2. Interspecific Competitionhomepages.wmich.edu/~malcolm/BIOS3010-ecology/... · BIOS 3010: Ecology...

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Dr. S. Malcolm BIOS 3010: Ecology Lecture 6: slide 1

BIOS 3010: Ecology Lecture 6: Processes: Interspecific competition

•  Lecture summary: – Definition &

examples. – Lotka-Volterra

model: •  Structure. •  Competition

coefficients. •  Predicted outcomes.

James P. Rowan, http://www.emature.com

Semibalanus balanoides

Chthamalus stellatus

Alan J. Southward, http://www.marlin.ac.uk/


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!


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.

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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)


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.


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

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7. Four outcomes of the Lotka- Volterra 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)


Figure 8.2: Intertidal distribution of two barnacle species


Figure 8.3: Intra- and interspecific competition in Paramecium spp.

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Figure 8.5: Competition between diatoms & silicate availability


Figure 7.18 (3rd ed.): Character displacement (mandible variation) in ant communities

Data for Veromessor pergandei


Figure 8.23: Character displacement (gill rakers) in sticklebacks

benthic limnetic

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Figure 8.25a: Character displacement of freshwater snail shell lengths

Hydrobia ulvae

Hydrobia ventrosa


Figure 7.4 (3rd ed.): Asymmetrical competition between cattail species in Michigan


Figure 8.7: Zero isoclines of the Lotka-Volterra competition equations for species N1 and N2

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Figure 8.9: Lotka-Volterra competition model outcomes


Barnacles, July 2006, Kintyre, Scotland

2. Interspecific Competitionhomepages.wmich.edu/~malcolm/BIOS3010-ecology/... · BIOS 3010: Ecology...

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