Interactions within and among microbial populations
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Interactions within and among microbial
populations
Terminology
• Intra-specific = interactions among members of the same species/population.
• Inter-specific = interactions among members of different species.
Terminology
• Density dependentdependent population regulation = a population dynamic in which growth rate is regulated by the densities of individuals.
• Density independentindependent population regulation = a population dynamic in which growth rate is regulated by factors not related to population size.
How can you tell if organisms are influencing each other?
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).
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)
Effect of interaction on:
Name if InteractionPopulation/Species A
Population/Species B
0 0
0 +
+ +
0 -
+ -
- -
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.
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.
Classic studies of resource competition by Gause (1934,
1935)
Paramecium aurelia
Paramecium bursaria
Paramecium caudatum
Classic studies of resource competition by Gause (1934,
1935)Gause found that interactions between
Paramecium aurelia and P. caudatum always ended in competitive exclusion.
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!
Classic studies of resource competition by Gause (1934,
1935)
In contrast, Paramecium bursia and P. caudatum could coexist.
Paramecium caudatumParamecium bursaria
Classic studies of resource competition by Gause (1934,
1935)
In contrast, Paramecium bursia and P. caudatum could coexist.
WHY?
Paramecium caudatumParamecium bursaria
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.
In contrast, both P. aurelia and P. caudatum ate the bacteria in suspension.
Classic studies of resource competition by Gause (1934,
1935)
Gause’s 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.
Gause’s 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
Competition (-/-) is central to both evolutionary theory and ecological theory:
• Competition can be a powerful
selection pressure.
• Competition structures of communities.
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.
Consider how Rick Lenski’s experiments support these ideas
Consider how Rick Lenski’s experiments support these ideas
intra-specific competition
character displacement - adaptive radiation
Ultimate results?SpeciationCommunity structure
Resource-ratio Competition Theory
In the mid 70’s and early 80’s Dave Tilman contributed to the development of resource-ratio competition theory.
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.
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
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 =
= 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.
= 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.
Asteronella formosa
Cyclotella meneghiniana
= 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
Which species is the better competitor for PO4 ?
Asterionella formosa Ks for PO4 = 0.04
Cyclotella meneghiniana Ks for PO4 = 0.25
Which species is the better competitor for PO4 ?
Asterionella formosa KAsterionella formosa Kss for PO for PO44 = = 0.04*0.04*
Cyclotella meneghiniana Ks for PO4 = 0.25
Which species is the better competitor for SiO2 ?
Asterionella formosa Ks for SiO2 = 3.9
Cyclotella meneghiniana Ks for SiO2 = 1.4
Which species is the better competitor for SiO2 ?
Asterionella formosa Ks for SiO2 = 3.9
Cyclotella meneghiniana KCyclotella meneghiniana Kss for SiO for SiO22 = 1.4*= 1.4*
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.
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.
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
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
Tilman predicted that…
Asterionella’s boundary between PO4 and SiO2 limitation should occur when [SiO2]/[PO4] = (3.9 / 0.04) = 97
When [SiO2]/[PO4] > 97 growth of Asterionella should be limited by PO4
When [SiO2]/[PO4] < 97 growth of Asterionella should be limited by SiO2
Tilman predicted that…
Cyclotella’s boundary between PO4 and SiO2 limitation should occur when [SiO2]/ [PO4] = 1.4 / 0.25 = 5.6
When [SiO2]/[PO4] > 5.6 growth of
Cyclotella should be limited by PO4
When [SiO2]/[PO4] < 5.6 growth of Cyclotella should be limited by SiO2
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] > 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?
Which species is the better competitor for PO4 ?
Asterionella formosa KAsterionella formosa Kss for PO for PO44 = = 0.04*0.04*
Cyclotella meneghiniana Ks for PO4 = 0.25
Tilman predicted that…
When [SiO2]/[PO4] < 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?
Which species is the better competitor for SiO2 ?
Asterionella formosa Ks for SiO2 = 3.9
Cyclotella meneghiniana KCyclotella meneghiniana Kss for SiO for SiO22 = 1.4*= 1.4*
Tilman conducted 73 competition studies in
chemostats…
At different [SiO2]/[PO4] levels and different flow rates.
Tilman conducted 73 competition studies in
chemostats…
At different [SiO2]/[PO4] levels and different flow rates.
Results conform to the predictions remarkably well!
Tilman conducted 73 competition studies in
chemostats… Fl
ow
rate
(volu
mes
per
day)
Nutrient ratio [SiO2]/[PO4]
1,000 97 10 5.6 1.0
Asterionella wins
Cyclotella wins
Stable
co-existence
Tilman conducted 73 competition studies in
chemostats… Fl
ow
rate
(volu
mes
per
day)
Nutrient ratio [SiO2]/[PO4]
1,000 97 10 5.6 1.0
Asterionella wins
Cyclotella wins
Stable
co-existenceAsterionell
a limited by SiO2
Cyclotella limited by PO4
Tilman conducted 73 competition studies in
chemostats… Fl
ow
rate
(volu
mes
per
day)
Nutrient ratio [SiO2]/[PO4]
1,000 97 10 5.6 1.0
Asterionella wins
Cyclotella wins
Stable
co-existence
Intra-specific competition
>Inter-specific competition
Effect of Interaction on:
Name if InteractionPopulation/Species A
Population/Species B
Neutralism 0 0
Commensalism 0 +
Mutualism + +
Amensalism 0 -
Predation, parasitism + -
Competition - -
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