Community Ecology

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Chapter 53

• Community Ecology

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Community

• Assemblage of populations in an area or habitat
• Species richness - of species
• Relative abundance - of species
• What causes each community to have a certain
  assemblage of species

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Individualistic vs. Interactive Hypotheses

• Individualistic assemblage based on need
• Interactive assemblage based on other species
• Individualistic assemblage is widely accepted for
  plant communities

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Animal models for assemblage

• Rivet model Paul and Anne Ehrlich suggested
  that species are like rivets in a planes wing,
  not all are needed but if you remove a few you
  weaken the wing
• Reincarnation of the interactive model
• Redundancy model Brian Walker
• Web of life is very loose
• If one predator disappears another predator takes
  its place in a community
• Most communities lie somewhere in between these
  extremes

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Interspecific Interactions

• Relationships among species in a community

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Competition

• Species fight for limited resources
• Detrimental to both species
• Competitive exclusion principle
• No 2 species can occupy the same niche
  continuously

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Competition What is a niche?

• Niche is an organisms way of life
• Sum total of organisms use of biotic and abiotic
  resources

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Competition Competitive exclusion principle
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Competition Resource Partitioning

• Natural selection favors changing your niche in
  some way to avoid extinction

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Competition Character Displacement

• Sympatric species those that live in the same
  area
• Allopatric species those that live in different
  areas
• Character displacement tendency for characters
  of sympatric species to be more diverse

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Predation

• One species benefits / the other is harmed
• Predation also includes herbivores and parasitism
• Predator adaptations
• Skills for finding and killing prey
• Plant adaptations
• Chemical toxins
• Spines and thorns
• Grit

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Predation cont.

• Animal defenses
• Camouflage cryptic coloration
• Poisons or smells
• Aposematic coloration
• Batesian mimicry
• Mullerian mimicry

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Predation Parasites and Pathogens

• Parasites feed on a host
• Normally do not kill the host
• Why?
• Endoparasites live within the host
• Ectoparasites feed externally
• Mosquitoes
• Parasitoidism insects lay eggs on the host, and
  the offspring feed on the host
• Pathogens typically bacteria viruses or
  protists

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Mutualism

• Benefits both species
• Arose from predator-prey or host-parasite
  relationships
• Adaptation to gain advantage from the organism
  you supply with materials

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Commensalism

• Benefits one species the other is unaffected
• Hitchhiking species
• Opportunistic feeders

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Coevolution and Interspecific Interactions

• Coevolution reciprocal evolutionary adaptations
  of two interacting species
• Change in one species acts as a selective force
  on another species.
• Bats and moths

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Trophic Structure

• Structure of a community depends on the feeding
  level

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Food Webs
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Food Webs vs. Food Chains

• Food webs are more accurate depictions of an
  organisms place in a community
• Animal may enter the chain at more than one place
• Bears eat squirrels (predator) and berries
  (herbivore)
• Animals at successive trophic levels tend to be
  larger

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Limiting length of Food Chains

• Most food chains (derived from food webs) are
  only 5 or 6 links long
• Inefficient energy transfer
• 10 of energy stored in organic matter is
  converted to organic matter in the next level
• 100 kg plant material to 10 kg herbivore material
  to 1 kg of primary carnivore
• Dynamic stability hypothesis
• Long chains are unstable
• Changes at lower levels are magnified at higher
  levels

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Energetic Hypothesis
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Dominant Species and Keystone Species

• Some species play strong roles in communities
  either
• Because of sheer numbers (dominant)
• Or pivotal role in community (keystone)
• Removal of dominant species usually has little
  effect due to less competitive species taking
  over
• American chestnut
• Removal of keystone species sometimes has a large
  affect

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Community Structure

• May be controlled either by bottom-up by
  nutrients or top-down by predators
• Consider
• V H (increase in vegetation increases
  herbivores)
• V H (increase in herbivores decreases
  vegetation)
• V H (Feedback flows in both directions)

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Bottom-Up Model

• Postulates that vegetation affects herbivores
• In this case nutrients (N) control community
  organization because nutrients control plant
  community (V), which control herbivore numbers
  (H), which control predator numbers (P)
• N V H P
• If nutrients are added then all trophic levels
  increase
• But if you add predators from a bottom-up
  community there is no affect

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Top-down Model

• Postulates that it is mainly predation that
  affects community structure
• Trophic cascade model
• Cascade of /- affects
• Decrease in P results in increase of H and
  decrease of V and increase of N

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Disturbance and Communities

• Tradition views of community structure
• Stability tendency of a community to reach and
  maintain an equilibrium, or relatively constant
  composition of species even in the face of
  disturbance
• Many communities on a local scale change is very
  common
• Communities are constantly changing

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Disturbances and Community Structure

• Disturbances storms, fire, floods, droughts,
  human activities
• Often create opportunities for species that have
  not previously occupied habitats to become
  established
• Example storm disturbance on coral reefs

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Disturbances A Good Thing

• Small scale disturbances sometimes enhance
  environmental patchiness, which can be important
• Also can prevent large scale disturbances
• Yellowstone National Park (1988)
• Some pines only release seeds after a fire

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Ecological Succession

• Changes in a community over time
• Primary Succession change in community which
  starts with nearly lifeless structure
• Secondary Succession starting with soil and
  other key abiotic factors

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Key processes involved in succession

• Facilitation early species may contribute to
  the appearance of later species by making the
  environment more favorable
• Inhibition early species inhibit later species
  which thrive in spite of, not because of, early
  species
• Toleration later species are neither benefited
  for hindered by early species

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Biodiversity of Communities

• Biodiversity measure of species richness
• Factors affects biodiversity
• Size of community
• Geographic location of community
• More diverse in the tropics
• Small islands are less diverse than large islands
  or those near continents

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Measure of Biodiversity

• One component of biodiversity is
• Species richness total number of different
  species in a community
• Relative abundance of different species
• Diagram
• Species richness is the same
• Relative abundance is different

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Species richness

• Declines along an equatorial-polar gradient
• Tropical habitats support more life
• Why?
• Tropical communities are older
• Climate tropics have more sunlight and more
  water

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Species Richness and Water Availability
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Species Richness and Communitys Geographic Size

• Biodiversity patterns also follow a species-area
  curve
• Larger the area the more species
• Larger areas offer more habitats than smaller
  ones
• Conservation biologists use species-area curves
  to predict the affect of habitat loss on a
  species

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Species Richness on Islands

• Islands provide excellent study tools for
  biogeography studies
• Islands also include mountain tops, natural
  woodland fragments (any patch surrounded by
  unsuitable habitat for island species)
• MacArthur and E.O. Wilson developed a hypothesis
  of island biogeography to identify important
  determinants of species diversity on islands.

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Island Biogeography

• Newly formed oceanic island will receive
  colonizing species from a distant mainland
• Factor affecting number of species that will
  inhabit the island
• Rate at which new species immigrate to the island
• Rate at which species become extinct on the
  island
• Island physical feature that affect these rates
• Size
• Distance from mainland
• Small islands have fewer immigrants and higher
  extinction rates
• Closer island will have more immigrants than a
  far island
• Immigration and extinction rates also depend of
  the number of species already on the island
• As species increase the immigration rate of new
  species declines because it is most likely that
  this species is already represented
• As species increase on the island extinction
  rates will go up because of the competitive
  exclusion principle

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Island Biogeography

• Graphic view of MacArthur and Wilsons
  predictions
• Eventually immigration will equal extinction due
  to factors of island dynamics

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