BIO226 : Predation/Competition : Competition : Lesson

Glossary terms that are important in this lesson: Allelopathy, asymmetry, competition coefficient, competitive coexistence, competitive exclusion, equilibrium, exploitation competition, interference competition, interspecific competition, intraspecific competition, isocline, keystone predator, Liebig's law of the minimum, limiting factors, Lotka-Volterra equation, parasitoid.

Competition occurs when individuals share a resource which is in short supply. Sometimes competition is a head-to-head confrontation, but more often one individual uses a resource before another individual gets to it and deprives the other of something it needs.

I. Competition

1. Introduction
   • Definition
   • Intraspecific competition
     • Reduces resources in a density-dependent manner: affects fecundity & survival
     • Evolutionary change when efficiency of resource use is genetically based
   • Interspecific competition
     • Competitive coexistence: both populations share some resources
     • Competitive exclusion: one population grows at a resource level which curtails the growth of the second

2. Resources
   • Definition
     • Substance or factor that is consumed by an organism
     • Availability of substance or factor affects population growth rates
     • Examples: food, water, space, resting sites, refuges
   • Conditions
     • Influence rates at which organisms consume resources
     • Cannot be consumed
     • Examples: temperature, pH, humidity, salinity, viscosity, buoyancy
   • Types
     • Renewable
       • Originates outside the system; consumers do not influence supply: sunlight
       • Source inside system; direct influence of consumers on supply: predator-prey
       • Source inside system; indirect influence of consumers on supply: part of a cycle
     • Nonrenewable
       • Occupied but not consumed: space
       • Consumed and not replaced: fossil fuels

Limiting resources (Justus von) Liebig's law of the minimum: population is restricted by the resource which is most limiting Limiting factors A. A single factor is limiting when growth is directly proportional to the supply of that factor B. Factors interact synergistically when more than one factor simultaneously restrict growth C. The population is saturated with this factor when it does not respond to changes in supply D. The population is inhibited when there is too much of the resource Examples: Cyclotella meneghiniana, Impatiens parviflora

4. Early Experiments with Competition
   • Species grown separately and together
     • Paramecium aurelia and P. caudatum (G.F. Gause)
     • Tribolium castaneum and T. confusum (Thomas Park)
   • Physical conditions are critical to the outcome
   • Relative performance of the two species under the same conditions determines the outcome
   • Direction of competition can change if physical conditions change
   • Competitive exclusion principle:
     • Two species cannot coexist on the same limiting resource
     • A resource is limiting when it is shared and in short supply

II. Theory of Competition

1. Intraspecific competition represented by the logistic equation
   • Carrying capacity (K) is the density of the population supported by the environment at equilibrium
   • Intrinsic growth rate (r) is the maximum growth rate under ideal conditions
   • Population density (N) varies between zero and K
   • Change in population density (dN/dt) is least when N approaches K

2. Interspecific Competition: Lotka-Volterra equation, modified logistic
   • Density of a second population (N_j)
   • Competition coefficient (a_ij) represents the effect of each individual of the second population on taking the resources used by the first population
   • Change in the density of the first population is affected by the density of the first population AND the density of the second population

Equilibrium Conditions Equilibrium occurs when dN/dt = 0 Then Ni = Ki - aijNj  (Note that, when dN/dt = 0, the differential equation simplifies to an equation for a straight line.) Ni = Ki when Nj = 0 (intercept on the x-axis) Nj = Ki/aij when Ni = 0 (intercept on the y-axis) Connect the two intercepts to obtain the equilibrium line for Population i Population i will grow to a density somewhere on its equilibrium line (its isocline)

4. Predictions
   • Population i will win if K_i > K_j/a_ji and K_j < K_i/a_ij
   • Populations will coexist if K_i < K_j/a_ji and K_j < K_i/a_ij
   • Coexistence occurs when competition coefficients are weak, i.e., when a_ija_ji < 1

III. Competition in Nature

1. Parasitoids
   • Braconid wasps of the genus Opius in Hawaii
   • Red and yellow scale insects in California citrus groves
   • Wasps of the genus Aphytis on scale insects on California citrus groves

2. Plants
   • Desmodium glutinosum and D. nudiflorum: growth was depressed by both intraspecific and interspecific competition but effects were asymmetrical
   • Vaccinium uliginosum, V. vitis-idaea, and Empetrum nigrum: negative effect on close competitors, but positive effect on E. nigrum

3. Nutrients
   • Hypothesis 1: Intensity of competition is greater when resources are scarce
   • Hypothesis 2: When resources are scarce, abiotic factors limit populations resulting in lower intensity of biotic interactions
   • Stipa capensis (desert annual in Israel) showed that competition intensity increased as resources became more abundant
   • Prairie grasses presented a hazy picture

4. Barnacles, voles, and lizards
   • Chthamalus stellatus and Balanus balanoides: physical aggression and physiological tolerance
   • Microtus pennsylvanicus and M. montanus: exclusion by aggressive behavior
   • Sceloporus merriami and Urosaurus ornatus: competition and physical factors

IV. Mechanisms of Competition

1. Classification
   • Exploitation competition: some individuals consume resources before other individuals do; expressed through differential survival and reproduction
   • Interference competition: some individuals directly inhibit others; interference competition is relatively frequent within species
     • Allelopathy: Salvia (sage) use chemicals which inhibits the growth of other vegetation

2. Asymmetry: response to competition by only one species
   • Chthamalus and Balanus
   • Predators and herbivores often involved
   • Patch reefs in the Caribbean Sea
     • Habitat of reef flat, reef slope and sand plain
     • Asymmetrical competition resulted when species that shared one resource (e.g. space or food) were differently specialized for other aspects of the environment (e.g. physical conditions or consumers)

3. Distantly related species
   • Sharing resources provides potential for competition

4. Influence of predation: keystone predator shapes the structure of biological communities
   • High diversity of grassland plant communities are maintained by grazing animals which prefer to eat superior competitors
   • Rocky coast of Washington: Sea star (Pisaster) predators limit populations of superior competitors resulting in high diversity of prey
   • Predatory newts preferred rapidly growing Scaphiopus tadpoles, resulting in survival of the inferior competitors as well as more food and better growth for all survivors

When you have completed this lesson, go on to Review Questions