VI. COMPETITION

d. Problems with L-V Models

- need to do a competition experiment first, to measure α’s, to predict outcomes of other competition experiments

- don’t know anything about the nature of the competitive interaction… what are they competing for?

 

  

INTERACTIONS AMONG POPULATIONS

VI. COMPETITION

A. Empirical Tests of Competition

B. Modeling Competition

1. Intraspecific competition

2. Lotka-Volterra Models

3. Tilman Models (1982)

): 

 

  

3.Tilman's Resource Models (1982)

): 

 

  

Isoclines graph population growth relative to resource ratios (2 resources, R1, R2) Consumption rate of Species A(Ca).

3. Tilman's Resource Models (1982)

): 

 

  

Isoclines graph population growth relative to resource ratios (2 resources, R1, R2) Consumption rate of Species A(Ca).

Resource limitation can occur in different environments with different initial resource concentrations (S1, S2).

So, in S2, the population becomes limited by the short supply of R2.

 

  

Species B requires more of both resources than species A.

 

  

Species B requires more of both resources than species A. So, no matter the environment and no matter the consumptions curves (lines from S), the isocline for species B will be "hit" first. So, Species B will stop growing, but Species A can continue to grow and use up resources.... this drops resources below B's isocline, and B will decline.

 

  

Species B requires more of both resources than species A. So, no matter the environment and no matter the consumptions curves (lines from S), the isocline for species B will be "hit" first. So, Species B will stop growing, but Species A can continue to grow and use up resources.... this drops resources below B's isocline, and B will decline. So, if one isocline is completely within the other, then one species will always win.

 

  

If the isoclines intersect, coexistence is possible (there are densities where both species are equilibrating at values > 1).

 

  

If the isoclines intersect, coexistence is possible (there are densities where both species are equilibrating at values > 1). Whether this is a stable coexistence or not depends on the consumption curves. Consider Species B. It requires more of resource 1, but less of resource 2, than species A. Yet, it also consumes more of resource 1 than resource 2 - it is a "steep" consumption curve. So, species B will limit its own growth more than it will limit species A.

 

  

If the isoclines intersect, coexistence is possible (there are densities where both species are equilibrating at values > 1). Whether this is a stable coexistence or not depends on the consumption curves. Consider Species B. It requires more of resource 1, but less of resource 2, than species A. Yet, it also consumes more of resource 1 than resource 2 - it is a "steep" consumption curve. So, species B will limit its own growth more than it will limit species A. This will be a stable coexistence for environments with initial conditions between the consumption curves (S3). If the consumption curves were reversed, there would be an unstable coexistence in this region.

3. Tilman's Resource Models (1982)

- Benefits:

1. The competition for resources is defined

 

  

3. Tilman's Resource Models (1982)

- Benefits:

1. The competition for resources is defined

2. The model has been tested in plants and plankton and confirmed

 

  

3. Tilman's Resource Models (1982)

- Benefits:

1. The competition for resources is defined

2. The model has been tested in plankton and confirmed

 

  

PO

4 (u

M)

SiO2 (uM)

Cyclotella

Asterionella

Cyclotella wins

Asterionella wins

Stable Coexistence

3. Tilman's Resource Models (1982)

- Benefits:

1. The competition for resources is defined

2. The model has been tested in plants and plankton and confirmed

3. Also explains an unusual pattern called the "paradox of enrichment"

 

  

3. Tilman's Resource Models (1982)

- Benefits:

1. The competition for resources is defined

2. The model has been tested in plants and plankton and confirmed

3. Also explains an unusual pattern called the "paradox of enrichment"

If you add nutrients, sometimes the diversity in a system drops... and one species comes to dominate. (Fertilize your lawn so that grasses will dominate... huh?)

 

  

If you add nutrients, sometimes the diversity in a system drops... and one species comes to dominate.

 

  

Change from an initial stable coexistence scenario (S1)to a scenario where species A dominates (S2).

3. The Nature of Competitive Interactions

1. Types:

- contest vs. scramble

): 

 

  

Contest (interference):Competition for access to resourceAllelopathy/territorialityIntrasexual selection

Scramble (exploitative):Direct consumption of resourceFeeding frenzies

3. The Nature of Competitive Interactions

1. Types:

- contest vs. scramble

- symmetrical vs. asymmetrical

- unusual that both competitors are equal across all habitats.

 

  

3. The Nature of Competitive Interactions

1. Types:

- contest vs. scramble

- symmetrical vs. asymmetrical

2. Competitive Outcomes: - Reduction in organism growth and/or pop. size (G, M, R)

- Competitive exclusion (N = 0)- Reduce range of resources used = resource partitioning. - If this selective pressure continues, it may result in a

morphological change in the competition. This adaptive response to competition is called Character Displacement

 

  

Character Displacement