BIOS 6150: Ecology Dr. Stephen Malcolm, Department of Biological Sciences Week 5: Interspecific Competition: Lecture summary: 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 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: N 1 & N 2 are the population sizes of species 1 & 2 r 1 & r 2 are the intrinsic rates of natural increase for spp 1 & 2 K 1 & K 2 are the carrying capacities for species 1 & 2. With competition coefficients α and β population size changes for the two competing species are: dn 1 /dt = r 1 N 1 ((K 1 -N 1 -α N 2 )/K 1 ), and, dn 2 /dt = r 2 N 2 ((K 2 -N 2 -β N 1 )/K 2 ) 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 N 2 on the y-axis plotted against N 1 on the x-axis (Figs. 8.7 & 8.9), When this is true for species 1, then, r 1 N 1 (K 1 -N 1 -α N 2 ) = 0, and K 1 -N 1 -α N 2 = 0, Therefore N 1 = K 1 -α N 2 : When N 1 = 0, N 2 = K 1 /α The result of pure interspecific competition at A in Fig. 8.7a. When N 2 = 0, N 1 = 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 (K 1 >K 2 /β therefore K 1 β >K 2 ), Even though intraspecific competition within species 1 is stronger than the interspecific effect of species 2 (K 1 /α > K 2 therefore K 1 > K 2 α ). (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. K 1 > K 2 α and K 2 > K 1 β - gives a stable equilibrium. BIOS 6150: Ecology - Dr. S. Malcolm. Week 5: Interspecific Competition Slide - 11
Figure 7.18 (3 rd 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 (3 rd 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