BIOS 6150: Ecology
Dr. Stephen Malcolm, Department of Biological Sciences
•  Week 5: Interspecific
Competition:
•  Lecture summary:
• 
• 
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Definition.
Examples.
Outcomes.
Lotka-Volterra model.
Semibalanus balanoides
James P. Rowan, http://www.emature.com
Chthamalus stellatus
Alan J. Southward, http://www.marlin.ac.uk/
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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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”
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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3. Interspecific competition between 2
barnacle species (Fig. 8.2 after Connell, 1961):
“Click for pictures”
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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4. Gause's Paramecium species
compete interspecifically (Fig. 8.3):
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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5. Tilman's diatoms exploitation/scramble (Fig. 8.5):
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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6. A caveat: “The ghost of competition past:”
•  Lack observed 5 tit species in a single British
wood:
•  4 weighed 9.3-11.4g and 1 weighed 20.0g.
•  All have short beaks and hunt for insect food on leaves
& twigs + seeds in winter.
•  Concluded that they coexisted because they exploited
slightly different resources in slightly different ways.
•  But is this a justifiable explanation? Did species change or were
species eliminated?
•  Connell (1980) emphasized that current patterns
may be the product of past evolutionary responses
to competition - “the ghost of competition past” !
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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7. 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 & 8.25)
•  (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 competitive exclusion in reed species.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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8. The Lotka-Volterra model of interspecific
competition:
•  Based on the logistic equation:
•  dN/dt = rN((K-N)/K)
•  after Volterra (1926) & Lotka (1932)
•  Where:
•  N1 & N2 are the population sizes of species 1 & 2
•  r1 & r2 are the intrinsic rates of natural increase for spp 1 & 2
•  K1 & K2 are the carrying capacities for species 1 & 2.
•  With competition coefficients α and β population
size changes for the two competing species are:
•  dN1/dt = r1N1((K1-N1-α N2)/K1), and,
•  dN2/dt = r2N2((K2-N2-β N1)/K2)
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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9. Competition coefficients:
α is the effect on species 1 of species 2
(also written as α 12):
•  If α <1 then interspecific competition has less
impact than intraspecific competition.
•  If α >1 then interspecific competition has more
impact.
β  is the effect on species 2 of species 1
(also written as α 21)
(Note: in text, p 235, equations 8.5, 8.6 & 8.7 incorrectly
show α 21 instead of α 12)
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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10. Lotka-Volterra competition model zero isoclines:
•  These zero population growth isoclines, where dN/
dt = 0 are shown in graphs of N2 on the y-axis
plotted against N1 on the x-axis (Figs. 8.7 & 8.9),
•  When this is true for species 1, then,
r1N1(K1-N1-α N2) = 0, and K1-N1-α N2 = 0,
•  Therefore N1 = K1-α N2:
•  When N1 = 0, N2 = K1/α
•  The result of pure interspecific competition at A in Fig. 8.7a.
•  When N2 = 0, N1 = K
•  The result of pure intraspecific competition at B in Fig. 8.7a.
•  To give the zero isocline of Fig. 8.7a.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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11. Four outcomes of the LotkaVolterra competition model (Fig. 8.9):
•  (a) Species 1 wins (competitive exclusion):
•  Spp. 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 ).
•  (b) Species 2 wins (competitive exclusion):
•  Converse of (a)
•  (c) Either species 1 or species 2 wins:
•  Interspecific competition greater in both species than intraspecific
competition - the outcome depends on starting densities.
•  (d) Coexistence:
•  Both species have less competitive effect on the other species than they
do on themselves.
•  K1 > K2α and K2 > K1β - gives a stable equilibrium.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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Figure 7.18 (3rd ed.): Character displacement
in the seed-eating ant Veromessor pergandei.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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Figure 8.23: Character displacement in benthic (l)
and limnetic (m) three-spined stickleback species.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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Figure 8.25: Character displacement in the mud snails
Hydrobia ulvae (E) and H. ventrosa (J) in
(a) Denmark and (b) Finland.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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Figure 7.4 (3rd ed.): Asymmetric competition between the
cattails Typha latifolia and T. angustifolia when growing
together (a, c) or separately (b).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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Figure 8.7: Zero isoclines generated by the
Lotka-Volterra equations (a) N1, (b) N2.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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Figure 8.9:
Outcomes of
competition
generated by
the LotkaVolterra
competition
equations.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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Barnacles, July 2006, Kintyre, Scotland
BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition
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