Interactions within and among microbial populations

1
Interactions within and among microbial
populations
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Terminology

• Intra-specific interactions among members of
  the same species/population.
• Inter-specific interactions among members of
  different species.

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Terminology

• Density dependent population regulation a
  population dynamic in which growth rate is
  regulated by the densities of individuals.
• Density independent population regulation a
  population dynamic in which growth rate is
  regulated by factors not related to population
  size.

4
How can you tell if organisms are influencing
each other?
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How can you tell if organisms are influencing
each other?

• Study the relationship between population growth
  rate and population density.
• If there is a relationship (either or -), then
  assume that the organisms are influencing each
  other, if not, there is no interaction
  (neutralism).

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How can you tell if organisms are influencing
each other?

• On the blackboard illustrate
• Positive density dependence (e.g. allee effect,
  mutualism)
• Negative density dependence (intra- or
  inter-specific competition)
• Density independence (neutralism)

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Effect of interaction on
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Competition

• A. Two major types
• 1. Interference competition Competition between
  two individuals/populations/species in which one
  physically or chemically excludes the other from
  a portion of habitat and hence from the resources
  that could be exploited there.
• 2. Resource competition Occurs when use of a
  resource by one individual/population/species
  reduces the availability of that resource to
  other individuals/populations.

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• Resource competition can only occur when
  population growth rates of both
  individuals/populations/species are limited by
  the same resource.
• If no resources are in limiting supply, then
  competition does not occur.

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Classic studies of resource competition by Gause
(1934, 1935)
Paramecium caudatum
Paramecium aurelia
Paramecium bursaria
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Classic studies of resource competition by Gause
(1934, 1935)

• Gause found that interactions between Paramecium
  aurelia and P. caudatum always ended in
  competitive exclusion.

12
Classic studies of resource competition by Gause
(1934, 1935)

• Gause found that interactions between Paramecium
  aurelia and P. caudatum always ended in
  competitive exclusion.

Within 14 days Paramecium aurelia WON!
13
Classic studies of resource competition by Gause
(1934, 1935)

• In contrast, Paramecium bursia and P.
  caudatum could coexist.

Paramecium bursaria
Paramecium caudatum
14
Classic studies of resource competition by Gause
(1934, 1935)
WHY?

• In contrast, Paramecium bursia and P.
  caudatum could coexist.

Paramecium bursaria
Paramecium caudatum
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Classic studies of resource competition by Gause
(1934, 1935)

• Because they inhabited different regions of the
  flask and ate different food.
• P. bursia fed on the bottom of the flask, and P.
  caudatum ate the bacteria in suspension.

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Classic studies of resource competition by Gause
(1934, 1935)

• In contrast, both P. aurelia and
• P. caudatum ate the bacteria in suspension.

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Gauses Principle (Law)

• When the niches of two species overlap, there
  will be competition and, if the overlap is
  extreme, there will be competitive exclusion.
• Niche the ranges of conditions and resources
  within which an organism or species persists,
  often conceived as a multidimensional space.

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Gauses Principle (Law)

• When the niches of two species overlap, there
  will be competition and, if the overlap is
  extreme, there will be competitive exclusion.

This is also called the Competitive Exclusion
Principle
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• Competition (-/-) is central to both evolutionary
  theory and ecological theory
• Competition can be a powerful selection pressure.
  
• Competition structures of communities.

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• Study the concept diagram that illustrates the
  relationships between
• intra-specific competition
• inter-specific competition
• character displacement (adaptive radiation)
• competitive exclusion
• Consider the outcomes of these processes in terms
  of proximate (immediate) and ultimate
  (evolutionary) effects
• speciation, community structure and local or
  global extinction.

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Consider how Rick Lenskis experiments support
these ideas
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Consider how Rick Lenskis experiments support
these ideas

• intra-specific competition
• character displacement - adaptive radiation
• Ultimate results?
• Speciation
• Community structure

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Resource-ratio Competition Theory

• In the mid 70s and early 80s Dave Tilman
  contributed to the development of resource-ratio
  competition theory.

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Resource-ratio Competition Theory

• This theory is based upon knowledge of how the
  per capita rate of change of a population depends
  upon the availability of a limiting resource.

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Resource-ratio Competition Theory

• This theory is based upon knowledge of how the
  per capita rate of change of a population
  depends upon the availability of a limiting
  resource.
• dN/Ndt

DRAW this relationship
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Resource-ratio Competition Theory

• This theory is based upon knowledge of how the
  per capita rate of change of a population depends
  upon the availability of a limiting resource.

dN/Ndt ?
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? ?max S/(Ks S)

• ?max the maximum per capita growth rate under
  conditions of resource saturation
• S the concentration of a growth-limiting
  resource
• Ks resource concentration at which growth
  occurs at half the maximum rate.

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? ?max S/(Ks S)

• This is analogous to Michaelis-Menten enzyme
  kinetics, and was applied to the growth rates of
  single species cultures of bacteria by Monod in
  1950.

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 Asteronella formosa
 Cyclotella meneghiniana
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? ?max S/(Ks S)

• Figure 1, Titman, 1976 Michaelis-Menton
  functions
• Asterionella formosa Ks for PO4 0.04
• Ks for SiO2
  3.9
• Cyclotella meneghiniana Ks for PO4 0.25
• Ks for
  SiO2 1.4

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Which species is the better competitor for PO4 ?

• Asterionella formosa Ks for PO4 0.04
• Cyclotella meneghiniana Ks for PO4 0.25

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Which species is the better competitor for PO4 ?

• Asterionella formosa Ks for PO4 0.04
• Cyclotella meneghiniana Ks for PO4 0.25

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Which species is the better competitor for SiO2 ?

• Asterionella formosa Ks for SiO2 3.9
• Cyclotella meneghiniana Ks for SiO2 1.4

34
Which species is the better competitor for SiO2 ?

• Asterionella formosa Ks for SiO2 3.9
• Cyclotella meneghiniana Ks for SiO2 1.4

35
If the growth of a species was potentially
limited by 2 nutrients

• its growth rate is determined by the
  concentration of the nutrient which leads to the
  lower growth rate.

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Tilman predicted that

• the boundary between growth rate limitation by
  SiO2 or by PO4 should occur when the
  concentrations of SiO2 and PO4 cause equal growth
  rates.

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Tilman predicted that

• the boundary between growth rate limitation by
  SiO2 or by PO4 should occur when the
  concentrations of SiO2 and PO4 cause equal growth
  rates.
• From the Michaelis-Menten equation, growth rates
  are equal when
• S1/(S1 K1) S2/(S2 K2)
• S1/S2 K1/K2

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Tilman predicted that

• From the Michaelis-Menten equation, growth rates
  are equal when
• S1/(S1 K1) S2/(S2 K2)
• S1/S2 K1/K2
• S1 concentration of SiO2
• S2 concentration of PO4
• K1 half saturation conc. for SiO2 limited growth
• K2 half saturation conc. for PO4 limited growth

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Tilman predicted that

• Asterionellas boundary between PO4 and SiO2
  limitation should occur when SiO2/PO4 (3.9
  / 0.04) 97
• When SiO2/PO4 gt 97 growth of Asterionella
  should be limited by PO4
• When SiO2/PO4 lt 97 growth of Asterionella
  should be limited by SiO2

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Tilman predicted that

• Cyclotellas boundary between PO4 and SiO2
  limitation should occur when SiO2/ PO4 1.4
  / 0.25 5.6
• When SiO2/PO4 gt 5.6 growth of Cyclotella
  should be limited by PO4
• When SiO2/PO4 lt 5.6 growth of Cyclotella
  should be limited by SiO2

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Tilman predicted that

• This resource utilization information can be used
  to predict the results of nutrient-based
  competition between these two species.
• When SiO2/PO4 gt 97 both species will be
  limited by PO4 but one species is more limited
  than the other.
• Which species is predicted to be the superior
  competitor under PO4 limitation?

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Which species is the better competitor for PO4 ?

• Asterionella formosa Ks for PO4 0.04
• Cyclotella meneghiniana Ks for PO4 0.25

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Tilman predicted that

• When SiO2/PO4 lt 5.6 both species will be
  limited by SiO2 but one species is more limited
  than the other.
• Which species is predicted to be the superior
  competitor under SiO2 limitation?

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Which species is the better competitor for SiO2 ?

• Asterionella formosa Ks for SiO2 3.9
• Cyclotella meneghiniana Ks for SiO2 1.4

45
Tilman conducted 73 competition studies in
chemostats

• At different SiO2/PO4 levels and different
  flow rates.

46
Tilman conducted 73 competition studies in
chemostats

• At different SiO2/PO4 levels and different
  flow rates.
• Results conform to the predictions remarkably
  well!

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Tilman conducted 73 competition studies in
chemostats
Stable co-existence
Asterionella wins
Cyclotella wins
Flow rate (volumes per day)
1,000 97 10
5.6 1.0
Nutrient ratio SiO2/PO4
48
Tilman conducted 73 competition studies in
chemostats
Stable co-existence
Asterionella wins
Cyclotella wins
Asterionella limited by SiO2
Flow rate (volumes per day)
Cyclotella limited by PO4
1,000 97 10
5.6 1.0
Nutrient ratio SiO2/PO4
49
Tilman conducted 73 competition studies in
chemostats

• Stable
• co-existence
• Intra-specific competition gt
• Inter-specific competition

Asterionella wins
Cyclotella wins
Flow rate (volumes per day)
1,000 97 10
5.6 1.0
Nutrient ratio SiO2/PO4
50
Effect of Interaction on