BIOS 6150: Ecology Dr. Stephen Malcolm, Department of Biological Sciences • Week 5: Interspecific Competition: • Lecture summary: • Definition. Semibalanus balanoides • Examples. James P. Rowan, http://www.emature.com • Outcomes. • Lotka-Volterra model.

Chthamalus stellatus Alan J. Southward, http://www.marlin.ac.uk/ BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition 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”

BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition Slide - 2 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 Slide - 3 4. Gause's Paramecium species compete interspecifically (Fig. 8.3):

BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition Slide - 4 5. Tilman's diatoms exploitation/scramble (Fig. 8.5):

BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition Slide - 5 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 Slide - 6 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 Slide - 7 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 Slide - 8 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 Slide - 9 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 Slide - 10 11. Four outcomes of the Lotka- Volterra 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 Slide - 11 Figure 7.18 (3rd ed.): Character displacement in the seed-eating ant Veromessor pergandei.

BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition Slide - 12 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 Slide - 13 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 Slide - 14 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 Slide - 15 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 Slide - 16 Figure 8.9:

Outcomes of competition generated by the Lotka- Volterra competition equations.

BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition Slide - 17 Barnacles, July 2006, Kintyre, Scotland

BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition Slide - 18