Community Interactions I

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Community Interactions I

• Competition

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Competition

• Consumers compete when
• 2 or more consumers utilize the same resource(s)
• levels of resource(s) are affected by consumption
• levels of resource(s) affect birth and death
  rates of consumers

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Resources

• Definition anything that is consumed and leads
  to population growth and is used up or reduced in
  abundance in the environment (D. Tilman)
• renewable - replenished within organisms
  lifetime (grass)
• nonrenewable - not replenished or takes longer
  than lifetime (soil)
• limiting resources - required for survival
  and/reproduction

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2 Types of Competition

• Intraspecific
• between individuals of same species
  (conspecifics)
• density-dependent mortality and fecundity
  (immigration, emigration?)
• Interspecific
• between individuals of different species
• density-dependent mortality and fecundity (based
  on density of competitor)

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Mechanisms of competition

• Exploitative (scramble)
• indirect interactions via resource
• early bird gets the worm plants for soil water
  (undergrads at a free food party)
• Interference (contest)
• direct interactions between individuals
• starlings a a bird feeder try to push each other
  off grizzlies along salmon river one plant
  shades another, (football game)
• Combination of both
• plants for water and space

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• Apparent competition
• indirect interaction via predators (R. Holt)
• native Hawaiian birds (honeycreepers) and
  invading songbirds (often escaped pets etc) via
  avian malaria and avian pox virus
• (undergrad bio majors at final exams via medical
  school admission boards

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Effects of Competition

• Evolutionary
• Strong effects on evolution
• Ecological
• Effects on distribution and abundance
• Competitive exclusion.

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Intraspecific Competition

• Leads to stable (?) regulation of population size
  within limits imposed by the environment
• bluegills in overstocked farm ponds
• Normally can reach dinner plate size.
• In overstocked ponds only reach 4-5 inches or
  less.
• Few if any reproduce.
• Decreased fecundity
• Self-thinning in plants
• Decreased survivorship

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• African buffalo
• Wet season food is abundant
• Dry season quality of food decreases.
• As food quality and quantity decline, scramble
  competition becomes keener.
• The more buffalo present, the less food
  available for each individual.

• As animals use up fat reserves, they become
  susceptible to disease.
• Older animals are more affected so adult
  mortality is
• density dependent influenced by rainfall and
  resource
• quality.
• Juvenile mortality is density independent, and is
  influenced
• by environmental stochasticity.

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Interspecific Competition

• Possible outcomes
• competitive exclusion extinction of one of the
  competitors
• coexistence
• ecological mechanisms
• evolutionary mechanisms

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Competitive Exclusion

• RECALL Ecological Niche
• Fundamental niche
• position in n-dimensional hyperspace
• broader than the --gt
• Realized niche
• smaller due to interactions with competitors,
  predators, etc.
• If two species are so similar in their
  requirements that they try to occupy the same
  niche this will lead to interspecific
  competition.
• The ultimate effects of competition are on
  survivorship and fecundity
• This is the Competitive Exclusion Principle

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Classic studies of resource competition by Gause
(1934, 1935)
Paramecium caudatum
Paramecium aurelia
Paramecium bursaria
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Classic studies of resource competition by Gause
(1934, 1935)

• Gause found that interactions between Paramecium
  aurelia and P. caudatum always ended in
  competitive exclusion.

Within 14 days Paramecium aurelia WON!
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Interspecific competition Paramecium

• Gause (Georgii Frantsevich Gause) Russian
  biologist
• P. caudatum and P. aurelia
• P. caudatum goes extinct
• Strong competitors, use the same resource (yeast)
• Competitive asymmetry
• Competitive exclusion

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Classic studies of resource competition by Gause
(1934, 1935)
WHY?

• In contrast, Paramecium bursia and P.
  caudatum could coexist.

Paramecium bursaria
Paramecium caudatum
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Interspecific competition Paramecium

• P. caudatum P. bursaria coexist
• P. bursaria is photosynthetic
• Competitive coexistence
• Apparently stable

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Classic studies of resource competition by Gause
(1934, 1935)

• Gause found that interactions between Paramecium
  aurelia and P. caudatum always ended in
  competitive exclusion.

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Classic studies of resource competition by Gause
(1934, 1935)

• Because they inhabited different regions of the
  flask and ate different food.
• P. bursia fed on the bottom of the flask, and P.
  caudatum ate the bacteria in suspension.
• P. bursaria carries mutualistic green algae
  (Chlorella)
• P. bursaria gets sugar from algal photosynthesis
• Keeps number of endosymbiontic algae constant
• Eats excess any algae
• Algae get a mobile home

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Classic studies of resource competition by Gause
(1934, 1935)

• In contrast, both P. aurelia and P. caudatum ate
  the bacteria in suspension.

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Roles of Competition

• Competition (-/-) is central to both evolutionary
  theory and ecological theory
• Competition can be a powerful selection pressure.
  
• Competition structures communities.

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Modified Environment

• P. aurelia vs. P. caudatum
• Replace liquid medium daily
• P. caudatum wins, P. aurelia extinct
• Competition and parameters depend on environment
• Multiple mechanisms of competition
• P. aurelia wins when it pollutes the medium
• P. caudatum wins by resource competition?

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Mechanism of coexistence

• Paramecium caudatum
• nonphotosynthetic feeds on yeasts only
• must be near surface (O2)
• Paramecium bursaria
• endosymbiotic algae photosynthesis produce O2
• can feed on yeasts at the bottom of the test tube
• Two species used different resources
• weak interspecific competition coexistence

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Tilmans diatom experiments
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Mechanisms of Competitive Exclusion

• Resource depletion
• Fewer resources --gt less growth --gt smaller size
  --gt less reproductive output
• Decreased survival through starvation increased
  exposure to predation (via increased foraging
  time)
• Decreased foraging efficiency lower quality
  food less net energy gain from food

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Mechanisms of competitive exclusion

• Space depletion less good quality space per
  individual or breeding pair
• Increased social interaction time consumes
  energy and foraging time
• Interspecific predation (cannibalism - eggs,
  juveniles)
• Increased predation due to increased predator
  efficiency - search image develops
• Increased immigration - uses energy, may end up
  in poorer habitat

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Mechanisms of Coexistence

• Ecological mechanisms that foster coexistence
• resource partitioning - two or more species use
  different or mostly different types of limiting
  resources
• sounds like species learn to get along/share
  WRONG
• species have limiting constraints
• trade-off between the ability to be efficient at
  acquiring one type of resource vs being able to
  acquire many types of resources

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• Ex. Warblers MacArthurs studies
• Ex. bumblebees
• Ex. Galapagos finches 3 species of seed-eating
  ground finches in the genus Geospiza (Lack)
• G. fulinosa small ground finch, eats small seeds
• G. fortis medium ground finch, eats medium seeds
  and small seeds
• both species compete for small seeds
• medium finch cant live on small seeds alone (not
  enough energy)
• small finches cant crack medium sized seeds very
  well
• Ex. Barnacles - (Connell) Be able to explain
  this in detail

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Competition among barnaclesCompetitive exclusion
affects distribution abundance

• Rocky intertidal zone
• adult barnacles immobile on rocks
• larvae settle on rocks from plankton
• Joseph Connell (1961) Ecology 42710-723

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Distributions of Balanus Chthamalus
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Chthamalus Balanus

• Larvae settle throughout much of the intertidal
• Chthamalus adults only in the high intertidal
• Balanus adults only in the mid low intertidal
• Hypothesis Balanus excludes Chthamalus
• Hypothesis Chthamalus cannot tolerate
  submergence
• Hypothesis Balanus cannot tolerate desiccation

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Experiments

• Rocks with larvae and young adults
• remove Balanus
• control count, no removal
• Rocks with young adults of one species
• transplant Balanus to high low intertidal
• transplant Chthamalus to high low intertidal
• Follow fates of marked individuals over years

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Experimental result 1
Undercut
Crushed

• Balanus individuals grow rapidly
• Shell undercuts or crushes adjacent Chthamalus
• Competition for space Balanus wins

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Experimental result 2

• Chthamalus survives well in the low intertidal
  only if Balanus is removed
• With Balanus present, Chthamalus is completely
  eliminated
• Distribution of Chthamalus is limited by
  interspecific competition with Balanus
• Local competitive exclusion

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Experimental results 3

• Balanus does not survive in the high intertidal,
  regardless of Chthamalus
• Balanus upper limit set by physical environment
• Desiccation
• Chthamalus has a refuge from competition, a place
  where it escapes effects of its competitor
• Chthamalus tolerates dry conditions

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Barnacles one example of the role of
interspecific competition

• Is interspecific competition common in nature?
• Is it often severe enough to cause competitive
  exclusion?
• How is exclusion avoided? Does competition cause
  natural selection?

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• Keystone Species
• Based on idea of the keystone in an arch.
• Keystone takes the weight of all the other stones
  in the arch.
• Without the keystone the entire structure
  collapses.
• Some species are so important to the functioning
  of an ecosystem that they are called keystone
  species

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• Keystone predators
• Pisaster sea star Robt. Paine
• rocky intertidal community on NW coast
• predator starfish Pisaster
• Pisaster predation on Mytilus prevents this
  mussel from taking over the space available on
  the rocks and excluding other species
• remove starfish - community is all mussels
• replace starfish - get mussels, barnacles, sea
  anemones, etc.
• Lobster on East coast
• sea otter on west coast
• Wyeomyia smithii in pitcher plant leaves

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Keystone Predator (Paine 1969)
Pisaster Starfish
Mussels
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Keystone Predator a species that preferentially
consumes and holds in check another species that
would otherwise dominate the system. Pisaster
keeps mussel population under control and
thereby increases species diversity of rocky
intertidal community.
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• Keystone predators
• Pisaster sea star Robt. Paine
• rocky intertidal community on NW coast
• predator starfish Pisaster
• Pisaster predation on Mytilus prevents this
  mussel from taking over the space available on
  the rocks and excluding other species
• remove starfish - community is all mussels
• replace starfish - get mussels, barnacles, sea
  anemones, etc.
• Lobster on East coast
• sea otter on west coast

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Rocky shoreline on Northwest coast Site of
Paines experiments Note mussel bed above
highest spray line. Diverse algal and
invertebrate community below spray line
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Mussels Mytilus
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Byssal threads
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Note mussel bed above highest spray line. Diverse
algal and invertebrate community below spray line
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chitons
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Gooseneck barnacles
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Sea stars The Keystone Predator Pisaster
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Mussels cleared by Pisaster - Barnacles beginning
to colonize
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Keystone Species

• Current usage
• A keystone species is any species whose impact on
  its community or ecosystem is large and
  disproportionately large relative to its
  abundance.
• Does not have to be a predator!

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Pacific Sea Otters
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Sea urchins
Kelp
Abundant
Rare
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Maintenance of keystone species is critical to
conserving ecosystems. PROBLEM?? We rarely
know which species are the keystones.
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White Bark Pine
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Fig Tree
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American Lobster
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Prairie Dogs
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Wolves
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Beaver
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American Alligators
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African Elephant
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Wyeomyia smithii
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Avoiding competitive exclusion

• Differences in resource use
• habitats, food, behavior
• Consider seed eating birds
• Morphology and resource use related
• Big bill ? big seeds
• Small bill? small seeds

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Competition can increase the differences between
species

• Competition between species can be intense when
  the two species are similar in form.
• This competition can result in character
  displacement.
• In character displacement, the forms of the
  competing species evolve to become more different
  over time, reducing the intensity of competition.

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Differences in beak sizes among species of
Galápagos finches provide evidence for character
displacement
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Quantitative traits Resource use
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Selection and competition
TIME
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Differences in Resource Use

• Low overlap can originate in 2 ways
• 1) Evolution in response to selection by
  competition
• 2) Independent of competition, pre-existing
  differences enable 2 species to coexist when
  they meet
• Resource partitioning use of different
  resources by potential competitors facilitates
  coexistence
• Includes both 1) and 2)
• Character displacement evolution of
  morphological differences where two species
  co-occur
• Includes only 1)

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Morphology Resource use

• What evidence exists to show that species with
  different morphology
• use different resources?
• compete less intensely?
• Example Anolis lizards
• Insectivorous, arboreal, Caribbean Islands
• Evidence for resource partitioning
• Probably not character displacement

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Caribbean Anolis

• St. Maarten
• A. gingivinus SVL41 mm
• A. wattsi SVL38 mm
• Competition experiment
• A. gingivinus A. wattsi
• less food in stomach
• lower growth rate
• compared to A. gingivinus alone

• St. Eustatius
• A. bimaculatus SVL53 mm
• A. wattsi SVL40 mm
• Competition experiment
• A. bimaculatus A. wattsi
• same amount in stomach
• same growth rate
• compared to A. bimaculatus alone

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Character DisplacementNiche Partitioning

• Birds
• Large Bill Size crack large seeds
• Small Bill Size crack small seeds
• Selection for resource partitioning
• Examine 2 species where they are
• together (sympatry)
• separate (allopatry)
• Predict species DIFFER more in sympatry

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Darwins Finches

• Galapagos Islands
• Different seed-eating finches on different
  islands
• Recently evolved from a common South American
  ancestor

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Bill Sizes of Darwins Finches
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Character Displacement

• Evolution of morphological divergence in places
  where two otherwise similar species occur
  together
• Usual hypothesis is that selection occurs due to
  competition
• For finches, presumably competition for seeds

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• Evolutionary Mechanism - The ghost of
  competition past?
• Character displacement/niche partitioning
• related to resource partitioning
• competition results in change in some
  physiological or morphological characteristic
• allows for utilization of niche in different ways
  to reduce competitition
• where two similar species co-occur some
  phenotypic characters are displaced so that the
  two species appear different
• in portions of range where the species are found
  alone, these same characters appear very similar

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• ex. 3-spined sticklebacks in small coastal lakes
  in British Columbia
• be able to describe this in detail
• relics left behind by postglaciation uplift 11kya
• one species appears to be evolving into 2
• in lakes where only 1 exists it is limnetic
  (occupies open water) and feeds on plankton and
  sediments/vegetation
• if 2 are present, one is limnetic is
  planktivorous, the other is benthic and feed on
  sediments and vegetation (gill rakers are
  different)

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• Galapagos finches (Lack)
• if only one species on island bill depth was 10mm
• 2 or more species of ground finches together on
  island - bill depth for smallest species was
  around 8mm bill depth for next largest was 12mm
  (no 10mm bills were found)
• Lack suggested these differences had evolved in
  response to competition