Lecture 9: Interspecific Competition

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Lecture 9 Interspecific Competition

• EEES 3050

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Competition

• In the past chapters, we have been discussing how
  populations grow and what factors determine that
  growth.
• What happens when you put populations of more
  than one species together?

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How do species interact?

• Competition
• Predation
• Herbivory
• Parasitism
• Disease
• Mutualism

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Interspecific Competition

• Competition
• When two species use the same limited resource to
  the detriment of both species.
• Assessment-some general features of interspecific
  competition
• Competitive exclusion or coexistence
• Tilmans model of competition for specific
  resources (ZINGIs)
• Coexistence reducing competition by dividing
  resources

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Assessment

• mechanisms
• consumptive or exploitative using resources
  (most common)
• preemptive using space, based on presence
• overgrowth exploitative PLUS preemptive
• chemical antibiotics or allelopathy
• territorial like preemptive, but behavior
• encounter chance interactions

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Modeling coexistence?

• Can we model the growth of 2 species?
• Remember logistic model?
• What is K?
• Now we add another factor that can limit the
  abundance of a species.
• Another species.

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Freshmen and donuts an example

• There is a room with 100 donuts what does a
  typical male freshmen do?
• First eat several donuts. (A male freshman can
  eat 10 donuts)
• Second rapidly tell friends
• But not too many!
• Third Room reaches carrying capacity at 10 male
  freshmen.
• So K10 for male freshmen.

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Freshmen and donuts an example

• What happens if a male and female discover the
  room at the same time?
• First eat several donuts. (A female freshman
  can eat 5 donuts)
• Second rapidly tell friends
• But not too many!
• Third Room reaches carrying capacity at ? males
  and ? females.
• What is the carrying capacity?
• It depends

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Lotka-Volterra

• Need a way to combine the two equations.
• If species are competing, the number of species A
  decreases if number of species B increases.
• Such that
• Where alpha is the competition coefficient
• Lotka-Volterra A logistic model of interspecific
  competition of intuitive factors.

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Freshman Example

• In a room we have 100 donuts.
• Need 10 donuts for each male freshmen.
• So K1 10
• Need only 5 donuts for each female freshmen.
• So K2 20
• If room is at K1 and 1 male leaves, how many
  females can come in?
• So, , where a 0.5
• And, , where B 2

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Possible outcomes when put two species together.

• Species A excludes Species B
• Species B excludes Species A
• Coexistence

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Changes in population 1
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Changes in population 1
Yellow both increase White both decrease
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Changes in population 2
Yellow both increase White both decrease
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Yellow both increase White both
decrease Green Sp 1 increase Brown Sp 2
increase
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Tilmans model

• Problems with Lotka-Voltera model?
• No mechanism
• Dr. Tilman developed a model based on resource
  use.

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1 no species can survive 2 Only A can live 3
Species A out competes B 4 Stable
coexistence 5 Species B out competes A 6 Only
B can live
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Lab Experiments?

• Gause using yeast and Birch using beetles.
• Results show both exclusion and coexistence
• It was hypothesized that the yeasts had enough
  differences to allow coexistence.
• i.e. the requirements of the 2 species are
  slightly different.

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Gauses hypothesis

• As a result of competition two similar species
  scarcely ever occupy similar niches
• Also called the competitive exclusion principle
• Complete competitors cannot coexist.
• Niche still controversy about the definition.
• 1) The role of a species in the community Elton
  1927.
• 2) a subdivision of the habitat. - Grinnell 1917.

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Back to Competition Coefficient

• Competition coefficient
• Intensity of competition from species.
• In our original donut example
• a 1/ ß
• Also, can read about Gauses yeast populations in
  book.

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Changes in population 1
K2
K1 10 K2 20 a 0.5 ß 2
K2/ß
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Back to Competition Coefficient

• Competition coefficient
• Intensity of competition from species.
• In our original donut example
• a 1/ ß
• However in systems that are more complex
• the coefficients are not necessarily reciprocals.
• And carrying capacity may not be purely
  determined by resource being competed for.

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Changes in population 1
K2
K1 10 K2 17 a 0.5 ß 3
K2/ß
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Niche Hutchinson

• Redefinition in 1958.

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Niche Hutchinson

• Redefinition in 1958.
• Two environmental variables, can produce an
  environmental space or a species niche.
• Can add many other environmental factors.
• n-dimensional hypervolume
• Or a species Fundamental Niche
• However, because competition can limit this
  fundamental niche, what we witness in nature is
  the
• Realized Niche

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Can 2 species exist in the same niche?
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Can 2 species exist in the same niche?
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Can 2 species exist in the same niche?
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Can 2 species exist in the same niche?

• Observation
• Several types of warblers live in the same tree
  species.
• Hypothesis based on competition theory
• Warblers will use different parts/areas of the
  trees.
• Experiment
• No experiment conducted, but observations can be
  made to test hypothesis.

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McArthur suggested competition to explain warbler
patterns

• Ghost of competition past.
• So how do species coexist?
• Different food resources, i.e. diet
  specialization
• What about plants?
• Plants usually need same resources, water,
  nutrients, light.
• What about phytoplankton?

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How do phytoplankton live in the same location?

• Phytoplankton
• Common pool of nutrients
• Often large number of species
• Same environment, i.e. amount of light,
  temperature.
• In many bodies of water, nutrients are limited.
• Reasons?
• Environmental instability
• Non-equilibrium system.

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Assumptions of competition theory

• life history characteristics of species are
  adequately summarized by the per capita growth
  rate of species
• deterministic equations are sufficient to model
  population growth, and environmental fluctuations
  need not be considered
• the environment is spatially homogeneous and
  migration is unimportant
• competition is the only important biological
  interaction and
• coexistence requires a stable equilibrium point.

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How to determine if interspecific competition has
occurred (or is occurring)?

• From Wiens (1989)
• Need a checkerboard distribution
• Species overlap in resource use
• Intraspecific competition occurs
• Resource is limited
• One or more species is limited
• Other hypotheses do not fit.

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Example where criteria 1 and 2 fit.
Feeds far from shore
Feeds near shore
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Test theory with plants

• Observation
• Plants all require light, nutrients and water.
• Plant often found together.
• Hypothesis
• Competition between plants ought to be common.
• Plants do worse with other plants.
• Experiment
• Results

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Experimental design.

• Compare the growth of annuals and shrubs in the
  Mojave Desert.
• 2 experiments
• Effects of annuals on shrubs
• Effects of shrubs on annuals
• How?

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Results Annual had positive benefits from
shrubs. Shrubs had negative benefits from shrubs.
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• Facilitation
• Where one or both species benefit (have a
  positive effect) due to the presence of the other
  species.

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Resource utilization curves

• Species may evolve to minimize the impact of
  competition.

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Character Displacement

• Definition
• In areas where species overlap, there has been a
  divergence between the two species, supposedly as
  a result of competition.

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Example Galapagos finches

• Theory
• According to displacement theory, species that
  are sometime found together will have a character
  that has changed compared to when the species are
  found by themselves.
• Observation
• There are three species of finches in the
  Galapagos that are sometimes found together and
  sometimes separate.
• Hypothesis
• There will be differentiation in bill size when
  species are on the same island
• Test (Not truly an experiment)
• Examine the bill size of three species.
• Results

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Results
Could we actually test this?
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Four criteria for determining character
displacement

• Change in mean value of the character in areas of
  overlap should not be predictable from variation
  within areas of overlap or areas of isolation.
• Sampling should be done at more than one set of
  locations
• Characters need to be heritable.
• Species must actually be competing for resource.

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r vs. K selected species

• What do r and K refer to?
• r growth rate
• K carrying capacity
• r selected
• Species that remain in the growth rate stage for
  most of their existence.
• K selected
• Where organisms remain near the carrying capacity
• Influenced more by competition.

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Is it really competition?

• How else could these ideas be framed?
• Conflict avoidance.
• Not survival of fittest, but perhaps least
  noticeable.
• Best hider.
• Path or least resistance