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Interspecific Interactions and the Ecology of Communities
Biological Communities
Community
-Species that occur at any particular locality
-Characterized by
*Species richness
-Number of species present
-Amazon Rainforest (highest richness)
*Primary productivity
-Amount of energy produced
-Interactions among members govern many ecological and evolutionary processes
Two vies of structure and functioning of communities
-Individualistic concept: A community is nothing more than an aggregation of species that
happen to occur together at one place
-Holistic concept: A community is an integrated unit; superorganism – more than the sum of its
parts
-Abundance of tree species along a moisture gradient in Santa Catalina Mountains of
Southeastern Arizona
Look at PP
-Each line represents the abundance of a different tree species
-Community composition changes continually along the gradient
-Sometimes the abundance of species in a community does change geographically in a
synchronous pattern
-Most ecologist today favor the individualist concept
-In communities, species respond independently to changing environmental conditions
-Community composition changes gradually across landscapes
-Ecotones: places where the environment changes abruptly
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
-Requirement for moisture and more
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
-Exploitative competition
*Consuming the same resources
Fundamental niche
-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
Other causes of niche restriction
-Predator absence or presence
-Absence of pollinators
-Presence of herbivores
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
...
barsaria
-Expected same results – one winner
-Both species survived by dividing resources
*Realized niche did not overlap too much
Resource Partitioning: dividing resources
Fundamental niches overlap but realized niche are different enough to survive

Resource partitioning among sympatric lizard species
Subdivided niche to avoid direct competition
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
Sympatric: Same; together in same place
Allopatric: Alone; all by itself
Predator-Prey
Predation
-Consuming of one organism by another
Predation strongly influences prey populations
Prey populations can have explosions and crashes
-White-tailed deer in Eastern U
...

-Introduction of rats, dogs, cats on islands
-New Zealand: Stephen island wren extinct because of a single cat
Predation and coevolution
-Predation provides strong selective pressure on the prey population
-Features that decrease the probability of capture are strongly favored
-Coevolution race may ensue
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 other
-Herbivores coevolve to continue eating the plants
Chemical defenses in animals
-Monarch butterfly caterpillars feed on milkweed and dogbane families
-Monarch incorporate cardiac glycosides from the plants for protection from predation

-Butterflies are eaten by birds, but the Monarch contains the chemical from milkweed that
makes the bird sick
Poison-dart frogs of the family Dendrobatidae produce toxic alkaloids in the mucus that covers their
brightly colored skin
Defensive coloration
-Insects and other animals that are poisonous use the warning coloration (aposematic)
-Organisms that lack specific chemical defenses
*Camouflage or cryptic coloration help nonpoisonous animals blend with their
surroundings
*Camouflages animals do not usually live together in groups
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 that distasteful model
-Batesian mimicry (lower proportions in comparison to distasteful species)
*mimic looks like distasteful species
-Mullerian mimicry
*several unrelated but poisonous species come to resemble one another
Species Interacions
Symbiosis
-2 or more kinds of organisms interact in more-or-less permanent relationships
-Potential for coevolution
-Three major types of symbiosis
*Commensalism
*Mutualism
*Parasitism
Commensalism benefits one species and is neutral to the other
-Spanish moss: hangs from trees
When commensalism may not be commensalism
Oxpeckers and grazing animals
-Oxpeckers eat parasites off of grazers

-Sometimes pick scabs and drink the blood
-Grazers could be unharmed by the insects the Oxpeckers eat
Mutualism benefits both species
-Coevolution: flowing plants and insects
*Ants and acacias
-Acacias provide hollow thorns and food
-Ants provide protection from herbivores
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
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
-Wasps, whose larvae feed on the body of the host, killing it
External parasite: the yellow vines are the flowering plant dodder; it is a parasite that obtains
Internal parasites
-Endoparasites live inside the host
-Extreme specialization by the parasite as to which host it invades
-Structure of the parasites might be simplified but



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