Interspecific Competition

Population interactions

Population Interactions, Two Species SystemResponse

Type ofInteraction

A B

Neutral 0 0Mutualism + +Commensalism + 0Amensalism - 0Parasitism + -Predation + -Competition - -

Note: 0 = no direct effect; + = positive effect on growth ofpopulation; - = negative effect on growth of population

Logistic Curve

Time

# individuals

Logistic model• Logistic equation

* dN/dt = rN((K-N)/K)

• Assumptions of the logistic model:* Each individual has identical ecological properties* Instantaneous response to environmental change* Constant upper limit to population size and rate of

gain is directly related to remaining gap* Limited space and constant food supply* Age distribution is stable

Lotka/Volterra model

• dN1/dt = r1N1[(K1-N1- 12N2)/K1] is the competition coefficient 12 is effect of species 2 on species 1

• dN2/dt = r2N2[(K2-N2- 21N1)/K2]• if 12 >1, then individuals of sp. 2 have a

greater inhibitory effect on individuals of sp. 1 than does species 1 on species 1.

• if 12 <1, then sp. 2 individuals have less effect

Lotka/Volterra assumptions

• Environment is homogeneous and stable

• Migration is unimportant

• Coexistence requires a stable equilibrium point

• Competition is the only important biological interaction

Model with isocline

See Fig. 12.1 and 12.2

a

Growth is 0 along the line

a

Species 1 growth Species 2 growth

Finding K/

• dN1/dt = r1N1[(K1-N1- 12N2)/K1

• if growth is 0, and r is >0, then the term (K1-N1- 12N2) must be 0

• therefore N1 = K1 - 12N2

• when N1 = 0, N2 = K1/12

a

Possible outcomes

Fig. 12.3

Possible outcomes

Fig. 12.3

Competition - 2 species

Competition - 3 species

Diffuse competition

• Effect of several species

Diffuse competition

• Effect of several species

• Exclusion may occur

How does coexistance occur?

• Unpredictable gaps: the poorer competitor is a better colonizer

• Unpredictable gaps: the pre-emption of space

• Fluctuating environments

• Ephemeral patches with variable life-spans

• Aggregated distributions

If we observe niche differences in the field, what does it mean?

• Current competition

• Evolutionary avoidance through competition

• Evolutionary avoidance (without competition)

DeWitt diagrams

• Plants sown at different densities (40:0, 30:10, 20:20, 10:30, 0:40)

• Seeds harvested