Terrestrial Ecology

1
Terrestrial Ecology

• Introduction to ecology
• Competitive interactions_1
• Competitive interactions_2
• Predator-prey interactions
• Plant-herbivore interactions_1
• Plant-herbivore interactions_2
• More complicated ecological interactions

2
Modelling competitive outcomes

• If
• 2 species live together
• use the same resource(s)
• what happens to their populations?
• Mathematical models for predicting outcomes
  include
• Lotka Volterra (1925, 1926)NOT their
  predator-prey models
• Tilman (1977)

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Tilmans Competition Model

• See Krebs 2001 Tilman 1977
• Based on
• Resource use
• Mechanisms of competition

Tilman D. (1977) Resource competition between
plankton algae an experimental and theoretical
approach. Ecology, 58, 338-348.
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1 species response to 2 resources

• NB/ different axes to Lotka-Volterra plots

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2 species response to 2 resources
Sp. B
Sp. A
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2 species competitive outcomes
Sp. B
Sp. A
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Another way of looking at it
Sp. B

• 2 resources phosphate, silicate

Sp. A
Tilman D. (1977) Resource competition between
plankton algae an experimental and theoretical
approach. Ecology, 58, 338-348.
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When were resources limited?

• 2 species Asterionella, Cyclotella
• 2 resources phosphate, silicate
• For each species
• Grow cultures singly
• Use range of Si/P ratios
• Use range of flow rates
• Determine when Si or P limited

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When were resources limited?
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Predicted responses to resources limited?
Asterioella
Cyclotella
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Trials to test predictions?

• 2 species A.f. Asterionella, C.m. Cyclotella
• 2 resources phosphate, silicate
• Each trial 30 days 76 trials total
• Start NA.f. NC.m. (same population size)
• Run under range of
• Si/P environments
• Flow rates
• Measure populations over time
• Compare with single species runs (controls)

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Example of results

• Resultsas predicted

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Summary of all results

• Results as predicted !

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Does model predict the real world?

• Collected water samples from Lake Michigan
• Measured Si/P ratio
• Measured relative abundance of Asterionella and
  Cyclotella

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Does model predict the real world?

• YES !
• 70 of variation explained by model

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Tilmans Model - summary

• Competition described by mechanism
• Resource availability
• Utilisation response by each species
• Emphasises role of resources
• in determining competitive outcomes
• Predicted
• Controlled lab world well
• Real world well too!

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Tilmans Model - summary

• Demonstrated co-existence
• When limiting resources differ
• Demonstrated competitive exclusion
• When utilisation capacity for a limited resource
  differs between species

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Competitive Exclusion Principle

• or Gauses Hypothesis
• complete competitors cannot co-exist
• (Hardin 1960)

so why such high diversity in the real world?
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Why such diversity?

• Competition rare resources not limiting
• Evolution ? competition

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Competition rare?

• Unpredictable environments ? stochastic events
  keep populations low ? low (no?) intra-
  inter-specific competition

• competition still could be important, just
  occasional

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Competition rare?

• Fluctuating environments ? alternating
  competitive advantage ? co-existence

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Competition rare?

• Unstable environments ? alternating competitive
  advantage ? co-existence

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Evidence of competition?

• Resource partitioning

• Both
• Consistent with competition(or 5 independent
  decisions?)
• Acts to ? competition

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Resource partitioning

• If it is due to current competition
• Then if exclude Species A
• Predict for Species B?

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Evolution ? competition?

• Character displacement(? characteristics) ?
• ? competition ?
• co-existence

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Character displacement
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Main points

• Defining ecological interactions
• By effects
• By mechanisms
• Types of competition
• Resource limited
• Interference
• Competition can be modelled
• Do models reflect the real world?
• Of not why not?
• Resource partitioning
• Evidence of competition?
• Can ? competition
• Character displacement
• Evidence of past competition?
• ? competition