Competition

1
Competition Coexistence

• Chapter 8

2
Competition Coexistence

• Weve seen how abiotic factors affect
  populations.
  
• Now lets focus on the effects of other
  individuals and other species.

3
Species Interactions

• Biotic interactions include
• Herbivory
• Predation
• Parasitism
• Competition
• Mutualism
• Commensalism

4
Species Interactions

• Herbivory, predation, parasitism
• Positive for one population.
• Negative for the other population.
• Batesian mimicry
• Mimicry of a non-palatable species by a palatable
  one.
  
• Positive for one population.
• Negative for the other population.

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Species Interactions

• Amensalism
• One-sided competition.
• One species has a negative effect on another, but
  the reverse is not true.
  
• Neutralism
• Coexistence of non-interacting species.
• Probably rare.

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Species Interactions

• Mutualism and commensalisms
• Symbiotic relationships.
• Species are intimately associated with one
  another.
  
• Mutualism both benefit.
• Commensalism beneficial for one neutral for
  the other.
  
• Not harmful, as is the case with parasitism.

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Species Interactions

• Competition
• Negative effect for both species.
• Interspecific competition between species.
• Intraspecific competition among members of a
  single species.
  

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Characterizing Competition

• Resource competition
• Organisms compete for a limiting resource.
• Common in invertebrates.
• Interference competition
• Individuals harm one another directly by physical
  force.
  
• Most common in vertebrates.

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Intraspecific Competition

• Quantifying competition in plants vs. animals.
• For plants, expressed as change in biomass.
• For animals, expressed as change in numbers.
• Plants can not escape competition.
• Animals can move away from competition.

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Intraspecific Competition

• Yoda (1963)
• Quantify competition between plants.
• Yoda's Law or self-thinning rule 3/2 power
  rule.
  
• Describes the increase in biomass of individual
  plants as the number of plant competitors
  decrease.
  

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Interspecific Competition

• Field experiments
• Organisms can interact with all other organisms.
• Natural variations in the abiotic environment is
  factored in.
  
• Laboratory experiments
• All important factors can be controlled.
• Vary important factors systematically.

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Interspecific Competition

• Laboratory experiments show how the outcome of
  interspecific competition can vary with changes
  in the abiotic and biotic environment.

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Interspecific Competition

• Thomas Park competition experiments.
• Tribolium castaneum and T. confusum flour
  beetles.
  
• Large colonies of beetles can be grown in small
  containers with dry food-medium.
  
• Large number of replications.

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Interspecific Competition

• Observed changes in population sizes over
  two-three years.
  
• Waited until one species became extinct.
• Cultures were infested with a parasite.
• T. confusum won 89 of the time.
• Without the parasite, T. castaneum won 67 of the
  time.
  
• Either way no 100 winner.

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Interspecific Competition

• Microclimate effects
• T. confusum did better in dry environments.
• T. castaneum did better in moist environments.

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Interspecific Competition

• Mechanism of competition - predation of eggs.
• Predatory tendencies varied with different
  strains.

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Interspecific Competition

• Parks experiments showed that the results of
  competition could vary as a function of at least
  four factors temperature, moisture, parasites,
  genetic strains.
• Stochasticity occurred even under laboratory
  conditions.
  
• Results in nature probably even more variable.

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Interspecific Competition

• In nature, interspecific competition may vary
  among habitats and areas.

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Interspecific Competition

• Assessing the importance of competition in
  natural systems.
  
• Remove species A and measure the response of
  species B.
  
• Difficult to do outside of laboratory.
• Migration problems.
• Krebs or cage effect numbers are artificially
  high.

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• Examples in nature
• Parasitic wasps used to control scale pest.
• Climate can alter competitive outcome.

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Interspecific Competition

• Well documented case of competition.
• All species introduced perhaps it should be
  expected that they would be more competitive than
  if they had evolved together.
  
• Study of competition important in real world
  situations.

25
Interspecific Competition

• Is the release of multiple species of biological
  control agents beneficial?
  
• Control of pests in agriculture is of paramount
  importance.
  
• Biological control is seen as a preferable
  alternative to chemical control.
  

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Interspecific Competition

• Should multiple species of biological control
  agents be released?
  
• Observe which enemy does the best job.
• Is this the best strategy?
• Intensive competition for the prey leads to lower
  effectiveness of the biological agents.
  
• Greater population establishment rate with fewer
  enemy species.
  
• Establishment rate of single-species releases
  were significantly greater than the simultaneous
  release of two or more species (76 vs. 50).

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Frequency of Competition

• Joe Connell (1983)
• Competition was found in 55 of 215 species
  surveyed.
  
• Effects of number of competing species
• Single pairs competition was almost always
  reported (90).
  
• Multiple species, competition was reported in 50
  of the studies.
  
• Differing opinions - Schoener (1983).

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Frequency of Competition

• Common flaws of Connells Shoeners studies
• Positive results tend to be more readily accepted
  into the literature.
  
• Scientists do not study systems at random - may
  work in systems where competition is more likely
  to occur.
  

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Frequency of Competition

• Failure to reveal the true importance of
  competition in evolution and ecological time
  
• Most organisms have evolved to escape competition
  and lack of fitness it may confer.
  
• Competition may only occur infrequently and in
  years where resources are scarce.
  

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• Patterns of competition

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Mechanisms of Competition

• Consumptive or exploitative using resources
  most common.
  
• Preemptive using space.
• Overgrowth one species growing over another
  limiting light or some other resource.

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Mechanisms of Competition

• Chemical production of toxins.
• Territorial defense of space.
• Encounter transient interactions directly over
  specific resources.
  

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Mechanisms of Competition

• Consumptive most common.
• Preemptive overgrowth primarily sessile
  organisms.
  
• Territorial and encounter primarily actively
  moving animals.
  
• Chemical terrestrial plants (and some sponges
  corals).

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Frequency of Competition

• Often, only one member of a pair of species
  responded to experimental manipulations.
  
• Asymmetric competition should be expected
  superior competitor may be limited by some other
  factor.
  
• Amensalism

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Frequency of Competition

• Differing views of competition
• Gurevitch et al. 1992
• Found no differences in competition between
  different habitat types but did find filter
  feeders and herbivores competed more than
  carnivores or plants.

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Frequency of Competition

• Grime 1979
• Competition unimportant for plants in
  unproductive environments.
  
• Tilman 1988
• Competition occurs across all productivity
  gradients resources involved may differ.
  
• Supported by Gurevitchs results.

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Modeling Competition

• The effect of competition on the growth of a
  population can be predicted by mathematical
  models.

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Modeling Competition

• Lotka-Volterra competition models are based on
  the logistic equation of population growth the
  s-shaped curve from chapter 6.

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Modeling Competition

• Four possible outcomes of the Lotka-Volterra
  competition equations
  
• Species 2 eliminated.
• Species 1 eliminated.
• Either species 1 or species 2 eliminated,
  depending on starting conditions.
  
• Both species coexist.

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Modeling Competition

• Test of Lotka-Volterra equations
• Populations of yeast are greater in pure cultures
  than in mixed cultures, although logistic growth
  curve is still followed.
  
• Carrying capacity is set by alcohol concentration
  a by-product that increases as the colonies
  grow.

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Modeling Competition

• The values obtained from the Lotka-Volterra
  equations were in general agreement with
  experimental results.

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Modeling Competition

• Deficiencies
• The maximal rate of increase, the competition
  coefficients, and the carrying capacity are all
  assumed to be constant.
  
• There are no time lags.
• Field tests of these equations have rarely been
  performed.
  
• Laboratory tests have shown divergence.

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Modeling Competition

• Mechanisms that drive competition are not
  specified in the Lotka-Volterra model.
  
• R - Tilman (1982, 1987) alternative.
• Need to know the dependence of an organism's
  growth on the availability of resources.
  

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Modeling Competition

• Theoretically, the species with the lowest R
  will outcompete all others.
  
• Sometimes there are several limiting factors,
  allowing coexistence.
  

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Modeling Competition

• Coexistence may occur in a spatially variable
  habitat, where if a species is out-competed in
  one area, it may win in another.
  
• Or, herbivores may graze vigorous competitors
  preventing them from taking over.

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Coexistence of Species

• Niche
• Grinnell (1918) a subdivision of a habitat that
  contains an organism's' dietary needs, its
  temperature, moisture, pH, and other
  requirements.
• Elton (1927) and Hutchinson (1958) an organism's
  role within the community.
  

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Coexistence of Species

• Gause two species with similar requirements
  could not live together in the same place.
  
• Gause's principle, known as competitive exclusion
  principle, where direct competitors cannot
  coexist.
  

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Coexistence of Species

• David Lack Competition and coexistence in about
  40 pairs of birds was mediated by habitat
  segregation.
  

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Coexistence of Species

• Examples of coexistence
• Darwin's finches on the Galapagos.
• Terns on Christmas Island (Ashmole 1968).

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Coexistence of Species

• Ranks for resource partitioning (Schoener 1974)
• Macrohabitat (55).
• Food type (40).
• Time of day or year (5).

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Coexistence of Species

• Hutchinson (1959)
• Seminal paper, "Homage to Santa Rosalia, or why
  are there so many kinds of animals?"
  
• Examined size differences for
• Sympatric species (species occurring together).
• Allopatric species (occurring alone).
• Hutchinson's ratio, 11.3.

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Coexistence of Species

• Criticism of Hutchinson
• Further tests showed no differences between
  species than would occur by chance alone.
  
• Size-ratio differences could have evolved for
  other reasons.
  
• Biological significance cannot always be attached
  to ratios, particularly to structures not used to
  gather food.
  

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Coexistence of Species

• Support of Hutchinson
• d/w analysis for separation on continuous
  resource sets.
  
• d distance between maxima.
• w measure of spread.

Resource partitioning Three species with
similar, normal resource utilization curves
utilize a resource supply K. Variables d
distance between means, w standard deviation of
resource utilization, and d/w resource
separation ratio.
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Division of resources along two niche dimensions.
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Coexistence of Species

• Resources can have a discontinuous distribution
  or occur in distinct units, like leaves on a
  shrub.
  

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Coexistence of Species

• Two insect species one feeds toward the top of
  the plant, the other feeds all over except at
  very top bottom.
  
• Overlap only on 2nd 3rd pairs of leaves.

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Coexistence of Species

• The niche overlap between the two insect species
  can be measured by the proportional similarity
  (PS).

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Coexistence of Species

• PS resource.
  
• PS 0.70 indicates competitive exclusion for
  single resource.
  

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Coexistence of Species

• Proportional similarity indices for two or more
  resources can be combined.
  
• Multiply separate PS values to determine overall
  PS value.
  
• Coexistence for two resources
• 0.70 x 0.70 0.49 or less.

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Summary

• Competition may be interspecific or
  intraspecific.
  
• Competition may be viewed as resource competition
  or interference competition.
  
• Intraspecific competition between plants may be
  described by the 3/2 self-thinning rule.
  

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Summary

• Outcome of competition can be influenced by
• Environmental conditions.
• The presence or absence of natural enemies.
• The genetic strain of the competitors involved.

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Summary

• Experimental studies show that in nature
  competition occurs between different types of
  organisms over a broad scale.
  
• Such studies focused on exotics and
  generalizations to natural ecosystems are
  questionable.
  

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Summary

• Competition between exotics and native species
• Serious consequences for natural ecosystems.
• Frequency of competition.
• 55 to 75 of species involved.
• Competition is often asymmetric.

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Summary

• Six mechanisms of competition
• Consumptive
• Preemptive
• Overgrowth
• Chemical
• Territorial
• Encounter

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Summary

• Lotka-Volterra model early competition model.
• Two species interaction.
• Four possible outcomes
• Species 1 becomes extinct.
• Species 2 becomes extinct.
• Either species 1 or species 2 becomes extinct
  based on starting conditions.
  
• Coexistence.

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Summary

• Competition is minimized and species can coexist
  if they use different resources.
  
• How much can they overlap?
• Hutchinson's 11.3 ratio.
• d/w values greater than unity.
• Proportional similarity (PS) values no greater
  than 70 (0.70).
  

69
Review

• Chapter 1 Why How to Study Ecology
• What is ecology?
• Four main areas of ecology
• Factors limiting population size
• Ecosystem functions
• Biodiversity
• Four hypotheses concerning diversity
• What are statistical tests?
• Correlation
• Types of experiments
• Spatial and Temporal Scales

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Review

• Chapter 2 Genetics Ecology
• How are new species formed?
• What is a mutation?
• Types of mutations
• Importance of genetic variation
• Inbreeding
• Genetic drift
• Neighborhoods
• 50/500 rule
• Effect of immigration

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Review

• Chapter 3 Extinction
• Definition
• What does fossil record show regarding
  extinction?
  
• Large scale extinctions 11-13 thousand years ago
  causes
  
• How do humans cause extinctions?
• Islands extinction
• Introduced species
• Characteristics of organisms that may contribute
  to likelihood of extinction
  
• Endangered species

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Review

• Chapter 4 Group/Individual Selection
• What is group selection?
• What is individual selection?
• Which is the favored theory?
• Altruism
• Kin selection including case study
• Many-eyes hypothesis
• Selfish herd theory

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Review

• Chapter 5 Life History Strategies
• Semelparous vs. Iteroparous
• Reproductive strategy and population age
  structure
  
• What happens when an age class is removed?
  (overfishing removes older, over browsing removes
  younger age class)
  
• Sex ratio what is it and how is it maintained
• Monogamy/polygyny/polyandry
• Resource based polygyny
• r-selected vs. K-selected species -
  characteristics
  

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• Chapter 6 Population Growth
• Life tables what are they what do they show
• Time-specific vs. age specific life tables
• Survivorship curves Types I, II, III
• Net reproductive rate what is it
• What happens if net reproductive rate is equal to
  one, greater than or less than 1?
  
• Geometric growth (what is it, shape of curve)
• Logistic growth (what is it, shape of curve)
• Assumptions problems
• Deterministic vs. stochastic models
• What are stochastic models based on, what do they
  introduce that was not present in deterministic
  models and why is this valuable?
  

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• Chapter 7 Physical Environment
• Distribution and abundance of many species are
  limited by abiotic factors what are they and
  how do they affect distribution and abundance?
  
• Liebigs law of the minimum
• Optimum range
• Ectotherms vs. endotherms
• Degree-days
• Fire before after arrival of Europeans
• Global warming
• Greenhouse effect
• Human influences on greenhouse effect
• Positive negative feedback
• Effect of climate change on natural communities

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• Chapter 8 Competition Coexistence
• Competition Interspecific vs. Intraspecific
• Types of biotic interactions
• Herbivory
• Predation
• Parasitism
• Competition
• Mutualism
• Commensalism
• Resource vs. interference competition
• 3/2 thinning rule
• What influences the outcome of competition?
• Competition between exotics natives
• Frequency of competition
• What are the 6 mechanisms of competition?
• Lotka-Volterra model outcomes
• Hutchinsons 11.3 ratio
• Proportional Similarity