14.4 Interactions Within Communities
• The theory that two species with similar
requirements cannot coexist in the same
community was proposed by Gause
- 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"
• 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.
• 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.
• An ecological relationship between two species,
where one species benefits and the other is harmed
is either parasitism or predation
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
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
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
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
• 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
• 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
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
• 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
• 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.
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
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
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