Download chapter 53

CHAPTER 53
COMMUNITY ECOLOGY
Section A: What Is a Community?
1. Contrasting views of communities are rooted in the individualistic and
interactive hypotheses
2. The debate continues with the rivet and redundancy models
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Introduction
• What is a Community?
• A community is defined as an assemblage of species
living close enough together for potential interaction.
• Communities differ
in their species
richness, the
number of species
they contain, and
the relative
abundance of
different species.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Contrasting views of communities are
rooted in the individualistic and interactive
hypotheses
• An individualistic hypothesis depicts a community
as a chance assemblage of species found in the same
area because they happen to have similar abiotic
requirements.
Fig. 53.1a
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• An interactive hypothesis depicts a community as
an assemblage of closely linked species locked in
by mandatory biotic interactions.
Fig. 53.1b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• These two very different hypotheses suggest
different priorities in studying biological
communities.
• In most actual cases, the composition of
communities does seem to change continuously.
Fig. 53.1c
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. The debate continues with the rivet and
redundancy models
• The rivet model of communities is a reincarnation of
the interactive model.
• The redundancy model states that most species in a
community are not closely associated with one
another.
• No matter which model is correct, it is important to
study species relationships in communities.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CHAPTER 53
COMMUNITY ECOLOGY
Section B1: Interspecific Interactions and
Community Structure
1. Populations maybe linked by competition, predation, mutualism, and
commensalism
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Introduction
• There are different interspecific interactions,
relationships between the species of a community.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Populations may be linked by
competition, predation, mutualism and
commensalism
• Possible interspecific interactions are introduced in
Table 53.1, and are symbolized by the positive or
negative affect of the interaction on the individual
populations.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Competition.
• Interspecific competition for resources can
occur when resources are in short supply.
• There is potential for competition between
any two species that need the same limited
resource.
• The competitive exclusion principle: two
species with similar needs for same limiting
resources cannot coexist in the same place.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The ecological niche is the sum total of an
organism’s use of abiotic/biotic resources in the
environment.
• An organism’s niche is its role in the
environment.
• The competitive exclusion principle can be
restated to say that two species cannot coexist
in a community if their niches are identical.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Classic experiments confirm this.
Fig. 53.2
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Resource partitioning is the differentiation of
niches that enables two similar species to
coexist in a community.
Fig. 53.3
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 53.2
• Character displacement is the tendency for
characteristics to be more divergent in sympatric
populations of two
species than in
allopatric
populations
of the same two
species.
• Hereditary changes
evolve that bring
about resource
partitioning.
Fig. 53.4
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Predation.
• A predator eats prey.
• Herbivory, in which animals eat plants.
• In parasitism, predators live on/in a host and
depend on the host for nutrition.
• Predator adaptations: many important feeding
adaptations of predators are both obvious and
familiar.
• Claws, teeth, fangs, poison, heat-sensing
organs, speed, and agility.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Plant defenses against herbivores include chemical
compounds that are toxic.
• Animal defenses against predators.
• Behavioral defenses include fleeing, hiding, selfdefense, noises, and mobbing.
• Camouflage includes cryptic coloration,
deceptive markings.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 53.5
• Mechanical defenses include spines.
• Chemical defenses include odors and toxins
• Aposematic coloration is indicated by
warning colors, and is sometimes associated
with other defenses (toxins).
Fig. 53.6
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Mimicry is when organisms resemble other
species.
• Batesian mimicry is where a harmless
species mimics a harmful one.
Fig. 53.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Müllerian mimicry is where two or more
unpalatable species resemble each other.
Fig. 53.8
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Parasites and pathogens as predators.
• A parasite derives nourishment from a host,
which is harmed in the process.
• Endoparasites live inside the host and
ectoparasites live on the surface of the host.
• Parasitoidism is a special type of parasitism
where the parasite eventually kills the host.
• Pathogens are disease-causing organisms that
can be considered predators.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Mutualism is where
two species benefit
from their interaction.
• Commensalism is
where one species
benefits from the
interaction, but other
is not affected.
• An example would
be barnacles that
attach to a whale.
Fig. 53.9
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Coevolution and interspecific interactions.
• Coevolution refers to reciprocal evolutionary
adaptations of two interacting species.
• When one species evolves, it exerts selective
pressure on the other to evolve to continue the
interaction.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CHAPTER 53
COMMUNITY ECOLOGY
Section B2: Interspecific Interactions and
Community Structure (continued)
2. Trophic structure is a key factor in community dynamics
3. Dominant species and keystone species exert strong controls on community
structure
4. The structure of a community may be controlled bottom-up by nutrients or
top-down by predators
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. Trophic structure is a key factor in
community dynamics
• The trophic structure of a community is determined
by the feeding relationships between organisms.
• The transfer of food energy from its source in
photosynthetic organisms through herbivores and
carnivores is called the food chain.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Charles Elton first
pointed out that the
length of a food
chain is usually four
or five links, called
trophic levels.
• He also recognized
that food chains are
not isolated units but
are hooked together
into food webs.
Fig. 53.10
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Food webs.
• Who eats whom in a
community?
• Trophic relationships
can be diagrammed in
a community.
• What transforms
food chains into
food webs?
• A given species may
weave into the web at
more than one trophic
level.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 53.11
• What limits the length of a food chain?
• The energetic hypothesis suggests that the length of
a food chain is limited by the inefficiency of energy
transfer along the chain.
• The dynamic stability hypothesis states that long
food chains are less stable than short chains.
Fig. 53.13
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
3. Dominant species and keystone species
exert strong controls on community
structure
• Dominant species are those in a community that
have the highest abundance or highest biomass (the
sum weight of all individuals in a population).
• If we remove a dominant species from a
community, it can change the entire community
structure.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Keystone species
exert an important
regulating effect
on other species
in a community.
Fig. 53.14
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• If they are removed, community structure is greatly
affected.
Fig. 53.15
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
4. The structure of a community may be
controlled bottom-up by nutrients or topdown by predators
• Simplified models based on relationships between
adjacent trophic levels are useful for discussing how
communities might be organized.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Consider three possible relationships between plants
(V for vegetation) and herbivores (H).
• VH
VH
V  H
• Arrows indicate that a change in biomass of one
trophic level causes a change in the other trophic
level.
• The bottom-up model postulates V  H linkages,
where nutrients and vegetation control community
organization.
• The top-down model postulates that it is mainly
predation that controls community organization
V  H.
• Other models go between the bottom-up and topdown extreme models.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CHAPTER 53
COMMUNITY ECOLOGY
Section C1: Disturbance and Community Structure
1. Most communities are in a state of nonequilibrium owing to disturbances
2. Humans are the most widespread agents of disturbance
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Introduction
• Disturbances affect community structure and
stability.
• Stability is the ability of a community to persist in
the face of disturbance.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Most communities are in a state of
nonequilibrium owing to disturbances
• Disturbances are events like fire, weather, or human
activities that can alter communities.
• Some are routine.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 53.16
• Marine communities are subject to disturbance
by tropical storms.
Fig. 53.17
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 53.17
• We usually think that disturbances have a negative
impact on communities, but in many cases they are
necessary for community development and
survival.
Fig. 53.18
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. Humans are the most widespread agents
of disturbance
• Human activities cause more disturbance than
natural events and usually reduce species diversity in
communities.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CHAPTER 53
COMMUNITY ECOLOGY
Section C2: Disturbance and Community Structure
(continued)
3. Ecological succession is the sequence of community changes after a
disturbance
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
3. Ecological succession is the sequence of
community changes after a disturbance
• Ecological succession is the transition in species
composition over ecological time.
• Primary succession begins in a lifeless area where
soil has not yet formed.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 53.19
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Mosses and lichens colonize first and cause the
development of soil.
• An example would be after a glacier has
retreated.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Secondary succession occurs where an existing
community has been cleared by some event, but the
soil is left intact.
• Grasses grow first, then trees and other
organisms.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Soil concentrations of
nutrients show changes
over time.
Fig. 53.20
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
CHAPTER 53
COMMUNITY ECOLOGY
Section D: Biogeographic Factors Affecting the
Biodiversity of Communities
1. Community biodiversity measures the number of species and their relative
abundance
2. Species richness generally declines along an equatorial-polar gradient
3. Species richness is related to a community’s geographic size
4. Species richness on an island depends on island size and distance from the
mainland
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Introduction
• Two key factors correlated with a community’s
biodiversity (species diversity) are its size and
biogeography.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Community biodiversity measures the
number of species and their relative
abundance
• The variety of different kinds of organisms that make
up a community has two components.
• Species richness, the total number of species in the
community.
• Relative abundance of the different species.
• Imagine two small forest communities with 100
individuals distributed among four different tree species.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Species
richness
may be equal,
but relative
abundance
may
be different.
Fig. 53.21
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Counting species in a community to determine
their abundance is difficult, especially for
insects and smaller organisms
Fig. 53.22
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. Species richness generally declines along
an equatorial-polar gradient
• Tropical habitats support much larger numbers of
species of organisms than do temperate and polar
regions.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 53.23
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• What causes these gradients?
• The two key factors are probably evolutionary
history and climate.
• Organisms have a history in an area where they
are adapted to the climate.
• Energy and water may factor into this
phenomenon.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 53.24
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
3. Species richness is related to a
community’s geographic size
• The species-area curve quantifies what may seem
obvious: the larger the geographic area, the greater
the number
of species.
Fig. 23.25
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
4. Species richness on islands depends on
island size and distance from the mainland
• Because of their size and isolation, islands provide
great opportunities for studying some of the
biogeographic factors that affect the species diversity
of communities.
• Imagine a newly formed island some distance from
the mainland.
• Robert MacArthur and E. O. Wilson developed a
hypothesis of island biogeography to identify the
determinants of species diversity on an island.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Two factors will determine the number of
species that eventually inhabit the island.
• The rate at which new species immigrate to the
island.
• The rate at which species become extinct.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 53.26
• Studies of plants on
many island chains
confirm their
hypothesis.
Fig. 53.27
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings