BIOS 3010: Ecology
Lecture 6: Processes: Interspecific competition
•  Lecture summary:
–  Definition &
examples.
–  Lotka-Volterra
model:
Semibalanus balanoides
James P. Rowan, http://www.emature.com
•  Structure.
•  Competition
coefficients.
•  Predicted outcomes.
Chthamalus stellatus
Dr. S. Malcolm
BIOS 3010: Ecology
Alan J. Southward, http://www.marlin.ac.uk/
Lecture 6: slide 1
2. Interspecific Competition:
–  Like intraspecific competition, competition
between species can be defined as:
•  "Competition is an interaction between individuals,
brought about by a shared requirement for a
resource in limited supply, and leading to a
reduction in the survivorship, growth and/or
reproduction of at least some of the competing
individuals concerned"
–  For example, Connell's 2 species of barnacle (asymmetrical
interference/contest) in Fig. 8.2 and slides
–  Gause's Paramecium species of Fig. 8.3 and,
–  Tilman's diatoms (exploitation/scramble) of Fig. 8.5
–  Connell's "the ghost of competition past" = the current
product of past evolutionary responses to competition!
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 2
3. Basic outcomes of competition:
•  These interactions illustrate the two basic outcomes of competition:
–  1) coexistence:
•  if two competing species coexist in a stable environment, then they
do so as a result of niche differentiation (of their realized niches) =
character displacement (Figs 7.18, 8.23 & 8.25a)
–  2) competitive exclusion - the "competitive exclusion
principle" or "Gause's principle:”
•  if there is no niche differentiation, then one competing species will
eliminate or exclude the other.
•  Thus exclusion occurs when the realized niche of the superior
competitor completely fills those parts of the inferior competitor's
fundamental niche which the habitat provides.
•  see Fig. 7.4 of exclusion in reed species.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 3
1
4. The Lotka-Volterra model of
interspecific competition:
•  Based on the logistic equation that describes sigmoidal population growth
as a result of intraspecific competition:
dN/dt = rN((K-N)/K)
–  (after Volterra, 1926 & Lotka, 1932) with the inclusion of the
competition coefficients α and β we can represent population size
changes for the two competing species as:
dN1/dt = r1N1((K1-N1-α N2)/K1)
–  and
dN2/dt = r2N2((K2-N2-β N1)/K2)
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 4
5. Competition coefficients:
•  The competition coefficient α is the effect on species
1 of species 2 (also written as α12):
–  If α <1 then interspecific competition has less impact than
intraspecific competition.
–  If α >1 then interspecific competition has more impact.
•  Conversely, β is the effect on species 2 of species 1
(also written as α21):
–  N1 & N2 are the population sizes of species 1 & 2.
–  r1 & r2 are intrinsic rates of natural increase for spp. 1 & 2.
–  K1 & K2 are the carrying capacities for species 1 & 2.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 5
6. Lotka-Volterra competition model
- zero isoclines:
–  Zero population growth isoclines (dN/dt = 0) are
shown in graphs of N2 on the y-axis plotted against
N1 on the x-axis in Figs. 8.7 and 8.9.
•  When this is true for species 1, then r1N1(K1-N1-αN2) = 0
and K1-N1-αN2 = 0
•  Therefore N1 = K1-αN2
•  When N1 = 0, N2 = K1/α (the result of pure interspecific
competition at A in Fig. 8.7a)
•  When N2 = 0, N1 = K1 (the result of pure intraspecific
competition at B in Fig. 8.7a) to give the zero
isocline of Fig. 8.7a
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 6
2
7. Four outcomes of the LotkaVolterra competition model:
•  From Figure 8.9 the 4 outcomes are expected to be:
–  1) species 1 wins (competitive exclusion)
(8.9a)
•  species 1 is a stronger interspecific competitor (K1 >K2/β, therefore K1β >K2)
even though intraspecific competition within species 1 is stronger than the
interspecific effect of species 2 (K1/α > K2, therefore K1 > K2α)
•  (converse of 1)
–  3) either species 1 or species 2 wins
(8.9c)
•  (interspecific competition greater in both species than intraspecific
competition - the outcome depends on starting densities)
–  4) coexistence
(8.9d)
•  (both species have less competitive effect on the other species than they do
on themselves: K1 > K2α, and K2 > K1β - gives a stable equilibrium)
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 7
Figure 8.2: Intertidal distribution of two
barnacle species
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 8
Figure 8.3: Intra- and interspecific
competition in Paramecium spp.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 9
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Figure 8.5: Competition between diatoms
& silicate availability
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 10
Figure 7.18 (3rd ed.): Character displacement
(mandible variation) in ant communities
Data for Veromessor pergandei
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 11
Figure 8.23: Character displacement
(gill rakers) in sticklebacks
benthic
Dr. S. Malcolm
limnetic
BIOS 3010: Ecology
Lecture 6: slide 12
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Figure 8.25a: Character displacement of
freshwater snail shell lengths
Hydrobia
ulvae
Hydrobia
ventrosa
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 13
Figure 7.4 (3rd ed.): Asymmetrical competition
between cattail species in Michigan
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 14
Figure 8.7: Zero isoclines of the Lotka-Volterra
competition equations for species N1 and N2
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 15
5
Figure 8.9: Lotka-Volterra competition
model outcomes
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 16
Barnacles, July 2006, Kintyre, Scotland
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 6: slide 17
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