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BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 1
BIOS 6150: Ecology Dr. Stephen Malcolm, Department of Biological Sciences • Week 3: Intraspecific Competition. • Lecture summary:
• Results in a negative outcome for all competitors.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 3
3. Definition of Competition:
• “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” (Begon et al., 2006, p. 132) • The ultimate effect of competition on an individual is a
decreased fitness contribution to the next generation (fewer offspring) compared with what would have happened had there been no competitors.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 4
4. Four characteristics of intraspecific competition:
• (1) Decrease in fitness: • The ultimate effect of competition is a decrease in the fitness of all
interactants (thus it is a “-,-” interaction): • Often via decreased survivorship or fecundity. • Fitness reduction must be measurable to conclude that competition occurred.
• (2) Limited supply of resources: • The resource for which individuals compete must be in limited supply.
• (3) Reciprocity: • Even if the detectable competition is either mostly one-sided, or
balanced, it must be reciprocal and have a negative impact on both interactants (symmetrical and asymmetrical).
• (4) Density dependence: • The probability of an individual being adversely affected increases with
increasing competitor density (in contrast to density independent effects).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 5
5. Extremes of intraspecific competition:
• Scramble (exploitation) and contest (interference) competition were first described as simplistic extremes by Nicholson (1954) in Australia. • Mortality (% or kcompetition due to competition) or
survivorship is plotted against logarithm of initial density to show degree of density dependence.
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6. Scramble & Contest Competition:
• Scramble competition: • Nicholson described scramble competition for dungflies
competing for the limited resources of cow feces. • Each member gathers a constant amount of resource at all densities.
Thus at high density there is insufficient resource and the whole population dies (slope b = ∞).
• Contest competition: • Where individuals of the population interfere or contest with
each others abilities to harvest resources, some survive. • Exact density-dependent compensation is thus described by a
mortality slope b = 1. • Figs. 5.1 & 5.2 from Begon et al. (2006) to show both kinds and
negative effects of competition in single species populations.
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7. Density dependence:
• Competition also increases with population density when mortality may increase or survivorship may decrease (Fig. 5.2). • The nature of density dependence can also
change with increasing density from: • density independence, through, • undercompensating density dependence, to, • exactly compensating density dependence, to, • overcompensating density dependence
(Figs 5.3, 5.4 & 6.5).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 8
8. Density dependence (continued):
• So intensities of both kinds of intraspecific competition increase with population density and change from density independence to density dependence. • Thus density dependent birth and mortality rates lead to
the regulation of population size at a stable equilibrium where births = deaths.
• This is the carrying capacity (K) at the population size sustainable by available resources as shown in Figs. 5.7 & 5.8.
• Density dependent population regulation generates the sigmoidal or S-shaped curve characteristic of intraspecific competition (see Fig 5.11).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 9
9. Density dependent growth:
• In addition to effects on numbers, competition also negatively influences growth: • This in turn influences numbers through reduced per capita
reproductive output.
• Rates of growth and rates of development can be reduced as shown in Figs 6.14 & 5.12: • But the total population biomass can remain the same, despite
individuals being smaller: • The “law of constant final yield” (exact compensation)
• Reproductive allocation can also shift with changing resource availability (Figs. 6.16 & 5.15): • Within genets, tiller growth was less variable and more regulated
than the genets themselves.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 10
10. k-values and density dependent mortality:
• k-values of mortality due to competition can define competition according to the slope b of the relationship (Fig. 5.16) of kcompetition plotted against the logarithm of initial density (density before the effects of competition).
• b = 0 density independence. • b < 1 undercompensating density dependence. • b = 1 (contest) exact density dependent compensation. • b > 1 overcompensating density dependence • b = ∞ (scramble) overcompensating density dependence
• see Fig. 2-3 from Hassell (1976) of scramble and contest competition. • k-mortality is shown in Fig 5.16 & k-fecundity in Fig. 6.20.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 11
11. Discrete breeding season model of intraspecific competition:
• Using: • R net reproductive rate • Nt population size at time t • Nt+1 population size at time t+1
• In the absence of competition, the model describes population increase simply as:
• Nt+1 = NtR and • Nt = NoRt
• This gives the exponential population growth across discrete
generations as in Fig. 5.18.
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12. Carrying capacity, limited resources and the effect of competition:
• At high density when the ratio of Nt/Nt+1 = 1 this is by definition the carrying capacity K.
• So in the presence of competition, the population rises to K as shown in Fig. 5.18. • according to:
• Nt+1 = NtR/1+(aNt) • where a = (R-1)/K
• so the unrealistic R in the first equation is now replaced by the more realistic R/(1 + aNt)
• as a and Nt increase so does the effect of competition and R is decreased.
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13. Density dependence of the model:
• The k-value for mortality due to competition is thus the difference between log NtR and log NtR/(1+aNt) and plotting these k values against log10Nt (Fig. 5.20) shows that the model exactly compensates with a slope b = 1.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 14
14. Incorporation of variable density dependence with b:
• A more realistic model of competition that incorporates a range of competitive regulation was derived by Maynard Smith & Slatkin (1973) in which they simply added the slope b of the k-value plotted against log initial density:
• Nt+1 = NtR/1+(aNt)b --- equation 5.18 (p.148) • in which b is the slope of mortality (k) against
population size (log10Nt) and, • a substitutes for (R-1)/K as before
(see Figs. 5.21 & 6.26 for real data). • Also generates realistic ranges of population
fluctuations (Fig. 5.22).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 15
15. Continuous breeding - the logistic equation:
• The model above was for discrete time steps described by a “difference” equation.
• For continuously breeding populations (birth and death continuous) we need a continuous form of the model using a “differential” equation. • So for exponential population increase the rate of population
increase is dN/dt and this speed of change is described in the absence of competition by:
• dN/dt = rN • where r is the intrinsic rate of natural increase which is lnR or lnRo/T
• So the continuous equivalent to Fig. 5.18 is shown in Fig. 5.23 and this is the differential form of the difference equation Nt = NoRt
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 16
16. Logistic limitation to a carrying capacity:
• The differential form of Nt+1 = NtR/1+(aNt) in Fig 5.18 is given by:
dN/dt = rN((K - N)/K) • This is the famous logistic equation. • This shows that exponential increase is
decreased to K by the logistic term (K - N)/K
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 17
17. Asymmetrical competition:
• Large vs small Impatiens in a woodland (Fig. 5.26): • Small plants did not grow and so the asymmetry
increased with time. • Root vs shoot competition in morning glory
Fig. 5.27 (Weiner expt.): • Root competition for nutrients resulted in most
biomass reduction, but shoot competition for light generated most size inequality and increase in asymmetry.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 18
Figure 5.1: Intraspecific competition among cave beetles for cave cricket eggs (a) scramble or exploitation, (b) contest or interference.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 19
Figure 5.2: Survivorship of red deer on the island of Rhum declines with lower birth rate and increased density.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 20
Figure 5.3:
Density dependent mortality in flour beetles changes from (1) density independence, to (2) undercompensating density dependence, to (3) overcompensating density dependence.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 21
Figure 5.4: Exact compensation in trout fry.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 22
Figure 6.5 (3rd ed.): Density dependent mortality in soybeans leading to overcompensation with time.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 23
Figure 5.7: Density dependent birth and mortality rates.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 24
Figure 5.8:
Differences between births and deaths (a), generate recruitment (b), and population increase to a carrying capacity (c).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 25
Figure 5.11:
Examples of S-shaped population increase for (a) Rhizopertha beetles on wheat, (b) wildebeest after a rinderpest outbreak, and (c)
willows after myxomatosis killed rabbit herbivores.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 26
Figure 6.14 (3rd ed.):
Effects of density on growth rate and size in (a) Rana tigrina frogs and (b) reindeer.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 27
Figure 5.12: Effects of intraspecific competition on growth and final biomass of populations of the limpet Patella cochlear.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 28
Figure 6.16 (3rd ed. – see Fig. 5.14, 4th ed.):
“Constant final yield” of plants sown at a range of densities for (a) subterranean clover, (b, c) the dune annual Vulpia fasciculata.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 29
Figure 5.15: Intraspecific competition in rye grass regulates the number of modules (tillers).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 30
Figure 5.16:
k-values to describe variable density dependent mortality in (a) a dune annual, (b) almond moth, (c) fruit fly, and (d) the moth Plodia interpunctella.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 31
Figure 2.3: (Hassell, 1976)
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 32
Figure 6.20, 3rd ed., see Fig. 5.17, 4th ed.):
k-values to describe density dependent reductions in fecundity in (a) limpets, (b) cabbage root fly, (c) grass mirid, and (d) plantain.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 33
Figure 5.18: Difference equation model to describe population increase in species with discrete generations.
11
12
16
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 34
Figure 5.21: Different intensities of intraspecific competition incorporated in equation 6.19.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 35
Figure 6.26 (3rd ed.):
Equation 5.18 fitted to data for different beetle species in the laboratory (a, b, c & e), and winter moths in the field (d).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 36
Figure 5.22: Range of population fluctuations for (a) values of b and R and (b) population size against time, generated by equation 6.19.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 37
Figure 5.23: Exponential and sigmoidal models of population increase against time for continuous breeding - the logistic model of population growth.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 38
Figure 5.26:
Asymmetric competition in the woodland plant Impatiens pallida in SE Pennsylvania.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 3: Intraspecific Competition Slide - 39
Figure 5.27: Root vs shoot competition in morning glory vines, Ipomoea tricolor.