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Ecology Interspecific Competition and the Niche

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Unformatted text preview: Interspecific Competition and the Interspecific Competition and the Niche The niche concept can be traced to an American, Joseph Grinnell, and to the English ecologist, Charles Elton. The Niche The Niche The Eltonian niche is based on the trophic position of an animal. Grinnell stressed the niche in terms of distribution over habitat types. Interspecific Competition and the Interspecific Competition and the Niche Grinnell also connected competitive exclusion with the niche: “It is axiomatic that no two species regularly established in the same fauna have the same niche relationships.” Interspecific Competition and the Interspecific Competition and the Niche Various authors have used the term niche differently resulting in informal versus formal definitions. Interspecific Competition and the Interspecific Competition and the Niche Informal: “The role or profession of an organism in the environment” (Krebs 1994). Informal: “The ecological role of a species in the community” (Ricklefs 1997). Interspecific Competition and the Interspecific Competition and the Niche Formal: “The limits, for all environmental features, within which individuals of a species can survive, grow and reproduce” (Begon et al. 1986). Interspecific Competition and the Interspecific Competition and the Niche Formal: “The ranges of many conditions and resource qualities within which the organism persists, often conceived as a multidimensional space” (Ricklefs 1997). Interspecific Competition and the Interspecific Competition and the Niche Formal definition comes from G.E. Hutchinson: the niche is conceived of as an N­dimensional hypervolume. Interspecific Competition and the Interspecific Competition and the Niche Consider important environmental features: Temperature, food sizes and types, pH, nutrient availability, And many other things. Interspecific Competition and the Interspecific Competition and the Niche Each can be a niche dimension: 1. For example, temperature: Minimum versus maximum temperatures within which a species can survive? Or pH: Minimum and maximum pH for survival? 3. Food sizes or types? 2. Interspecific Competition and the Interspecific Competition and the Niche Each is a niche dimension. Picture a two dimensional niche, based on temperature and pH minimum and maximum values. Two Dimensional Niche Two Dimensional Niche 14 pH 0 0o C Temperature 30o C Niche Dimensions Niche Dimensions Now add a third dimension, food sizes. You now have a 3­dimensional polygon Three Dimensional Niche Three Dimensional Niche Food Sizes 14 pH 0 0o C 30o C Temperature Interspecific Competition and the Interspecific Competition and the Niche This volume is the fundamental niche of this species for those 3 niche dimensions. We can continue to add other niche dimensions, as many as N­dimensions. Interspecific Competition and the Interspecific Competition and the Niche Thus, Hutchinson represented the fundamental niche as an N­dimensional hypervolume. Interspecific Competition and the Interspecific Competition and the Niche The fundamental niche is the largest ecological niche that an organism or species can occupy. It is based on interactions with the environment, but is in the absence of competitive interactions. By contrast, the realized niche is the niche after competitive interactions Interspecific Competition and the Interspecific Competition and the Niche The realized niche is the portion of the fundamental niche occupied after interactions with other species. The realized niche is part of, but smaller than, the fundamental niche. Assume Species 2 is the superior competitor; the Assume Species 2 is the superior competitor; the fundamental niche is the rectangle for species 1, but only the yellow area is the realized niche. 14 pH Species 1 Species 2 0 0o C Temperature 30o C Interspecific Competition and the Interspecific Competition and the Niche Niche breadth is the distance through a niche along a particular dimension in niche space. Niche overlap is the joint use of a resource or niche dimension by two or more species Niche Breadth versus Niche Overlap Niche Breadth versus Niche Overlap Pink is Niche Overlap 14 pH Species 1 Species 2 0 0o C Temperature 30o C Interspecific Competition and the Interspecific Competition and the Niche When comparing fundamental niches, we find that overlap occurs along all niche axes. How closely can niches overlap before competitive exclusion occurs? Competitive exclusion is observed most often between closely related species, which means species whose niches are very similar. Interspecific Competition and the Interspecific Competition and the Niche Species with very similar niches are most likely those unable to coexist. This lead to the “bottle” experiments of Gause and others. Gause’s Paramecium Experiments Number per 0.5 ml sample 600 400 P. aurelia P. caudatum 200 0 0 2 4 6 Time in days 8 10 12 Gause’s Paramecium Experiments Number per 0.5ml sample 400 300 P. aurelia P. caudatum 200 100 0 0 2 4 6 Time in days 8 10 12 (Birch 1953) (Birch 1953) Interspecific Competition and the Interspecific Competition and the Niche Gause proposed: “Two species cannot exist unless they are doing things differently.” Rephrased as: “No two species can occupy the same ecological niche.” Interspecific Competition and the Interspecific Competition and the Niche Finally, after Hutchinson proposed his idea of the fundamental niche as an N­dimensional hypervolume, and the idea of the realized niche as the post­competitive niche, Garrett Hardin proposed the Competitive Exclusion Principle. The Competitive Exclusion Principle The Competitive Exclusion Principle “Species which are complete competitors, that is whose niches overlap completely, cannot coexist indefinitely.” Interspecific Competition and the Interspecific Competition and the Niche At about the same time, that eminent ecologist, Dr. Seuss, waxed poetic about the competitive exclusion principle in his book, On Beyond Zebra! The Lotka­Volterra Competition The Lotka­Volterra Competition Equations Attempts have been made to model competition. The most famous are the early equations (1926) by Lotka and Volterra. They simply modeled competition as a decrease in the growth rate of a species when its competitor is present. The Lotka­Volterra Competition The Lotka­Volterra Competition Equations They used the logistic equation, which is already an equation that models intraspecific competition. By adding the second species the equations become a model of intra­ plus interspecific competition. The Lotka­Volterra Competition The Lotka­Volterra Competition Equations In these equations we have the following: N1 = Number of individuals of species 1 N2 = Number of individuals of species 2 r1 = intrinsic rate of increase of species 1 r2 = intrinsic rate of increase of species 2 K1 = carrying capacity of species 1 K2 = carrying capacity of species 2 The Lotka­Volterra Competition The Lotka­Volterra Competition Equations α12 = the competition coefficient, the effect of species 2 on species 1 α21 = the competition coefficient, the effect of species 1 on species 2 t = time The Lotka­Volterra Competition The Lotka­Volterra Competition Equations K1 − α1,1 N1 − α1, 2 N 2 dN1 / dt = r1 N1 K1 Since α11 =1.0, this is usually written as: K1 − N1 − α 1, 2 N 2 dN1 / dt = r1 N1 K1 The Lotka­Volterra Competition The Lotka­Volterra Competition Equations K 2 − α 2, 2 N 2 − α 2,1 N1 dN 2 / dt = r2 N 2 K2 K 2 − N 2 − α 2,1 N1 dN 2 / dt = r2 N 2 K2 Final Traditional Version Final Traditional Version K1 − N1 − α 1, 2 N 2 dN1 / dt = r1 N1 K1 K 2 − N 2 − α 2,1 N1 dN 2 / dt = r2 N 2 K2 The Lotka­Volterra Competition The Lotka­Volterra Competition Equations The usual value of the interspecific competition coefficient is: 0 < α < 1 Why? The Lotka­Volterra Competition The Lotka­Volterra Competition Equations An α = 0 would mean no competition; no niche An overlap. overlap. An α < 0 would imply mutualism. An An α = 1.0 is the implied value for intraspecific An competition. competition. The Lotka­Volterra Competition The Lotka­Volterra Competition Equations Since niche overlap with your own species is Since much greater than with other species, and the alpha value for intraspecific competition is 1.0, the alpha for interspecific competition should interspecific be less than one. be The Lotka­Volterra Competition The Lotka­Volterra Competition Equations Since α11 = 1.0, Since 0 < α12< 1 0 < The Lotka­Volterra Competition The Lotka­Volterra Competition Equations: Solutions? To solve the L­V equations one would need to estimate all of the K and r values. Then to get the alphas you would need to run a series of experiments. Therefore, we do not have an independent value of alpha. The Lotka­Volterra Competition The Lotka­Volterra Competition Equations One way to deal with these differential equations is to do an “equilibrium” analysis. By this we mean we set the differential equations equal to zero. Equilibrium Analysis Equilibrium Analysis K1 − N1 − α1, 2 N 2 dN1 / dt = 0 = r1 N1 K1 K 2 − N 2 − α 2,1 N1 dN 2 / dt = 0 = r2 N 2 K2 Equilibrium Analysis Equilibrium Analysis Uninteresting possible solutions: 1. r1 or r2 = 0 would mean one or both species is not viable in this environment. 2. N1 or N2 = 0 would mean competitive exclusion or one or both species is simply not present. 3. K1 or K2 = 0 is undefined for one or both species and they are not viable in this environment. Equilibrium Analysis Equilibrium Analysis This leaves us with: 0 = K 1 − N 1 − α 1 ,2 N 2 0 = K 2 − N 2 − α 2 ,1 N 1 Equilibrium Analysis Equilibrium Analysis A. A graphical analysis of these equations produced the following four possible solutions, depending on the values of K and alpha. Deterministic results: Species one always wins (competitive exclusion). Equilibrium Analysis Equilibrium Analysis B. Species two always wins (competitive exclusion). C. The two species always coexist (coexistence; no competitive exclusion). Equilibrium Analysis Equilibrium Analysis D. Stochastic Results: There is a possibility of coexistence, but it is unstable. Competitive exclusion is the most likely outcome. But it is not predictable which species will win. The result depends on initial conditions. Park’s Beetle Experiments Park’s Beetle Experiments Tempo C % “Climate” Relative Percent of replicate experiments in Humidity which one species wins Tribolium Tribolium confusum 0 castaneum 100 34 70 Hot­moist 34 30 Hot­Dry 90 10 29 70 Warm­Moist 14 86 29 30 Warm­Dry 87 13 24 70 Cold­Moist 71 29 24 30 Cold­Dry 100 0 Resource Based Competition Resource Based Competition David Tilman (1976, 1987) and others pointed out that the Lotka­Volterra equations were “phenomenological” and not “mechanistic”. Resource Based Competition Resource Based Competition That is, competition coefficients are merely measures of the effect of one species on the growth rate of another. They are estimated from experiments in which two species are grown together. Resource Based Competition Resource Based Competition Therefore they are not independently derived values that allow one to predict coexistence or competitive exclusion, or in the latter case, which of two species should win. They merely describe the “phenomenon” of competition. Resource Based Competition Resource Based Competition Furthermore, a competition coefficient does not help determine the mechanism of competition; we have no information on what resource the species might be competing for. Resource Based Competition Resource Based Competition If competition is really concerned with a resource in short supply, we need to understand what the resource is and how each species is using it before we can understand the potential competitive interaction. Resource Based Competition Resource Based Competition Tilman was a particularly strong advocate of a mechanistic approach to competition. He developed what is now known as resource based competition theory. In so doing he brought together ideas from a variety of disciplines, including microbiology, enzyme kinetics and agricultural chemistry. Resource Based Competition Resource Based Competition For example, the idea that population growth is constrained by the depletion of critical resources can be traced to the agricultural chemist Liebig (1840) and his law of the minimum. Liebig’s Law of the Minimum (1855) Liebig’s Law of the Minimum (1855) “Under steady state conditions, the population size of a species is constrained by whatever resource is in shortest supply.” Resource Based Competition Resource Based Competition In the resource based approach to competition, we need to couple the availability of resources to population growth. Tilman and others introduced the concept of R*. R* is the level of the resource needed to balance mortality. If the resource is provided at the rate R*, then dN/dt = 0, growth just balances mortality, and the population maintains itself. Resource Based Competition Resource Based Competition Tilman (1976) realized that an independently derived R* could be used to predict population dynamics and, ultimately, competitive interactions. Resource Based Competition Resource Based Competition Tilman first used the Michaelis­Menton enzyme kinetics equation, which is normally employed to describe the relationship between cellular metabolism and substrate concentrations. Resource Based Competition Resource Based Competition This equation had also been used by microbiologists to describe the growth rate of bacteria on organic substrates (Monod 1950). R µ = µ max R + Kµ Resource Based Competition Resource Based Competition Where: μ = the growth rate μmax= the maximum growth rate R = the concentration of the resource Kμ = the concentration of the resource producing half maximal growth rate. That is, the “half saturation” constant. Resource Based Competition Resource Based Competition We can replace μ by dN/dt and μmax by bN where b is the maximum birth rate. If we add a mortality rate we have: bNR dN / dt = −m Ki + R Resource Based Competition Resource Based Competition The per capita rate is found by dividing both sides by N: bR dN / Ndt = −m Ki + R Resource Based Competition Resource Based Competition Setting the per capita rate = 0 and solving for R* we have the following equation, where r, the intrinsic rate of increase, equals b – m. mK i R* = = mKi / r b−m The R* ­ Rule The R* ­ Rule According to the R*­rule: 1. 2. 3. For a given resource, three parameters determine which species will win in competition. These are: r the intrinsic rate of increase m the mortality rate Ki the half saturation constant for the given resource, i The R* ­ Rule The R* ­ Rule When two species are competing for the same resource, the species with the lower R* value will win. This species can exist with a lower amount of the resource, while the other species declines in population. Per capita growth Per capita growth as a function of Per capita growth as a function of resource availability. Resource supply Per capita growth Per capita growth as a function of resource availability, but with a constant mortality rate. R* = the amount of resource producing a per capita growth rate of zero. Population growth Mortality rate R* Resource supply Population and Resource Dynamics Population and Resource Dynamics R* Population size Resource Time R* = Kim/r R* = K Population Ki g/l X 10­6 m/hr b/hr r = (b–m)/hr R* g/l X 10­6 1 4.0 0.05 0.25 0.20 1.00 2 4.1 0.05 0.50 0.45 0.46 3 6.5 0.05 0.50 0.45 0.72 4 20.0 0.05 0.25 0.20 5.00 Hanson and Hubble (1980) in Science on 4 strains of bacteria The R* Rule The R* Rule In this example, population 2 will competitively displace the other three populations, 3 will outcompete 1 and 4, and 1 will defeat 4. R* Values for 3 Populations and Three R* Values for 3 Populations and Three Resources Population Resource 1 Resource 2 Resource 3 1 2.5 1.0 3.0 2 5.0 5.0 2.0 3 1.0 2.5 5.0 If only one resource is in short supply: If only one resource is in short supply: competitive exclusion Population Resource 1 Resource 2 Resource 3 1 2.5 1.0 3.0 2 5.0 5.0 2.0 3 1.0 2.5 5.0 Resources 1, 2 and 3 simultaneously in Resources 1, 2 and 3 simultaneously in short supply: result is coexistence Population Resource 1 Resource 2 Resource 3 1 2.5 1.0 3.0 2 5.0 5.0 2.0 3 1.0 2.5 5.0 Field Studies of Competition Field Studies of Competition The classic study on competition in the field was done by Joseph Connell (1961). He studied the distribution of two species of barnacles found in the intertidal zone. He observed that most intertidal zones show a striking vertical zonation. High Spring Tide High Neap Tide Low Neap Tide Low Spring Tide Field Studies of Competition Field Studies of Competition For the two species of barnacles of interest, there is a great deal of overlap in the portions of the intertidal where the larval stages settle. But the adults have a completely non­ overlapping distribution. Field Studies of Competition Field Studies of Competition Semibalanus (previously Balanus) balanoides occupies most of the intertidal zone from the high water neap tide to the low water spring tide. Chthalamus stellatus is only found from the high water neap to the high water spring tide zones. Semibalanus balanoides Chthamalus Semibalanus balanoides Chthamalus stellatus Field Studies of Competition Field Studies of Competition What explains this non­overlapping distribution of adults? 1. Differences in fundamental niches? 2. Interspecific competition? 3. Both? Field Studies of Competition Field Studies of Competition 1. The answer is: Both. Competition. In all areas below the mean high neap tide zone, Semibalanus is the superior competitor. It either overgrows or undercuts Chthalamus, eliminating it. Field Studies of Competition Field Studies of Competition 2. But Semibalanus is unable to survive the long hours exposed to drying winds and sun in the zone between high neap and high spring tide zones. 3. The fundamental niche of Semibalanus does not include that zone. Field Studies of Competition Field Studies of Competition For Chthalamus, however, the fundamental niche includes the entire intertidal zone. When exposed to competition with Semibalanus, however, the realized niche is only the high tide zone between the neap and spring tides. Field Studies of Competition Field Studies of Competition Second example: crayfish in the genus Orconectes. O. immunis is the “pond or papershell” crayfish. O. virilis is the “stream” crayfish. Why? Typically where they are found. Field Studies of Competition Field Studies of Competition O. immunis, the papershell crayfish, occurs widely in the prairie region and along the floodplains of the Mississippi and Missouri rivers. It is almost always found over a mud bottom in turbid waters that fluctuate drastically in area and depth. Field Studies of Competition Field Studies of Competition Typical habitats used by O. immunis are shallow sloughs and the isolated pools of prairie creeks. This crayfish retreats to burrows in late summer as the habitats in which it occurs dry up. Field Studies of Competition Field Studies of Competition The native range of the northern crayfish, O. virilis, encompasses all of the prairie region and a band of streams along the northern and western border of the Ozarks. In the prairie region this crayfish is very abundant rocky streams. O. immunis O. immunis O. virilis Field Studies of Competition Field Studies of Competition However, field and laboratory studies have found that the “pond crayfish” actually prefers stream habitats. On the other hand, the “stream crayfish” cannot actually live in ponds. Field Studies of Competition Field Studies of Competition Differences in substrate (gravel versus mud) and oxygen content (high versus low for ponds versus streams) determine who lives where. Field Studies of Competition Field Studies of Competition Both species prefer gravel and high oxygen content of streams. However, O. v...
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