Population Interactions

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Community Interactions
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Vocabulary
A community consists of all the interacting
populations within an ecosystem
An ecosystem is all the organisms and their
nonliving environment within a defined area.
A niche is an organisms habitat, way of life, and
physical environment.
An organisms habitat is were it lives.
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Interactions
When populations interact with one another, they
influence each other ability to survive and
reproduce. They are agents of Natural Selection.
Competition between different species is called
inter-specific competition.
Competition between the same species is called
intra-specific competition.
No two species occupy the exact same niche.
The competitive exclusion principle states if you
put two species with the same niche together then
one will out compete the other and kill it off.
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Predator - Prey
Prey and predator populations both follow
cyclical trends
When predators get too numerous, prey population
drops and predator resources are thus depleted
When prey get too numerous, disease and other
density-dependent factors decrease the population
During the population drops only the least fit
individuals will die.
A change in the prey population illicit evolution
in the predator population and vice versa.
This Process is Co-Evolution
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Predator-Prey population trends
Prey and predator populations both follow
cyclical trends. Note that the predator lags
behind the prey then overshoots, causing its
population to crash.
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Niche

• This encompasses all the aspects of an organism
  way of life., it including
• the physical home or habitat
• all the physical factors like temperature, pH,
  type of soil, etc.
• how the organism gets its supply of energy
• its predators, prey and interactions with other
  organisms.

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Same Niche Competitive Exclusion
Gause (1934) If two organisms are competing for
the same resources, one will be better at
acquiring those resources.
One organism will therefore win out over another
if they are in the same niche. This is called
competitive exclusion.
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Seashore Gastropod Competition
Snails of the intertidal zone will eat algae.
Niches overlap but they are never the same. Rough
periwinkles are able to live high in the splash
zone avoiding direct competition with common
periwinkles and predation by crabs. Both
periwinkles can be observed in the same location
---------------gt Common periwinkles prefer the
lower portions of the seashore.
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Barnacle Competition
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Other Interactions
Symbiosis is a close relationship between
different species over an extended period of time.

• Symbiosis (living together)
• Parasitism (still predator and prey)
• Predator much smaller than prey
• Mutualism (everybody wins)
• Partnerships with other species
• Commensalism (at least somebody wins)
• One species gains, the other loses nothing

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Parasitism
Parasitism is a symbiotic relationship in which
one organism benefits by feeding on the other.

• Nematodes in cod fish
• tape worms on salmonids
• tapeworm larvae in snails
• But what about evolution? Are parasites who
  damage or kill their host the parasites that are
  always selected for? Can you anticipate
  evolutionary change with the above parasites?

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Newfoundland Example Tapeworm from salmonid
gut. Parasite under trout skin.
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Mutualism
Mutualism is a symbiotic relationship in which
both species benefit.
And...
Bacteria in our gut
Mitochondria
Nitrogen fixing bacteria legumes
Domesticated plants and animals
In fact, almost every known organism has some
mutualistic relationship with at least one other
organism...
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Commensalism
Commensalism is a symbiotic relationship in which
one organism benefits while the other is not
harmed.
Newfoundland Example Barnacles on Whale Skin
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Keystone Species
In some communities a certain species, called a
keystone species plays a major role in
determining community structure.
Removing a keystone species dramatically alters
the community.
Eg. The predatory Star fish
It eats the mollusks that feed on plankton
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Community Interactions
Have a look at the movies that detail
interactions on the seashore and intertidal
organisms.