Download Chapter 53 outline

A
community is any assemblage of
populations in an area or habitat.
 Species richness- the number of species
contained within a community
 Relative abundance- a measure of how
common or rare a species is in a
community
 Contrasting
views of communities are
rooted in the individualistic and interactive
hypotheses
• Two views on the question of how to account for species
found together as members of a community emerged in
the 1920s and 30s based on observation of plant
distribution
• An individualistic hypothesis of community structure
was proposed by H.A. Gleason it asserts that members of
a community are found together simply because they
share similar abiotic requirements such as temperature,
amount of rainfall and type of soil
• The Interactive hypothesis was advocated by F.E.
Clements who insisted that a community was a closely
intertwined group of individuals who were linked by
mandatory biotic interactions that cause the community
to work together as a unit
• The individualistic hypothesis predicts that
communities should generally lack discrete
geographic boundaries because each species
will be distributed according to its tolerance
ranges for abiotic factors, and communities
should change continuously with the addition or
subtraction of any particular specie
• The interactive hypothesis predicts that species
should be clustered into discrete communities
with noticeable boundaries due to the presence
of one species greatly influencing the presence
of another species
 The
Rivet Model
• Proposed by Paul and Anne Ehrlich ,the rivet
model of communities is an extension of the
interactive model, it suggests that most of the
species in a community are associated tightly
with other species in a web of life.
• Species are analogous to rivets because not all
the rivets are required to hold the wing together,
but if someone started removing rivets, we’d be
concerned with the welfare of the next flight.
 The
Redundancy Model
• According to this model, most of the species in a
community are not tightly associated, the web of
life is loose.
• An increase or decrease in one species has little
effect on other species which operate
independently
• Most communities lie somewhere in the middle
of these two extremes
 Interspecific
interactions are
relationships
between species of
a community
 Populations may
be linked by
competition
predation
mutualism and
commensalism
Interaction
Effects on Population
Density
Competition
(-/-)
The interaction is
detrimental to both
species
Predation (+/-)
Including parasitism
The interaction is
beneficial to one
species and
detrimental to the
other
Mutualism (+/+)
The interaction is
beneficial to both
species
Commensalism (+/0)
One species benefits
from the interaction
but the other is
unaffected
Interspecific
competition for
resources can occur
when resources are in
short supply
 If two populations do
compete for a resource
the result may be a
reduction in the
density of one or both
species or the
elimination of one of
the two competitors

 The
competition exclusion principle
• G.F. Gause studied the effects of interspecific
completion and determined that two species
cannot coexist in the same community if there
niches are identical
• The ecological niche is the sum total of a
species’ use of the biotic and abiotic resources
in its environment or the organism’s ecological
role
• Ecologically similar species can live in the same
community if their niches differ in one or more
specific ways
• Resource partitioning
is the differentiation
in niches that enables
similar species to
coexist in a community
• Character
displacement is the
tendency for
characteristics to be
more divergent in
sympatric populations
of the same two
species
Species adapt to eating
different types of seeds
 Herbivory-
herbivore eats part of a plant
 Parasitism-a parasite lives on or in its
host and depends on the host species for
nutrition
 Predation is a large factor in adaptive
evolution, natural selection refines the
adaptations of both predator and prey
 Predator
Adaptations
• Most predators have acute senses that enable them
to locate and identify potential prey
• Claws , teeth, fangs, stingers, and poison help catch
and subdue prey
 Plant
Defenses Against Herbivores
• Chemical toxins often in combination with
antipredator spines and thorns are a plant’s main
defense against being eaten into extinction
• Examples include morphine, opium, nicotine, and
mescaline.
• Also, plants produce flavors that may be distasteful
to herbivores like cinnamon, and peppermint
• Behavioral defenses include alarm
•
•
•
•
calls which bring in many individuals
of the same species
Feeing is a common antipredator
response
Cryptic coloration is a passive defense
that makes potential prey difficult to
spot in its surroundings
Some animals have mechanical of
chemical defenses against predators
like porcupines and skunks or frogs
that can synthesize toxins
Aposematic coloration is a warning to
predators, many animals with chemical
defenses are brightly colored
• In Batesian mimicry a
harmless species
copies the appearance
of a harmful model
• In Müllerian mimicry
two or more harmful
species resemble each
other , the predator
learns more quickly to
avoid prey with this
appearance due to the
large amount of them,
and both species
benefits
• Parasitism is a symbiotic
relationship in which the
parasite derives its
nourishment from another
organism, it’s host which is
harmed in the process
• Parasites that live within
their host are
endoparasites
• Parasites that feed on the
external surface of the host
are ectoparasites
• In parasitoidism insects lay
their eggs on a living host
the larvae then feed on the
body of the host eventually
killing it
 Mutualism
is an
interspecific
interaction that
benefits both species
• Examples include
nitrogen fixation by
bacteria in the root
nodules, and the
digestion of cellulose
by micro organisms in
the digestive systems
of termites
 Commensalism
is an
interaction between
species that benefits
only one species for
example, algae or
barnacles that attach
to whales
 Coevolution
refers to reciprocal
evolutionary adaptations of two
interacting species. A change in one
species acts as a force on another
species, in which counteradaptation in
turn acts as a selective force on the first
species
 Food
chains link the trophic levels from
producers to top carnivores. Branching
food chains form food webs. A
communities total energy input limits the
length of its food chains
Dominant species are the most abundant
species in a community and dominance
is achieved if a species has a great deal
of competitive ability
 Keystone species are relatively rare
species that exert a disproportionate
influence on the community structure

 The
bottom up model proposes that
nutrients and produces are the main
determinants of community structure
 The top down model proposes that
predators control herbivores who in turn
control producers, thus power comes
from the trophic level above
 Most
communities are in a state of
nonequilibrium
• Evidence suggests that disturbance and
nonequilibrium instead of stability and
equilibrium are normal for most communities
 Humans
are the most widespread agents
of disturbance
• Humans create the greatest disturbances in
communities usually reducing species diversity.
• Humans also prevent some naturally occurring
disturbances like fire
• Primary succession
occurs when no soil
exists when
succession begins;
secondary succession
will begin in an area
where soil remains
after a disturbance
 Community
biodiversity measures the
number of species and their relative
abundance
• The simplest measure of biodiversity is the
number of species in a community (species
richness)
• species may be rare or common in a community
so relative abundance is a factor in biodiversity
 Species
richness generally declines along
an equatorial-polar gradient
• Species richness is much greater in the tropics than
in the temperate and polar regions.
• Climate is the best explanation for this biodiversity
gradient through its impact on energy and water
 Species
richness is related to a community’s
geographic size
 Species richness on islands depends on
island size and distance from the mainland
• A hypothesis of island biogeography maintains that
species richness on an ecological island levels off at
some equilibrium point where new immigrations are
balanced by extinctions
• The hypothesis predicts that species richness is
directly proportional to island size and inversely
proportional to distance of the island from the source
of colonizers