Transcript Population Interactions

Population Interactions
• Competition for Resources:
– Exploitative competition: Both organisms competing for the same
resource(s).
– Interference competition (amensalism): Organism exert direct,
negative effects on another (allelochemical and allelopathy)
– Competitive interactions can get interesting when two species
compete for more than one resource with differing capabilities.
• Predation (mortality):
– Prey population declines when growth rates slows below
predation rate (and other mortality terms)
– Predator Avoidance:
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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.
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.
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)
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 piscivore, 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.
3. Spring Circulation
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Low but increasing temperature
Mixing water column with low stability
Low (but variable and increasing) light availability
high nutrient availability (why?)
Rapid growth and increases in phytoplankton biomass,
particularly diatoms. Often represents period of highest
annual biomass.
Increasing light is dominant contributing factor; zooplankton
grazing remains low for now.