species
involved and on climatic and
edaphic (soil) factors.
Mature forest
Bare land
Grasses and
herbaceous plants
Pioneer community
(annual grasses)
Shrubs and
small trees
Young fast
growing trees
Pioneer Communities

A succession proceeds in
stages, until the formation of a
climax community, which is
stable until further disturbance.

Early successional (or pioneer)
communities are characterized
by:
Simple structure, with a small
number of species
interactions
Pioneer community, Hawaii
Broad niches
Low species diversity
Broad niches
Climax Communities

In contrast to early
successional
communities, climax
communities typically
show:
Complex structure,
with a large number of
species interactions
Climax community, Hawaii
Narrow niches with
typically large old
growth plants.
High species diversity
Large number of species interactions
Stages
– rock  lichen  small shrubs 
large shrubs  small trees  large
trees
 Land
Can We Predict the Path of
Succession, and is Nature in Balance?
 The
course of succession cannot be
precisely predicted.
 Succession
involves species competing for
enough light, nutrients and space which will
influence it’s trajectory.
ECOLOGICAL STABILITY AND
SUSTAINABILITY



Inertia (persistence): the ability of a living system
to resist being disturbed or altered.
Constancy: the ability of a living system to keep
its numbers within the limits imposed by available
resources.
Resilience: the ability of a living system to
bounce back and repair damage after (a not too
drastic) disturbance.
ECOLOGICAL STABILITY AND
SUSTAINABILITY
 Having
many different species appears to
increase the sustainability of many
communities.
 Human
activities are disrupting ecosystem
services that support and sustain all life and
all economies.