Lecture Outline: Interspecific Competition

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Lecture Outline Interspecific Competition

• Review of key concepts

• Theoretical models Lotka-Volterra equations

• Competition in variable environments

• Case study desert rodents

• Case studies interference competition between
  canids

• Competition-dispersal tradeoffs and coexistence

• Apparent competition

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Interspecific Competition

• An interaction between members of two or more
  species that, as a consequence either of
  exploitation of a shared resource or of
  interference related to that resource, has a
  negative effect on fitness-related
  characteristics of at least one of the species
  (Wiens 1989).

• Exploitation (scramble) competition individuals
  have free access to resources and use of those
  resources by some individuals diminishes their
  availability to other individuals.
• Interference (contest) competition Some
  individuals are denied access to resources by the
  (often aggressive) actions of others. Usually
  incorporates a spatial component.

• Intensity of competition is the proximate,
  physiological, behavioral effect on individuals.
• Importance of competition is the ecological or
  evolutionary consequences of those effects.

3
Niches and resource partitioning

• Niche environmental factors that influence the
  growth, survival, and reproduction of a species
• Hutchinson n-dimensional hypervolume
• Fundamental niche (no competitors) vs. realized
  niche (restricted due to presence of competitors).

4
Resource limitation

• The fact that two species share a resource says
  nothing about resource limitation.

• Strongest evidence of resource limitation comes
  from experiments.

5
Evidence for interspecific competition
(from Wiens 1989)
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Lotka-Volterra Competition Model

• Model developed independently in 1920s and 1930s
  by Alfred Lotka and Vito Volterra that formed the
  framework for many competition studies.

• Extension of basic logistic growth model for one
  population to a system of two competing species.

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Lotka-Volterra Competition Model

• Start with logistic growth model for each of the
  two species.
• Population growth of species 1 depends on
  population size of species 1.
• Population growth of species 2 depends on
  population size of species 2.

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Lotka-Volterra Competition Model

• a is a measure of the effect of species 2 on
  growth of species 1.
• ß is a measure of the effect of species 1 on
  growth of species 2.

• Competition coefficients measure strength of
  interspecific competition effects relative to
  intraspecific competition.
• If a gt 1, then competitive effect of species 2 on
  population growth of species 1 is greater than
  that of an individual of species 1.
• If a lt1, then competitive effect of species 2 on
  population growth of species 1 is less than that
  of an individual of species 1.

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Lotka-Volterra Competition Model

• In general, model predicts coexistence of two
  species when interspecific competition is weaker
  than intraspecific competition for both species.
• Otherwise, one species is predicted to exclude
  the other eventually.

• These are equations for straight lines called
  isoclines of zero population growth, where
  everywhere along the line population growth is
  stopped.
• (dN1/dt 0 and dN2/dt 0)

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Lotka-Volterra Competition Model
Isocline for species 1
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Outcomes of Lotka-Volterra Competition Model
Case 1

• Isoclines do not cross and isocline for species 1
  lies above that of species 2.
• Species 1 wins (species 2 excluded) with
  equilibrium for species 1 at its carrying
  capacity.

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Outcomes of Lotka-Volterra Competition Model
Case 3

• Isoclines cross
• Intraspecific competition is stronger than
  interspecific competition.
• Stable coexistence at equilibrium.

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Shortcomings of Lotka-Volterra Competition Model

• Includes all of the same limitations as the
  logistic growth model, including equilibrium
  approach.
• Resources are limiting.
• Competition coefficients and carrying capacities
  are constant in time and space.
• Density dependence is linear (nonlinear isoclines
  have complex stability properties).
• No spatial variation.
• No age or sex structure.

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• During 1960s and 1970s, ecologists generally
  thought that communities were in equilibrium and
  their structure mainly was determined by
  competitive interactions.

• While studying breeding bird communities in Great
  Basin shrubbsteppe, John Wiens concluded that
  conditions of resource limitation required by
  competition theory occurred only intermittently
  in the shrubsteppe.

• Wrote important paper that espoused a
  non-equilibrium view of nature in which
  environmental variations weaken the effects of
  competition (sensu Andrewartha and Birch).

• Populations might spend much time responding from
  periodic ecological crunches and little time at
  resource-defined equilibriums.

Wiens, JA. 1977. On competition and variable
environments. American Scientist 65590-597.
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Case study Competition among desert rodents

• Long-term experimental study of interspecific
  competition in rodent community in Chihuahuan
  Desert conducted by Jim Brown and colleagues.

• Do large granivorous rodents limit the abundance
  of smaller granivorous rodents?

(Heske, Brown, and Mistry. 1994. Ecology
75438-445)
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Representative participants
Large granivores (Dipodomys) Banner-tailed
kangaroo rat
Insectivorous rodents Grasshopper mouse
(Onychomys)
Small granivores Pocket mouse
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Treatments initiated in 1977
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Response of individual small granivore species
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Their Conclusions

• Continuous presence of competition between
  kangaroo rats and small granivores over 13-yr
  study despite large, species-specific
  fluctuations in abundance suggests that
  competition is pervasive in community.

• No indication that competition only occurred
  intermittently when resources were particularly
  scarce.

(Heske, Brown, and Mistry. 1994. Ecology
75438-445)
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Interference competition between canids

• General pattern for spatial displacement of
  smaller species by larger ones

• Coyote territories tend to occur on periphery or
  outside of wolf territories

• Fox territories tend to occur on periphery or
  outside of coyote territories

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Interference competition between canids

• Similar pattern for swift fox and coyotes in
  Texas.

Kamler et al. 2003. Can. J. Zool. 81168-172.
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Competition-dispersal tradeoffs

• How do so many species coexist in one area?

• Empirical evidence mostly for plants where
  tradeoff can be between energy allocated to roots
  and energy allocated to seeds.
• Some evidence for marine invertebrates and
  perhaps ants.
• Terrestrial wildlife?

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Apparent Competition

• Process that results in a decrease in the
  population growth of two prey species that do not
  compete for the same resource but do share the
  same natural enemy.
• Increase in abundance of predator from consuming
  prey species 1 increases the harm it does to prey
  species 2.
• Indirectly, prey species 1 negatively affects
  prey species 2 and vice versa.
• In theory, often this leads to the exclusion of
  one of the prey species.

Holt, RD. 1977. Theoretical Population Biology
12197-229.
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Exploitative Competition
Apparent Competition
-
-
Predator
Prey
Prey
Prey
Prey
Resource
-
-

• Apparent competition also is called competition
  for enemy-free space

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Apparent Competition
caecal nematode
Pheasants
Grey partridge

• Does intensity of parasite infection in partridge
  reflect parasite burden of pheasants the previous
  year?
• Are the parasite infections sufficient to
  influence partridge condition?

(Tompkins et al. 2000. Ecology Letters 310-14)
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Apparent competition mediated via shared parasites