14.4 Interactions Within Communities

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14.4 Interactions Within Communities

• The theory that two species with similar
  requirements cannot coexist in the same community
  was proposed by Gause

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• - Habitat the food, water, shelter and space
  requirements determining where an organism lives,
  its "address
• - Niche the role of the organism in its
  environment where it lives, and how it "fits in"
  to the community structure, its "job"

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• Human populations are theoretically capable of
  living anywhere on this planet with abundant
  resources and no competition. This is a statement
  of our fundamental niche
• Humans do not make use of the total space
  available to live on the planet and find
  themselves crowded into smaller areas than
  theoretically possible. This is a statement of
  our realized niche.

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• In many ways, a science classroom can be
  considered a model of an ecosystem. The students
  rely on the teacher for knowledge and guidance
  the teacher would not have a job without the
  students. Neither can exist without the other.
  Ecologically, this relationship demonstrates
  mutualism.
• In many ways, a science classroom can be
  considered a model of an ecosystem. A student
  copies another student's work, contributing
  nothing. The student who did the work feels hurt
  and used. Ecologically, this relationship can be
  described as parasitism.

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• An ecological relationship between two species,
  where one species benefits and the other is
  harmed is either parasitism or predation

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resource partitioning may increase the chance of
species success.

• - the occupying of non-overlapping niches by
  organisms requiring similar resources
• - reduces competition, making resources
  available

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character displacement may provide a survival
advantage

• - traits of species occupying the same
  geographic range are more different than traits
  in the same species geographically separated
• - reduces competition by letting species sharing
  a region to occupy different niches

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It can be argued that, while not good for the
victim, predation has a positive effect on the
prey species.

• - predators remove the old, weak, and unhealthy,
  raising the general strength of the population
• - predators help prevent the prey species from
  exceeding their food supply

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Prey species often develop passive defense
mechanisms that reduce the chance of them being
eaten.

• - morphological (e.g., spines, hooks, needles,
  etc.)
• - chemical ( taste, toxic, smell, etc.)
• - camouflage
• - hiding
• - mimicking dangerous species

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• The coral snake and the king snake are very
  difficult to tell apart. One is poisonous, the
  other is not. This is an example camouflage

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• When attacked, the sea cucumber ejects its
  intestines. The predator attacks these allowing
  the animal an opportunity to escape. It will grow
  a new digestive system. This is an example of
  active defence

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Commensalism - some biologists argue that it does
not exist

• - a relationship in which one organism benefits,
  and the other is unaffected
• - very difficult to determine if "unaffected"
  species benefits, is harmed or is truly not
  affected without direct observation
• Hermit crabs live in shells left when snails
  die. The shell provides protection for the crab.
  This is an example of commensalisms and a defence
  mechanism

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• Termites eat wood, but cannot digest cellulose.
  In their gut live a protist that can digest
  cellulose, but is unable to survive outside the
  termite. If the protists are removed, the termite
  will starve. This is an example of obligatory
  mutualism

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• Organisms introduced into a region that is not
  their natural habitat and in which they have few
  natural predators are known to ecologists as
  exotics or nonindigenous species
• Nonindigenous species presently causing concern
  by affecting naturally-occurring species in
  Ontario include purple loosestrife and zebra
  mussels.

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Purple loosestrife (Lythrum salicaria., a
European perennial, is establishing populations
in aquatic ecosystems in North America, where it
is disrupting interactions between native
species. Researchers at the University of Guelph
are studying a European beetle as a method of
biological control.a. What general assumptions
are being made concerning the European beetle?

• - beetles eat purple loosestrife
• - beetles do not eat native plants
• - beetles are not harmful to native animals

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b. What will happen to the beetle population if
it is successful in controlling loosestrife?

• - as loosestrife population declines, beetle
  population declines
• - beetles will not start to eat native plant
  species

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c. Describe the ecological concerns if the
assumptions prove to be incorrect.

• - beetles will eat native plants, interfering
  with interactions between native species
• - beetles will over-populate, there being no
  natural predators
• - one problem will have been traded for another