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Population Interactions

Population Interactions. Competition for Resources: Exploitative competition : Both organisms competing for the same resource(s); e.g. nutrients, light, prey. Interference competition (amensalism): Organism exert direct, negative effects on another (allelochemical and allelopathy)

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Population Interactions

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  1. Population Interactions • Competition for Resources: • Exploitative competition: Both organisms competing for the same resource(s); e.g. nutrients, light, prey. • Interference competition (amensalism): Organism exert direct, negative effects on another (allelochemical and allelopathy) • Competitive interactions can get interesting when two species with differing capabilities compete for more than one resource.

  2. Two species competing for Si and P resources. Curves represent growth rate under given nutrient concentrations. • Note that the two species differ in their abilities to compete for different resources. • Species 1 needs higher [Si] to survive competition. • Species 2 needs higher [P] to survive competition.

  3. Population Interactions • Predation (mortality): • Prey population declines when growth rates slows below predation rate (and other mortality terms) • Predator Avoidance: • Mechanical defenses: spines, filaments, gelatinous aggregates. • Chemical defenses: allelochemical and allelopathy (taste nasty) • Life history defenses: growth rate / reproduction tradeoff • Behavioral defenses: diel vertical migrations (e.g. zooplankton) • Predator-Prey (Functional Response) Models.

  4. Predator-Prey Models • Type I: e.g. Lotka-Volterra. For a given predator density, prey consumption increases linearly with prey density. • Type II: e.g. Holling disc equation. Includes search and handling time of prey, following structure of Michaelis-Menton equation. (e.g. microbes, zooplankton) • Type III: Introduces concept of “learning” and increase in predator efficiency with increase in prey density. (e.g. fish)

  5. Trophic Cascades • Interactions at higher levels of the food chain have a cascading influence down through lower levels. • Bottom-up control: Primary production is controlled by limitations of abiotic factors (light, nutrients, etc.) • Top-down control: Primary production is controlled by predation on herbivores. • Trophic cascades in aquatic systems; e.g. piscivores and phytoplankton biomass. • With piscivores, larger population of zooplankton crustaceans, graze down phytoplankton. • Removal shifts dominance to planktivorous fish and loss of large zooplankton and switch to rotifers; phytoplankton bloom that are resistant to rotifer grazing.

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