Transcript Community Ecology Chapter 56

Community Ecology
Chapter 56
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Biological Communities
• Community: all the organisms that live
together in a specific place
– Evolve together
– Forage together
– Compete
– Cooperate
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Biological Communities
• Communities can be characterized either
by their constituent species or by their
properties
– Species richness: the number of
species present
– Primary productivity: the amount of
energy produced
• Interactions among members govern many
ecological and evolutionary processes
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Biological Communities
• Interactions in a community
– Predation
– Mutualism
• Assemblage: the species included are only
a portion of those present in the community
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Biological Communities
• Two views of structure and functioning of
communities
– Individualistic concept: H.A. Gleason;
a community is nothing more than an
aggregation of species that happen to
occur together at one place
– Holistic concept: F.E. Clements: a
community is an integrated unit;
superorganism-more than the sum of its
parts
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Biological Communities
Most ecologists today
favor the individualistic
concept
• In communities, species
respond independently
to changing
environmental
conditions
• Community composition
changes gradually
across landscapes
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Biological Communities
• Abundance of tree
species along a moisture
gradient in the Santa
Catalina Mountains of
Southeastern Arizona
• Each line represents the
abundance of a different
tree species
• Community composition
changes continually
along the gradient
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Biological Communities
• Sometimes the
abundance of species
in a community does
change
geographically in a
synchronous pattern
• Ecotones: places
where the
environment changes
abruptly
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Ecological Niche
• Niche: the total of all the ways an
organism uses the resources of its
environment
– Space utilization
– Food consumption
– Temperature range
– Appropriate conditions for mating
– Requirements for moisture and more
Billock
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Ecological Niche
• Interspecific competition: occurs when
two species attempt to use the same
resource and there is not enough resource to
satisfy both
• Interference competition: physical
interactions over access to resources
– Fighting
– Defending a territory
– Competitive exclusion: displacing an
individual from its range
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Ecological Niche
• Fundamental niche: the entire niche
that a species is capable of using,
based on physiological tolerance limits
and resource needs
• Realized niche: actual set of
environmental conditions, presence or
absence of other species, in which the
species can establish a stable
population
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Ecological Niche
J.H. Connell’s classical study of barnacles
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Ecological Niche
• Other causes of niche
restriction
– Predator absence or
presence
• Plant species
– Absence of pollinators
– Presence of herbivores
Billock
Billock
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Ecological Niche
Principle of competitive exclusion: if
two species are competing for a limited
resource, the species that uses the
resource more efficiently will eventually
eliminate the other locally
• G.F. Gause’s classic experiment on
competitive exclusion using three
Paramecium species shows this principle
in action
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Ecological Niche
• Niche overlap and coexistence
• Competitive exclusion redefined: no
two species can occupy the same niche
indefinitely when resources are limiting
• Species may divide up the resources,
this is called resource partitioning
• Gause found this occurring with two of
his Paramecium species
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Resource
partitioning
among
sympatric
lizard
species
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Ecological Niche
• Resource partitioning is often seen in
similar species that occupy the same
geographic area
• Thought to result from the process of
natural selection
• Character displacement: differences
in morphology evident between
sympatric species
– May play a role in adaptive radiation
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Ecological Niche
Character displacement in Darwin’s
finches
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Ecological Niche
• Detection of interspecific competition
can be difficult
– If resources not limited there may be
no competition
– Small versus large population size
– May be environmental conditions that
cause the decline of a species, not
competition
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Ecological Niche
• Experimental studies of competition
• Seed-eating rodents and Kangaroo rats
– 50m x 50m enclosures
– Enclosures had openings large
enough for seed-eating rodents but
not the Kangaroo rats
– Monitor the number of small rodents
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Ecological Niche
Detecting interspecific competition
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Ecological Niche
• Interpreting field data: Negative effects of
one species on another do not
automatically indicate the existence of
competition
– Adults may prey on juveniles of the
other species
– Presence of one species may attract
predators that prey on both species
• Experimental studies are not always
feasible
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Predator-Prey
• Predation: consuming of one organism by
another
• Predation strongly influences prey
populations
• Prey populations can have explosions and
crashes
– White-tail deer in Eastern US
– Introduction of rats, dogs, cats on islands
– New Zealand: Stephen Island wren
extinct because of a single cat
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Predator-Prey
Predator-prey in the
microscopic world
When the prey
(Paramecium) are
used up in the test
tube, the predator
(Didinium) also dies
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Predator-Prey
• Introduction of prickly-pear cactus in
Australia
– The cactus population exploded
– Introduced the predator a moth
(caterpillar eats the cactus) and the
population came under control
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Predator-Prey
• Predation and coevolution
– Predation provides strong selective
pressure on the prey population
– Features that decrease the probability
of capture are strongly favored
– Predator populations counteradapt to
continue eating the prey
– Coevolution race may ensue
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Predator-Prey
• Plants adapt to predation (herbivory) by
evolving mechanisms to defend
themselves
– Chemical defenses: secondary
compounds
• Oils, chemicals to attract predators
to eat the herbivores, poison milky
sap and others
– Herbivores coevolve to continue
eating the plants
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Predator-Prey
Insect herbivores well suited to their plant
hosts: cabbage butterfly
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Predator-Prey
• Chemical defenses in animals
– Monarch butterfly caterpillars feed on
milkweed and dogbane families
– Monarchs incorporate cardiac glycosides
from the plants for protection from
predation
– Butterflies are eaten by birds, but the
Monarch contains the chemical from the
milkweed that make the birds sick
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Predator-Prey
Blue Jay learns not to eat Monarch
butterflies
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Predator-Prey
Poison-dart frogs
of the family
Dendrobatidae
produce toxic
alkaloids in the
mucus that covers
their brightly
colored skin
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Predator-Prey
• Defensive coloration
– Insects and other animals that are
poisonous use warning coloration
– Organisms that lack specific chemical
defenses are seldom brightly colored
– Camouflage or cryptic coloration help
nonpoisonous animals blend with
their surroundings
– Camouflaged animals do not usually
live together in groups
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Predator-Prey
Inchworm caterpillar closely resembles a twig
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Predator-Prey
• Mimicry allows one species to capitalize
on defensive strategies of another
– Resemble distasteful species that
exhibit warning coloration
– Mimic gains an advantage by looking
like the distasteful model
– Batesian mimicry
– Müllerian mimicry
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Predator-Prey
• Batesian mimicry
– Named for Henry Bates
– Discovered palatable insects that
resembled brightly colored, distasteful
species
– Mimics would be avoided by
predators because they looked like
distasteful species
– Feed on plants with toxic chemicals
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Predator-Prey
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Predator-Prey
• Müllerian mimicry
– Fritz Müller
– Discovered that several unrelated but
poisonous species come to resemble
one another
– Predator learns quickly to avoid them
– Some predators evolve an innate
avoidance
• Both mimic types must look and act like the
dangerous model
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Predator-Prey
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Species Interactions
• Symbiosis: two or more kinds of
organisms interact in more-or-less
permanent relationships
• All symbiotic relationships carry the
potential for coevolution
• Three major types of symbiosis
– Commensalism
– Mutualism
– Parasitism
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Species Interactions
• Commensalism benefits one species
and is neutral to the other
– Spanish moss: an epiphyte hangs
from trees
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Species Interactions
• When commensalism may not be
commensalism
• Oxpickers and grazing animals
– Oxpickers eat parasites off of grazers
– Sometimes pick scabs and drink
blood
– Grazers could be unharmed by the
insects the oxpickers eat
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Species Interactions
• Oxpickers on an
impala
• Is it commensalism,
parasitism or
mutualism ?
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Species Interactions
• Mutualism benefits both
species
• Coevolution: flowering
plants and insects
Ants and acacias
– Acacias provide
hollow thorns and food
– Ants provide
protection from
herbivores
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Species Interactions
• Not all ant and acacia relationships are
mutualism
• In Kenya, several species of ants live on
acacias
– One species clips the acacia
branches to prevent other ants from
living in the tree
– Clipping branches sterilizes the tree
– A parasitic relationship
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Species Interactions
• Parasitism benefits one species at the
expense of another
• External parasites:
– Ectoparasites: feed on exterior
surface of an organism
– Parasitoids: insects that lay eggs on
living hosts
• Wasp, whose larvae feed on the
body of the host, killing it
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Species Interactions
External parasite: the yellow vines are
the flowering plant dodder, it is a parasite
that obtains its food from the host plant it
grows on
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Species Interactions
• Internal parasites
– Endoparasites: live inside the host
– Extreme specialization by the parasite
as to which host it invades
– Structure of the parasite may be
simplified because of where it lives in
its host
– Many parasites have complex life
cycles involving more than one host
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Species Interactions
• Dicrocoelium dendriticum is a flatworm
that lives in ants as an intermediate host
with cattle as its definitive host
• To go from the ant to a cow it changes
the behavior of the ant
• Causing the ant to
climb to the top of a
blade of grass to be
eaten with the grass
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Species Interactions
• Ecological processes have interactive
effects
– Predation reduces competition
• Predators choice depends partly on
relative abundance of the prey options
• Superior competitors may be reduced
in number by predation
• This allows other species to survive
when they could have been out
competed
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Species Interactions
Starfish eat barnacles, allowing other species
to thrive instead of being crowded out by the
explosive population of barnacles
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Species Interactions
• Parasitism may counter competition
– Parasites may affect sympatric species
differently, changing the outcome of
interspecific interactions
– Flour beetles and a competition
experiment
• Without a parasite T. castaneum is
dominant
• With the parasite: T. confusum is
dominant
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Species Interactions
• Indirect effects: presence of one
species may affect a second by way of
interactions with a third species
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Species Interactions
• Keystone species: species whose effects
on the composition of communities are
greater than one might expect based on
their abundance
• Sea star predation on barnacles greatly
alters the species richness of the marine
community
• Keystone species can manipulate the
environment in ways that create new
habitats for other species
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Species Interactions
Beavers construct dams and transform
flowing streams into ponds, creating new
habitats for many plants and animals
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Succession and Disturbance
• Primary succession: occurs on bare,
lifeless substrate
– Open water
– Rocks
• Organisms gradually move into an area
and change its nature
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Succession and Disturbance
Primary succession on glacial moraines
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Succession and Disturbance
• Secondary succession: occurs in
areas where an existing community has
been disturbed but organisms still
remain
– Example: field left uncultivated
– Forest after a fire
• Succession happens because species
alter the habitat and the resources
available in ways that favor other
species entering the habitat
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Succession and Disturbance
• Three dynamic concepts in the process
– Tolerance: early successional species are
characterized by r-selected species tolerant
of harsh conditions
– Facilitation: early successional species
introduce local changes in the habitat. Kselected species replace r-selected species
– Inhibition: changes in the habitat caused
by one species inhibits the growth of the
original species
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Succession and Disturbance
• Animal species in a community can also
change over time
• Krakatau island
– Volcanic eruption
– Fauna changed in synchrony with the
vegetation
– Changes in animals affect plant
occurrences; pollination, animal
dispersion
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Succession and Disturbance
Succession after a volcanic eruption
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Succession and Disturbance
• Communities are constantly changing as
a result of
– Climatic changes
– Species invasions
– Disturbance events
• Nonequilibrium models that emphasize
change rather than stability are used to
study communities and ecosystems
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Succession and Disturbance
• Intermediate disturbance hypothesis:
communities experiencing moderate
amounts of disturbance will have higher
levels of species richness than communities
experiencing either little or great amounts of
disturbance
– Patches of habitat will exist at different
successional stages
– May prevent communities from reaching
the final stages of succession
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Succession and Disturbance
• Disturbance is common,
rather than exceptional
in many communities
• Understanding the role
that disturbances play in
structuring communities
is an important area of
ecology
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