6a. Interspecific Competition

mutualism: interactions that benefit both populations

commensalism: one species benefits and the other is neither benefited or harmed (epiphytes)

predation: killing and consuming of prey

parasitism: organism (parasite) lives or on another (host) from which it obtains food

interspecific competition: adverse effects on both populations

ecological character displacement: unusual dissimilarity among sympatric species relative to allopatric, in features such as body size or resource utilization

ecological compression: when a species successfully colonizes a new area, realized niche of some already established species is restricted further

ecological release: species realized niche expands in absence of competitors

  • do not exist alone in nature
  • interspecific competition develops when a resource needed by both populations is in limited supply
  • individuals within a population experience intraspecific competition = with members of their own species = notion of K in logistic growth
  • but individuals also experience interspecific competition = with other species
    • to greater or lesser extent

realized niche: niche without considering competitors

fundamental niche: niche considering competitors

Lotka-Volterra

  • model was derived independently by Lotka and Volterra
  • describes a situation in which there is non predatory competition between two species for a limited resource
  • based on logistic growth
  • two species living in the same space affect each other
  • one species does not equal the other species, we need a conversion factor
  • alpha = conversion factor for expressing species 2 in terms of species 1
  • beta = conversion factor for expressing species 1 in terms of species 2
  • all the space for species 1 is used up when there
    • K1 individuals of species 1
    • K1/alpha individuals of species
    • a combination of species 1 and 2 that puts species 1 on its zero isocline = not growing
  • only three outcomes are possible
    • both species coexist
    • species 1 becomes extinct
    • species 2 becomes extinct
  • in a situation where neither species has a very strong effect on the other, we might expect them to coexist
    • these ideas can be explored mathematically or graphically
    • superimose both zero isoclines and add vector arrows
    • four possible outcomes
N1 wins
  • no equilibrium because species 1 is able to increase in a zone where species 2 other must decline
  • leads to extinction of species 2
N2 wins
  • no equilibrium because species 2 is able to increase in a zone where species 1 must decline
  • leads to extinction of species 1
Stable Equilibrium
  • if diagonal isoclines cross, their equilibrium point may result in a stable equilibrium
    • vector lines directed towards intersection point
Unstable equilibrium
  • in this case, the equilibrium is unstable
    • sensitive to initial numbers of both species
    • mutual non-invadability
    • vector arrows point away from the equilibrium point

Natural Populations

  • Elton (1927)
    • The niche of an animal means its place in the biotic environment - role of animals in the environment
    • ex. large carnivore niche
    • ex. arboreal herbivore
  • Niche (2008)
    • range of conditions necessary for persistence of the species, and its ecological role in the ecosystem
  • Gause (1934)
    • as a result of competition, two similar species scarcely ever occupy really similar niches
    • for two species to coexist, they must possess ecological differences
    • Lotka-Volterra: too much niche overlap = extinction of one species

Competitive Exclusion Principle

  • Competitive exclusion principle: complete competitors cannot coexist

  • Field naturalists were the first to question competitive exclusion principle

  • closely related species are often seen living in the same habitat (warblers, small mammals)

  • ecological paradox of competition: How can we reconcile extinction of competitors in lab and coexistence of competitors in field

  • MacArthur found that they fed in different canopy posts and had slightly different nesting dates

Resource Partitioning
  • species sharing same habitat can coexist by utilizing different resources
    • ex. diff food size
    • ex. diff times
    • ex. diff areas
  • one species, no competitor, utilizing a resource, seeds of various sizes
  • second species enters this area, and its resource utilization curve shows overlap, then we have interspecific competition
  • this overlap can force a reduced range of resource use and direct competition will be reduced
  • two species will coexist
  • ecological character displacement allows coexistence by causing species to use environment differently
  • ecological compression
  • ecological release
  • when species is without competitors, it occupies the largest possible ecological niche = fundamental
  • competition causes a population to function within a realized niche