Competition

1
Competition Evolution and Resources
2
Lecture Goals

• Define and discuss
• evolutionary / ecological effect
• character displacement
• competitive release
• Identify potential risks in the interpretation of
  "natural experiments"
• Outline Tilman's model of competiton and explain
  how it differs from the Lotka-Volterra model

3
Competition as an Evolutionary Force

• Types of competitive outcomes
• 1. Competition gtgt local extinction gtgt fewer
  competitors
• (ecological)
• 2. Competition gtgt differentiation gtgt less
  competition (evolutionary)
• How can we distinguish between these?
• Are observed species mixes due to 1 or 2 in
  any particular case?
• Beware the invocation of the ghost of competition
  past! (unproved explanation)

4
Natural Experiments

• Competitive Release
• when apart - species have larger realized niche
  (niche expansion)
• when together - species have smaller realized
  niches
• Character Displacement
• observable morphological differences emphasized
  when species occur together
• physiological behavioral traits could also be
  displaced
• By Observation, By Manipulation

5
Competitive Release?

• Two source populations of Trifolium (clover)
• with and without Lolium, a grass (T.grass and
  T.alone)
• Best Growth by T.alone transplanted to grass site
  with both grass and clover spp. removed
• T.grass did as well with and without Lolium (no
  effect)
• T.grass performed better than T.alone with Lolium
• Release only for T.alone , BUT not in regular
  competition
• Less competition had evolved for T.grass
  (Evolutionary)

6
Character Displacement

• 2 similar species of 3-spine stickleback
  (Gasterosteus aculeatus ???) from 2 historic
  introductions
• gill rakers 'strain' food
• when in same lake, species have different gill
  rakers (lengths, spacing) habitats (limnetic
  and benthic)
• when a single species is in a lake, gill rakers
  are of intermediate form, feeding in both
  habitats

7
Mechanisms Resources
How are niches differentiated?

• Spatially (movement, distribution - both micro
  and macro)
• with Typha spp., cattails separate by water depth
  
• Temporally (with season/time of day)
• anuran tadpoles Hyla crucifer Bufo woodhousii -
  4-6 week offset in feeding periods
• Conditionally (using same resources)
• "winner" varies with situation
• result in either Spatial or Temporal separation

8
Tilmans Model ofCompetition
Note more realism leads to more complex models...
9
Explicit Resources

• The Lotka-Volterra competition model implies
  resources through its parameters.
• How does K imply a resource?

Tilman developed and tested more complex models
that explicitly considered variation in resource
levels being exploited by competing consumers.
10
Underlying Principles of Tilmans Model

• PRINCIPLE I The resource will be reduced to a
  level equal to the lower tolerance of the
  exploiting species.
• How is balance maintained?
• PRINCIPLE II The competitor that can tolerate
  lower resource levels will ultimately dominate in
  the exploitation of a single resource.

11
Now, Tilman must model the dynamics of both the
resources and the competitors (4 variables
instead of two).
We will examine his model graphically, rather
than focus on the mathematics.
Note plots are now presented in resource space
instead of species space as in the phase
diagrams for the Lotka-Volterra model. They
illustrate changes in resource availability.
12
Two Resources
What happens to resources and exploiting Species
1 below and left of the Species 1 isocline?

• Source is the original, unexploited level of the
  resources
• Tilman has shown there is only one equilibrium S
  in his model.

13
Competitive Exclusion
How does this graph suggest competitive
exclusion? Which species will be excluded?
Will the location of the Source" resource levels
influence the outcome?
Can you picture the location of the isoclines
associated with the success of Species2?
14
Coexist?
(- - -) lines are defined by the resource decline
functions of X and Y.
The intersection of isoclines is a potential
equilibrium, contingent upon original resource
levels (source).
15
Testing the Model

• Diatom species competing for silicate and
  phosphate
• predicted and observed outcome of different
  initial supply points
• generally, predictions held

16
Questions
Though "character displacement " refers to
morphological features, what other traits may be
influenced by competition? Are these evolutionary
effects? Why?
What are the chief differences between the
Lotka-Volterra model of competition and Tilman's?
How would you decide which was 'better'?