Chapter 53: Community Ecology

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Chapter 53 Community Ecology

• Trophic Structure, Predation, and Competition

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Biological Community
Assemblage of populations of various species
living close enough for potential interaction
Interspecific interactions
Symbiosis
Competition
Disease
Herbivory
Predation
Interspecific Competition species compete for a
resource thats in short supply
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Competition
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Predation, Parasitism, Herbivory
/- interaction predator kills eats prey
/- interaction herbivore eats plant, kills it
Predator
Prey
Herbivore
Plant
Have senses that enable them to locate prey
Self-defense mechanisms camouflage, bright
coloration, mimicry
Have special sensors to recognize appropriate
food
Chemical weapons/ thorns protect them
/- interaction one organism (parasite) gets
nourishment from other organism (host), which is
harmed
parasitoidism
ectoparasite
endoparasite
Insect lays eggs in/on living host, leaving the
larvae to feed on body, and kill it
Feed on external surface of host
Live within hosts body
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Disease, Mutualism, Commensalism
Inflict lethal harm, can limit populations
/0 interaction 1 species benefits, the other
is not affected hard to find true example in
nature
/ interaction both species help each other
Pathogen disease-causing agent unlike parasite
microscopic, lethal /- interaction
Species Diversity
Variety of diff. kinds of organisms that make up
the community
Species Richness
Total of different species
Proportion each species represents of total
individuals in the community
Relative abundance
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Trophic Structure
Eventually to decomposers
Feeding relationships between organisms
Food Chain
Energetic Hypothesis length of food chain is
limited by inefficiency of energy transfer along
chain
Dynamic Stability Hypothesis long food chains
are less stable than short food chains
population fluctuations at lower levels are more
profound at higher levels, causing potential
extinction of high level predators
Food Web food chains are linked together 1
species, such as plant, is eaten by several
species
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Species With Large Impacts
Biomass total mass of all individuals in a
population
Dominant species
Keystone species
Not necessarily abundant exert strong control on
community by their ecological roles
Species that collectively have highest biomass
OR most abundant
foundation species facilitators cause
physical changes that benefit community, by
increasing survival and reproduction
Ecosystem engineers
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Bottom-Up and Top-Down Models
Top-Down (Trophic Cascade) Model
Bottom-Up Model
Model of community organization
Predation controls community organization
predators control herbivores, which control
plants, which control nutrient levels
Mineral nutrients control community organization
nutrients control plant s, which control
herbivore s, which control predator s
Polluted State
Restored State
Fish Abundant Rare
Zooplankton Rare Abundant
Algae Abundant Rare
Biomanipulation
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Disturbance
An event that changes a community, removes
organisms from it, and alters resource
availability
storm
Not always negative
fire
flood
drought
overgrazing
Burned trees release nutrients no trees to block
sunlight for new plants
Human Disturbance harmful, reduces species
diversity has contributed to famines by
overgrazing land
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Ecological Succession
Transition in species composition of a biological
community, usually following a disturbance of
the community
Primary Succession occurs in virtually lifeless
area, with no organisms or soil only autotrophic
prokaryotes are present
Secondary Succession occurs where an existing
community was cleared by disturbance, that leaves
soil intact first plants to recolonize grow
from wind-blown/animal-borne seeds
Factors Correlated With a Communitys Species
Diversity
Size
Geographic Location
Greater areamore diverse habitats, more species
Tropical habitats have the most species tropical
habitats are older (long growing season), and
greatest evapotranspiration
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Island Equilibrium Model (immigration
rateextinction rate)
Factors Affecting of Species That Will Inhabit
Island
New Species Immigration Rate
Species Extinction Rate
Influenced By
of Species Already Present
Island Size
Islands Distance from Mainland
new colonizers unlikely to reach small island
low immigration rate, high extinction rate (less
resources)
Species
immigration rate (those reaching island are less
likely to represent species not already there)
Extinction rate (competitive exclusion)
Islands near mainland high immigration rate, low
extinction rate (new colonizers sustain presence
of species)
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Integrated and Individualistic Hypotheses
Integrated Hypothesis (Clements)
Individualistic Hypothesis (Gleason)
Describes a community as an assemblage of closely
linked species, locked into association by
mandatory biotic interactions
States that species are found in the same area
simply because they happen to have similar
abiotic requirements
These interactions cause the community to
function as an integrated unit
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Rivet and Redundancy Models
Rivet Model
Redundancy Model
Suggests most of the species in a community are
associated tightly with particular other species
in a web of life
Exact opposite of the rivet model
Suggests species in a community are redundant
species operate independently, arent affected
if one species increases/decreases Ex. if one
pollinator disappears, then another species will
do the job
the Ehrlichs agree with Clements
Walker agrees with Gleason
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DDT Affects Trophic Structure
DDT and other toxins, when in tissues and fats of
an organism, can accumulate and be passed onto
the next level of the food chain, affecting many
organisms. Rachel Carson states in Silent Spring
that the concentration of DDT in a low level
organisms fat is a lot lower than that of a
higher level organism on the food chain
Zooplankton other primary consumers, secondary
consumers, etc. are affected
If phytoplankton is infected
Everything occurring at low food chain levels is
magnified at high food chain levels, causing
possible extinction of top level predators
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Pollution Affects Trophic Structure
Similar effect of DDT If organism is affected by
pollution, it can spread through food chain,
greatly affecting top-level predators
If water is polluted and filled with garbage, the
fish living there drink bad water and arent
getting the right nutrients
Make fish sick, and all those who eat them
fishs predators, fishs predators predators,
and even human who eat fish for dinner
If one organism is infected by either DDT,
pollution, or by several other possibilities, the
harms done to that animal certainly dont stop
there
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DDT Impacts Predation
The spraying will cover the soil and the
nutrients it contains.

• The producers absorb the DDT and are eaten by the
  consumers.

• The predators will continue to consume the DDT
  laden prey, accumulating the DDT in their body
  fats.

imbalance of predators to prey will cause the
ecosystem to go out of balance
The reduction of 4th level predators, for
example, will cause
overabundance of the 3rd level prey, which
consume so much 2nd level prey
Top-Down Model
Decrease 2nd level prey, so theres not as many
organisms to eat primary producers
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Effects of Global Warming on Predation
As the environment becomes increasingly warmer
each year, organisms might not be able to adapt
or migrate to other areas
The introduction of organisms disrupts and
changes the ecosystem of the environment.
This migration will bring new predators and new
prey, which would alter the trophic structure.
If organisms die, their predators would starve,
and there would be an increase in the dying
organisms prey
Altering the trophic structure would change the
predator-prey relationships the top level
carnivores may not be the top level carnivores
anymore, having an effect like the top-down model
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DDT Affects Competition

• DDT can increase competition
• DDT may kill many organisms of a species that two
  other species compete for.
• Because the resource is in short supply,
  interspecific competition between the two
  competing species will increase, hurting both
  species

eventually eliminating one of the two competing
species (competitive exclusion).
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DDT Affects Competition

• DDT can also lessen competition

Reproduction Rates DDT influences reproduction
rates Ex. Eagles in Silent Spring by Rachel
Carson -DDT stored in eagles fatty cells cause
the deposition of calcium in their eggshells to
be hindered.
Biological Magnification DDTs concentration
with each link in the food chain. By eating
DDT-contaminated food, DDT accumulates in
carnivores, which may be enough to kill the
carnivores.
Less carnivores some organisms populations from
lower trophic levels (top-down model), providing
more of those organisms for the surviving
carnivores (more food for them).
Weak eggs, so many parent eagles accidentally
crushed the egg during incubation some eggs also
didnt hatch
eagle population the population of eagles
prey increased, so competition for those prey
isnt as intense.
less competition among remaining carnivores for
resources.
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Global Warmings Impact on Competition
Some organisms may not be able to adapt fast
enough to the rapidly changing conditions of the
world
Species may become extinct its predators may
starve/be eliminated its prey is more abundant
Have edge over other species to get resources,
reproduce rapidly lead to local elimination of
interior competitor (not able to obtain resources)
Some organisms may adapt well to the new
conditions caused by global warming
If species is extinct or adapts, a competitor is
still eliminated, leading to less competition for
resources
Survivors reproduce, pass genes onto offspring
over evolutionary time, may evolve to new species
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Presented By Tom Doherty
Kelly Terlizzi
Kerena Thomas Andrew Tom
A.P. Biology Pd. A 9/15/08