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Community Ecology
Community Interactions
• Interspecific interactions are relationships
between different species
• May include competition, predation,
herbivory, and symbiosis
• + and – indicate whether the species benefit
or are harmed in the interaction, 0 is neutral
Competition
• Interspecific competition occurs
when individuals of different species
compete for the same resources (-/-)
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•
This relationship can lead to
competitive exclusion; i.e. the species
that is less successful will be
eliminated from the environment
True competition is rare because of
ecological niches
• Differences in niches allows organisms to
survive even if they are similar as long as
the niches are not identical (resource
partitioning)
Predation
•
One species benefits by
killing and eating another
(+/-)
•
There are a variety of
adaptations to aid both
predator and prey
• Predators: heat-sensing
organs, toxins, speed,
agility, camouflage, etc.
• Prey: hiding, fleeing,
forming herds or schools,
alarm calls, cyptic and
aposematic coloration,
etc.
Mimicry
•
Batesian Mimicry
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•
Mullerian Mimicry
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•
A harmless species that has
coloration similar to a harmful
species
Two or more unpalatable and
harmful species resemble each other
becoming something similar to
aposematic coloration
Predators can also use mimicry
•
Some snapping turtles have tongues
that resemble worms
Herbivory
•
The eating of plants or alga by
other organisms (+/-)
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Predator adaptations include
chemical sensors to determine
the plant’s nutritional value or if
the plant is toxic; specialized
teeth and/or digestive systems
Prey adaptations include toxins,
such as strychnine, nicotine,
tannins, selenium toxins;
chemicals that produce tastes
that are unpalatable to many
animals, such as cinnamon,
cloves and peppermint;
chemicals that cause abnormal
development; thorns or spines
Symbiosis
•
Symbiosis occurs when species live in direct
contact with one another forming a
relationship that may have beneficial,
harmful or neutral effects
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•
•
Parasitism (+/-) – a parasite derives its nutrients
from a host, harming the host in the process
(ticks, lice, etc.)
Mutualism (+/+) – an interaction that benefits
both species (acacia and ants)
Commensalism (+/0) – an interaction that
benefits one species while having no apparent
effect on the other (cattle egrets and water
buffalo)
Species Diversity
•
Species diversity is
determined by
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Species richness – the
number of different species
in the community
Relative abundance – the
proportion each species
represents of all individuals
in the community
Shannon diversity index is
used to determine the
diversity of a community in
a numerical way. This
allows for a comparison of
different communities.
Trophic Structure
• The structure and dynamics of a
community are largely related to the
feeding relationships
•
These can be represented in food chains and
food webs
• It has been documented that most food chains and
webs are limited to 5 or fewer trophic levels
• Energetic hypothesis – limited by inefficiency of
energy transfer from one trophic level to another
(~10%)
• Dynamic stability hypothesis – limited because
longer food chains are less stable and less able to
rebound from population fluctuations
• Most data currently supports the energetic
hypothesis
Impact of Species
•
Certain species have important
roles and large impacts in given
environments
•
Dominant species are abundant
and have the highest biomass in
the community
• May be related to ability to outcompete for water or nutrients or
lack of predators (such as is seen
with invasives)
• Ex: Historically – the American
chestnut
•
Keystone species are important
because of their ecological roles
• Ex: sea otters, Pisaster ochraceus
•
Foundation species cause physical
changes in the environment
through their behavior or biomass
• Ex: beavers, Juncus gerardi
Bottom-Up and Top-Down Controls
• Bottom-up model
•
Unidirectional influence from lower to higher trophic
levels
• The presence or absence of mineral nutrients (N) controls the
plant numbers (V), which control herbivore numbers (H), which
control predator numbers (P)
• NVHP
• Change at the bottom will effect everyone else, but change at the top
will not effect those at lower trophic levels significantly
• Top-down model (Trophic cascade model)
•
Says the opposite
• NVHP
• Change at the top will cause alternating (+/-) effects as it moves
down the trophic levels
• Removal of the top predator will increase the number of herbivores,
decrease phytoplankton, cause nutrient levels to rise
Disturbance and Species Diversity
•
A disturbance is an event that
changes a community by
removing organisms from it
or altering resource
availability
•
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Nonequilibrium model –
communities are constantly
changing after being affected
by disturbance
Intermediate disturbance
hypothesis – moderate levels
of disturbance can create
conditions that foster greater
species diversity than low or
high levels of disturbance
Ecological Succession
•
A gradual replacement of
species over time
•
Primary succession occurs
where this has never occurred
before on soils that are not yet
formed (volcanic and glacial
areas)
• Begins with prokaryotes and
protists, lichens and mosses
follow, allowing the formation
of shallow soils. Grasses,
shrubs and eventually trees can
then move in as the soils
become more mature.
•
Secondary succession occurs in
an area that has been disturbed,
but the maturity of the soils have
not be disturbed
Biogeography and Species Diversity
•
The latitude of a community and
the area it occupies tend to play
large roles in species diversity
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The tropics tend to be more
diverse than areas closer to the
poles
• Related to climate and
evolutionary history because
growing seasons are about 5
times longer in tropical areas
than in tundra, thus speciation
events can occur more frequently
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Species-area curve: all other
factors being equal, the larger the
geographic area of a community,
the more species it has
Pathogens
• Pathogens are now
recognized as having a huge
impact on ecosystems and
their stability.
• Pathogens are naturally
occurring but are often being
transmitted to new locations
because of human activities
•
The flow of these pathogens is
of great interest as humans are
also being effected