Interspecific Competition

1
Interspecific Competition
2
Interspecific Competition

• between ? 2 species
• within same guild/trophic level
• same resources/set of resources
• mutually negative interaction (-/-)
• decrease in fitness (e.g., fecundity) presumed to
  cause reduced abundance
• does not involve predation

3
Types of Competition Studies

• Observational
• negative correlations between species
• attributed to present competition or past
  (ghost of competition past)
• cant determine cause and effect
• other factors may be involved

4
Types of Competition Studies

• Observational Comparison to null model
• compare observed patterns to those generated by
  chance alone
• statistical comparison
• challenge is to formulate the appropriate null
  model

5
Types of Competition Studies

• Experimental
• addition/removal studies
• manipulate presence and/or density of would-be
  competitors
• must account for density effects
• provides strong inference (strong evidence for
  or against)
• cannot be done with many species

6
6 Mechanisms of Competition(after Schoener 1983)

• 1) Consumptive one species competes with
  another by consuming a shared resource

Brown and Davidson 1977
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6 Mechanisms of Competition

• 2) Preemptive occupation of physical habitat by
  one species, thereby excluding another

8
invasive Salvinia
Swamp Forest Understory
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6 Mechanisms of Competition

• 3) Overgrowth one species grows over another

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6 Mechanisms of Competition

• 4) Chemical chemical warfare
• allelopathy in plants has not been convincingly
  demonstrated
• growth inhibitors in animals

Bare zones in California chaparral
Allelopathy among shrubs (Muller 1969)
11
6 Mechanisms of Competition

• 5) Territorial aggressive behavioral exclusion

Gray reef shark
12
6 Mechanisms of Competition

• 6) Encounter nonterritorial encounters between
  foraging species
• wasted time/energy that couldve been devoted to
  reproductive output

Example parasitoid wasp species
13
Models of Competition

• descriptive
• mechanistic

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Exponential Growth

• J curve
• dN/dt rN
• r intrinsic rate of increase

15
Logistic Growth Model

• S curve
• dN/dt rN(1-N/K)
• dN/dt - population growth rate
• r - per capita rate of increase
• K - carrying capacity
• growth rate decreases as approaches K
• max. population size occurs when dN/dt 0

Resistance
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Models of Competition

• Lotka (1925) Volterra (1926)
• descriptive model
• developed with mobile animals in mind
• extended logistic model to include two-species
  competition

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Lotka-Volterra Model

• dN1/dt r1N1(K1-N1-?12N2)/K1
• dN2/dt r2N2(K2-N2-?21N2)/K2
• main difference is a competition coefficient ?ij
  effect of species j on species i
• How much does species j utilize the carrying
  capacity of species i?

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Lotka-Volterra cont

• if species I and J are equivalent competitors,
  ?ij ?ji 1 rarely happens
• if ?ij lt 1 means effect of species j is less than
  effect of species i on its own members
• if ?ji lt 1 means effect of species i is less than
  effect of species j on its own members

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Zero Isoclines and State-space Graphs

• dN/dt or growth rate of selected species is set
  to 0 and you solve for N
• give values of N1 and N2 that yield zero
  population growth for each species
• the x axis represents abundance of species 1, and
  the y axis represents the abundance of species 2
• points represent a combination of abundances of
  species 1 and 2

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Zero Isoclines

• population is increasing left of isocline (below
  K) and decreasing right of isocline (above K)
• isocline for species 1 represents a combination
  of abundances of the two species where species 1
  population does not increase or decrease

equivalent of sp 2, no sp 1
all sp 2, no sp 1
equivalent of sp 1, no sp 2
all sp 1, no sp 2
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Different Outcomes
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Arrangements of 2 Isoclines (1)

• competitive exclusion of species 2 by species 1
• population of species 2 goes from 0 to negative
  under conditions in which species 1 can increase

stable equilibrium
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Arrangements of 2 Isoclines (2)

• competitive exclusion of species 1 by species 2
• population of species 1 goes from 0 to negative
  under conditions in which species 2 can increase

stable equilibrium
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Arrangement of 2 Isoclines (3)

• both species have achieved zero growth (isoclines
  cross) and stable coexistence (initial abundances
  do not matter)

stable equilibrium
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Arrangement of 2 Isoclines (4)

• isoclines cross, but whether species coexist
  depends on initial abundances of the species
  unstable equilibrium

stable equilibrium
unstable equilibrium
stable equilibrium
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Lotka-Volterra Summary

• assumptions
• no migration
• K and ?ij are constants
• stable coexistence is possible only when
  intraspecific competition is greater than
  interspecific competition

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Mechanistic Models of Competition

• incorporate resources
• express competition coefficients carrying
  capacities as rates of utilization resource
  renewal
• Under what conditions do we find coexistence of
  species?

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The R Rule

• R - concentration of a resource when a
  population of a single species grown alone
  reaches its equilibrium density
• winner of competition is determined by which
  consumer species produces the lower value of R
  in the absence of the other
• Essentially, who can maintain population at the
  lowest level of the limiting resource(s)?

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Tilmans Models of Competition

• multi-consumer, multi-resource models
• average mortality rate of each species
• assumed to be independent of density resources
• supply rates of limiting nutrients
• population growth rates as a function of nutrient
  supply rates
• assumed to level off at high rates due to
  saturation
• competition occurs through the effect of each
  species on the consumed resources (consumptive)

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Zero-growth IsoclinesCompetitive Exclusion

• region 1
• below minimum concentration needed to balance
  growth mortality
• both go extinct
• regions 2 3
• species A wins (lowest R)

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Zero-growth IsoclinesCompetitive Exclusion
A
B

• region 1
• below minimum concentration needed to balance
  growth mortality
• both go extinct
• regions 5 6
• species B wins (lowest R)

1
6
5
Supply Rate Resource Y
Supply Rate Resource X
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Reading Assignment

• Tilman, D. 1985. The resource ratio hypothesis
  of plant succession. American Naturalist 125
  827-852.
• Well discuss this paper next Monday, February 18.

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Crossing of Zero-growth IsoclinesStable
Coexistence

• region 1 both go extinct
• species A resource Y limits it most
• species B resource X limits it most

A
B
CA
1
2
3
4
CB
Supply Rate Resource Y
5
6
Supply Rate Resource X
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Crossing of Zero-growth IsoclinesStable
Coexistence

• CA and CB consumption vectors or ratio in which
  the 2 resources consumed by each consumer
• species A B consume resource that limits it the
  most at a greater rate than it consumes the
  non-limiting resource
• region 4 stable coexistence

A
B
CA
1
2
3
4
CB
Supply Rate Resource Y
5
6
Supply Rate Resource X
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Crossing of Zero-growth IsoclinesUnstable
Coexistence

• region 1 both go extinct
• each species consumes resource that limits the
  other species the most at a greater rate than it
  consumes the resource most limiting to it
• region 4 unstable coexistence

A
B
CB
1
2
3
4
CA
Supply Rate Resource Y
5
6
Supply Rate Resource X
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Competition between Algal Species
Cyclotella

• two diatom species
• two resources phosphate and silica
• R is lower for silica than phosphate in
  Cyclotella
• phosphate limits Cyclotella the most

R 0.6
PO4 (?M)
R 0.2
SiO2 (?M)
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Competition between Algal Species
Asterionella

• two diatom species
• two resources phosphate and silica
• R is lower for phosphate than silica in
  Asterionella
• silica limits Asterionella the most

R 1.9
PO4 (?M)
R 0.01
SiO2 (?M)
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Outcome of Competition
CCyclotella

• 1 - both go extinct
• 23 - Cyclotella wins
• 4 - stable coexistence
• 56 Asterionella wins

CAsterionella
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Neighborhood Models of Competition

• Tilmans models worked for phytoplankton
• resources more homogeneous
• dont work well for terrestrial plant species
• spatial relationships are important to
  competitive outcome in plants
• 2 main types of models
• simulations that keep track spatially of plants
• analytical models that capture essence of
  spatially constrained competition

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Neighborhood Models of Competition

• plants compete within neighborhoods
• focal plant responds to competitors within a
  surrounding area

focal plant
1
1
3
2
2
1
1
2
3
3
1
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Neighborhood Model of Intraspecific Competition
within Arabidopsis thaliana
Number of Seeds/Plant
Number of Neighbors

• Pacala and Silander (1985)
• fecundity reduced with number of neighbors

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Two Species Neighborhood Model of Competition

• Pacala (1986) - 2 annual plant species without
  seed dormancy
• density of neighbors affects fecundity
• main point similar to Lotka- Volterra
  coexistence where intraspecific competition gt
  interspecific competition

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Coexistence of Species

• some species fail to coexist
• those that do coexist, have interspecific
  differences in resource use
• even ecologically similar species differ to some
  degree

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Meanings of Niche (1)

• Grinnell (1914) coined the term
• no two species of birds or mammals will be found
  to occupy precisely the same niche
• Hardin (1960) competitive exclusion principle
• complete competitors (i.e., those that compete
  for EXACTLY the same resources in the same way)
  CANNOT coexist
• thus, species that do coexist must differ in
  resource utilization
• niche or resource partitioning, species packing

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Meanings of Niche (2)

• Elton (1926)
• what place a species occupies in a community
• functional role of a species
• Hutchinson (1957)
• range of physical biological conditions
  required by a species
• n-dimensional hypervolume each axis corresponds
  to an individual physical or biological variable

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Resource Partitioning

• species that coexist differ in some aspect of
  their lifestyle (n-dimensional hypervolume)
• MacArthur (1958)
• foraging differences of 5 warbler species in
  New Hampshire
• partitioning resources by specializing on
  different structural strata in the forest

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Fundamental Realized Niches

• fundamental
• physiological response curve, pre-interactive
• range of conditions in which a species can occur
  in the absence of competitors
• absence of other species in general, including
  facilitators
• realized
• ecological response curve, post-interactive
• range of conditions over which a species occur in
  the presence of competitors
• range will be reduced because competitive
  exclusion in areas of overlap with competing
  species

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Fundamental Realized Niches

• after Mueller-Dombois and Ellenberg (1974)
• competitors constrain species Z to its ecological
  response curve (realized niche)

Species Z
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Regeneration Niche

• Grubb (1977)
• one more way species can partition up the
  physical and biological hypervolume
• differences in phenology, timing of germination,
  microsite specialization