Community Ecology

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Community Ecology
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Communities

• A community is a group of organisms of different
  species that live in a particular area

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Individualistic Hypothesis vs. Interactive
Hypothesis

• Individualistic Hypothesis
• A community is a chance group of species found in
  the same area because they have similar abiotic
  requirements
• Integrated (Interactive) Hypothesis
• A community is a group of closely linked species
  locked together in mandatory biotic interactions
  that cause the community to function as an
  integrated unit

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Interspecific Interactions

• Interspecific interactions are interactions that
  occur between populations of different species
  living together in a community
• There are 4 major interspecific interactions
• Predation (and parasitism)
• Competition
• Commensalism
• Mutualism

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Predation-Parasitism
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Predation (and Parasitism)

• ( -)
• The interaction is beneficial to one species and
  detrimental to the other
• Predation
• When a predator eats its prey
• Example in picture

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Predation (and Parasitism)

• Parasitism
• Predators that live on or in their hosts, usually
  feeding off their body tissues or fluids
• Usually do not kill their hosts
• Examples in picture (tick, leech)

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Parasitism

• One organism (the parasite) gets its nourishment
  from another organism (the host), which is harmed
  in the process
• Endoparasites
• Live within host tissues (tapeworms)
• Ectoparasites
• Feed on external surfaces (mosquitoes)
• Parasitoidism
• Insect lays eggs on or in a host. The eggs feed
  on the host . . . eventually killing it

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Disease

• Pathogens are similar to parasites (typically
  bacteria, viruses or fungi)

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Plant Defenses Against Herbivores

• Plants Fight Back!
• Plants have 2 major mechanisms by which they
  defend themselves against being eaten
• Mechanical Defenses
• Thorns, hooks, etc.
• Chemical Defenses
• Poisons

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Plant Defenses Against Herbivores

• Chemical Defenses
• Produce chemicals that are distasteful or harmful
  to an herbivore
• Morphine (opium poppy)
• Nicotine (tobacco)

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Animal Defenses Against Predators

• Animals defend themselves against predators
  passively (hiding) or actively (fleeing)
• Cryptic coloration (camouflage) makes prey
  difficult to spot
• Aposematic coloration (warning coloration)warns
  predators not to eat animals that may be toxic or
  may sting.

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Animal Defenses Against Predation

• Mimicry
• When one species imitates or mimics another
• Batesian mimicry
• When one edible or harmless species mimics an
  bad-tasting (unpalatable) or harmful species
• Example hawkmoth mimics a snake

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Animal Defenses Against Predation

• Mimicry
• Mullerian mimicry
• Two species, both of which are unpalatable (taste
  bad) or harmful, resemble each other
• Example monarch butterfly (unpalatable) and
  queen butterfly (unpalatable) resemble each other

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Competition
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Interspecific Competition

• (-/-)
• Competition between organisms of different
  species for a particular limited resource
• The Competitive Exclusion Principle
• Two species with similar needs for the same
  limiting resources cannot coexist in the same
  place
• Niches may overlap but they may not be identical.
• Niche sum total of a species use of the biotic
  and abiotic resources in an environment

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Ecological Niches

• An organisms niche is the specific role it plays
  in its environment
• All of its uses of biotic and abiotic resources
  in its environment
• Example oak tree in a deciduous forest
• Provides oxygen to plants, animals, etc.
• Home for squirrels
• Nesting ground for blue jays
• Takes water out of the soil
• Etc., etc.

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Fundamental v. Realized Niche

• Fundamental Niche includes resources an organism
  could theoretically use (if no competition)
• Realized Niche includes resources it actually
  does use given competition from other species.

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Resource Partitioning

• Similar species develop ways to partition/divide
  resources in order to coexist.

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Character Displacement

• Evidence for competition can sometimes be
  determined by looking at closely related species
• Allopatric (geographically separate) are
  morphologically similar and use similar resources
• Sympatric (overlapping geography) show
  different morphology and use different resources
• Character displacement the trend of sympatric
  species to be more divergent than allopatric
  species

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Symbiosis

• A close relationship between two organisms.

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Symbiosis

• Parasitism (,-)
• Already discussed
• Commensalism (,neutral)
• Mutualism (,)

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Commensalism

• (0) relationship
• One partner benefits, the other is not affected
• Examples
• Sea anemone and clownfish
• Clownfish gets a place to live, sea anemone is
  not affected

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Mutualism

• () relationship
• Both partners benefit from the relationship
• You scratch my back, Ill scratch yours
• Examples
• Ants acacia tree
• tree provides high protein food in beltian
  bodies habitat for nests inside thorns ant
  protects against predators
• Mycorrhizae-fungal extentions on plant roots
• Plant gets increased water/nutrition, fungi gets
  food
• Hummingbirds flowers
• Hummingbirds get food, flowers can reproduce

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SUMMARY

• Relationship Organism 1 Organism 2
• Commensalism 0
• Mutualism
• Parasitism -
• Predator Prey -
• Competition - -

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Evolutionary component

• Many of the relationships discussed could be a
  result of coevolution
• Each species influences the heritable traits of
  another,closely associated, species

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Community Structure
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Community structure

• Community structure describes the make up and
  interactions of the species in a community.
• Many times this is a result of 2 factors
• Species diversity
• Feeding relationships

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Species diversity

• Species diversity if made of two components
• Species richness is the total number of different
  species in a community
• Relative abundance is the proportion of each
  species that makes up the community

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Who eats who?

• A trophic structure describes the feeding
  relationship between organisms in a community
• Feeding relationships always start with some sort
  of primary producer (generally a photosynthetic
  organism)
• Then you will have primary consumers (herbivores)
  and various secondary and tertiary consumers
  (carnivores)
• Eventually, the cycle ends with decomposers

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Food chains

• Food Chains- A single pathway of energy
  relationships among organisms in an ecosystem

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Energy transfer

• The arrows DO NOT merely show what gets eaten
• The purpose of the arrows is to show where the
  energy is going
• Scientists refer to eating as an energy transfer,
  because when one organism eats another, the main
  goal is to get energy from the organism.
• SO, the arrow points at the organism that GETS
  the ENERGY (the organism doing the eating)

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Limits of food chain length

• A food chain is usually only a few links long
• Can be as few as 2 to as many as 5 OR more
• Why?
• One hypothesis is the energetic hypothesis the
  length of a food chain is limited by the
  inefficient transfer of energy from one organism
  to the next (only about 10)
• Food chains with more photosynthetic organisms
  should be longer because you have MORE starting
  energy

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Limits of food chain length

• Second hypothesis is the dynamic stability
  hypothesis long food chains are less stable
  than short food chains
• The more organisms involved in a food chain the
  more potential for variation
• Extinction, migration, climatic changes
• With more species, you have more chances to
  disrupt the food chain

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Food web

• Food chains are a very inaccurate depiction of
  feeding relationships in an ecosystemFood webs
  are more accurate
• Food webs are interrelated food chains of an
  ecosystem

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Large impact

• In communities, certain species may have a larger
  impact on the community structure than other
• They may be highly abundant
• Play a pivotal role in maintaining the balance in
  a community

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Dominant Species

• Dominant Species
• Species in a community that have the highest
  abundance or highest biomass
• These species have a powerful effect on the
  distribution and eating patterns of all other
  species in a community
• Possible reasons for a dominant species
• Dominant species is most competitive in acquiring
  limited resources
• Dominant species is most successful at avoiding
  predators OR disease
• This may be the reason invasive species can take
  over a community that lack their natural
  predators and pathogens

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Keystone Species

• Keystone Species
• Important to a community because of their
  ecological roles (niches), not by numbers
• When these species become extinct, or scarce, the
  entire community changes and usually many other
  species are affected

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Sea otters and sea urchins
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Keystone Species- Pisaster

• When a species of starfish (pisaster) that feeds
  on mussels was removed from an intertidal zone,
  the mussel began to dominate and eat other
  species (decreasing biodiversity)

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Foundation Species

• Some organisms exert their influence by altering
  the environment
• This changes the landscape and alters the
  structural dynamics of the environment
• They may act as facilitators that have a positive
  affect on the community (protect from salt
  variations, maintain soil cohesion)
• They may also be harmful . . . What species do
  you think has been altering the environment the
  most?

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Species interactions

• Two simple ways to explain the effects of
  organisms in a community
• Bottom-up model lower levels of the food web
  influence the levels above them (producers are
  the most influential and the higher levels, top
  consumers, have the least influence)
• Top-down model the opposite of above
• There are also many intermediates of these two

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Ecological Disturbances
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Disturbance

• Classic view of communities
• Communities are in a state of equilibrium unless
  seriously disturbed by outside influences (they
  are stable)
• Constant composition of species
• Newer model is the nonequilibrium model
• Communities are constantly changing in response
  to disturbances
• Disturbance anything that changes the
  community, removes organisms, or alters the
  natural resources (shocker humans have the
  highest impact)

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Ecological Succession

• Ecological succession is a change in the species
  that live in a given area over a period of time
• One community replaces another
• Primary succession occurs in places where soil
  is not yet formed (bare bedrock)
• Secondary succession occurs in places where
  there is soil, but where some disturbance has
  eliminated the previous community (fire, tidal
  wave, natural disaster)

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Ecological Succession
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Ecological Succession

• The first organisms to inhabit an area undergoing
  succession are known as pioneer organisms
• These are usually small organisms (bacteria,
  lichens, algae, etc.)
• The ecosystem goes through a number of stages,
  with each new stage usually consisting of larger
  organisms than the last one
• Once a community has become stable and is not
  changing much, it is known as a climax community

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Causes of Ecological Succession

• There are 3 major causes of ecological
  succession
• Human Activities
• - logging, mining, development, etc.
• Natural Disasters/Disturbances
• - fires, volcanic eruptions, etc.
• 3. Natural Competition Among Species
• - Fictitious example
• - turtles and frogs both eat crickets
• - frogs are faster, turtles are slower
• - frogs eat more crickets, turtles starve
• - turtle population dies out, frog population
• gets bigger

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Biodiversity
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Biodiversity

• Two factors are usually associated with species
  diversity
• Geographic location
• Geographic size
• In general, fewer organisms on islands, than on
  main continent

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Tropical diversity

• In general, there is a larger diversity of
  species in tropical regions than in temperate or
  polar regions
• Possible reasons
• Evolutionary history in general, tropical
  regions are generally older than other biomes
  (more consistent climate means fewer major
  disturbances)

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Tropical diversity

• Climate is another reason for more diversity in
  the tropics
• Fairly consistent solar input
• Abundant rainfall
• Evapotranspiration measures how much water
  remains in a biome as opposed to the loss due to
  transpiration
• In the tropics, much of the water remains in the
  biome