Interactions among populations II: Dynamics of competition between species

Question 1

Define ecological community

Answer 1

group of species that occur together in space and/or time and compete for the same limiting resources

Question 2

Give Gause’s principle of competitive exclusion

Answer 2

two species cannot coexist on a single limiting resource if other ecological factors remain constant.

Question 3

Discuss Connell’s barnacles

Answer 3

• Chthamalus is competitively excluded from the lower shore by Balanus
• persists in the upper shore because it is more tolerant towards dessication

Question 4

Discuss Gause’s Paramecium

Answer 4

• both Paramecium aurelia and Paramecium bursaria eat bacteria
• when grown together in a well-stirred mixture, P. aurelia outcompetes P. bursaria
• if the mixture is not stirred, both species persist (P. bursaria mostly confined to the anoxic bottom of the beaker)
• P. bursaria, with its photosynthetic symbionts, generates oxygen internally
• P. aurelia outcompetes P. bursaria under aerobic conditions, but P. bursaria can persist in anoxic conditions where P. aurelia cannot

Question 5

Describe Fenchel’s snails (Hydrobia)

Answer 5

• Hydrobia ulvae and Hydrobia ventrosa
• most places you only find one species or the other; appear almost indistinguishable
• when they are sympatric, there is character displacement
• H. ulvae becomes significantly larger than H. ventrosa

Question 6

Describe Tilman’s diatoms

Answer 6

• photosynthetic, planktonic organisms
• unique silica cell wall
• can be grown in culture such that silica is the limiting factor

Question 7

R*

Answer 7

concentration of resource at population equilibrium

Question 8

Describe R*

Answer 8

• theory of competition for a single limiting resource: R*
• level to which the concentration of a limiting resource is reduced by an equilibrial monoculture of a species
• minimum resource concentration a species requires for positive population growth

Question 9

What are the assumptions of R*?

Answer 9

• species compete for a single limiting resource
• resource is labile and the system is well-mixed

Question 10

Give an example of a well mixed system

Answer 10

nitrogen in soil

Question 11

Describe the R* prediction

Answer 11

species with the lowest R* for the limiting resource is the superior competitor

Question 12

How to measure R*?

Answer 12

• a variety of ways
• individuals against each other
• seed v seed
• ability to invade a monoculture of the other species

Question 13

Give a two-species population model that includes both the density of intra (N1) and interspecific (N2) competitors

Answer 13

• dN1/dt = r1N1(K1-N1-alpha12N2)/K1
• interspecific competitors are weighted by a competition coefficient (α12).

Question 14

“alpha values”

Answer 14

specify the effect of interspecific competition.

Question 15

When does Lotka-Volterra coexistence occur

Answer 15

𝐾1>𝐾2 𝛼12
𝐾2>𝐾1 𝛼21
- if K1 = K2, when interspecific competition coefficients are < 1 (interspecific competition is weaker than intraspecific competition)