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IV. Community Ecology (Who's who and who's not)

B. Predation (Consumer-Resource Interactions)

  Models of Predation

Lotka and Volterra proposed independently:

dN1/ dt = r1N1 - PN1N2 where

N1= density (or population size) of the prey

P = coefficient of predation

N1N2= probility of encounter between predator and prey

dN2/ dt = P2N1N2 - d2N2  
N2 = density (or population size) of the predator

P2= coefficient of expressing effectiveness of the predator

d2= mortality rate of the predator

What do these equations assume about factors that affect growth rate of prey (i.e. do they not consider)?
What do these equations assume about factors that affect reproductive rate of predators (i.e. do they not consider)?
Other models have also been developed.

One outcome- Ossillation of predator and prey (when prey high and predator low, predator increases.....)
  Is this modeling an open system or a closed system?  Is this equilibrium or non-equilibrium coexistence?  What is the 'feedback' that maintains predator-prey coexistence?  Are cyclic population changes observed in nature?



Experiments in Predation Gause, using the ciliates Paramecium and Didinium, could not create a real system that would mimic predator-prey cycles predicted by Lotka-Volterra.


However, the system could be maintained either by occasional re-introduction of prey or by providing a refuge in form of bottom sediment. In other words, an open system required to maintain predator-prey.


How important is predation in ecological systems?

Ecological examples:
Effects on density:

Effects on morphology of prey:
Brooks and Dodson - size selective fish eliminate large-bodied zooplankton species.

  Effects on evolution of predators and their prey: 

The evolutionary importance of predation is clearly demonstrated in the sophisticated predator and prey adaptions.  Co-evolution is very intense in these interactions because improved adaptation of one species will directly affect other, resulting in adaptations that influence the ability of predators to detect prey.

Examples of predator-prey adaptations in coloration:


  • Chemical interactions such as toxic prey, and responses of prey to substances released by predators.


    For example, how might length of spines change in the prey rotifer Brachionus in the presence of fitrates from the predacous rotifer Asplanchna?

  • 'Swamping' predators including synchonizing the timing of reproduction and movement.

    If competition is prevalent, then why don't predators drive prey extinct?
    Predatory-Prey Coexistence

    Based on the theory of natural selection, would predators evolve strategies to 'manage' prey populations in such a way as to prevent their elimination?

    e.g. Huffaker (1958) used 2 species of mites (predator Typhlodromus occidentalis and prey Eotetranychus sexmaculatus) on oranges and added barriers to dispersal (rubber balls, paper, vasoline).  

    For example, given:
    • 100 light bulbs - on or off
    • For each bulb in a second, the probability of turning from on to off is 0.5 (local extinction)
    • For each bulb in a second, the probability of turning from off to on is 0.5 only if a connected bulb is on (will not go extinct if migration occurs), but is 0.0 if connected bulbs are off

      Where bulbs are analogous to patches of habitat and a bulb going off is analogous to extinction of a species from that patch.

      Starting with all bulbs on:

    • If no bulbs are connected (no migration), all are off in 1 s (global extinction)
    • If all bulbs are connected (migration to any patch possible, all off in 1022 years (universe is only 1010years old).
    • If each bulb connected to only 10 other bulbs, all off in 17 minutes.
    Caswell (1978) develops a such a model for predator and 3 prey species that compete - no species went extinct even  after 1000 generations.

      Other types of "predation"
    By definition, are the following 'predator-prey' interactions?
    Cannibalism could also be considered as interference intraspecific competition with the added benefit of added nutrition. So if such a behavior eliminates competitors and delivers high-quality prey, why isn't cannibalism more common in nature?


    Compared to carnivore foraging strategies,  nutritional  value is often more important in herbivore food choice relative to choice based on  availability.  Why?

    Choice both between plant species and within a plant (young leaves tend to be more nutritious).

    Do herbivore-plant interactions always fit the general definition of predator-prey interaction?

    Moderate grazing can stimulate biomass production, especially in grasses where new tissue from the meristem is close to the ground and not grazed.

    Why is fruit so tasty, nutritious, and brightly advertized?


    Prey (plant) defense can also be quite sophisticated:

    • Trees can transfer chemical deterents to damaged leaves (one insect predator counters this by ringing an area to prevent transfer)
      • Individual trees of the same species can differ in defensive chemicals making it difficult for scale insect pest to evolve  consistently effective methods of attack (Edmunds and Alstad 1978 from Connell 1980).

    Actual act of heribivory and predation may be relatively rare events. How can these be studied? (And how do we study organism that live in environments that are less hospitable to humans?) 
    NOAA's Hydrolab habitat

    Bottom line on species interactions:  There are lots of possible interactions among multiple species with a community and in most communities there are many species.  Given this complexity, are there generalizations that can be made in the study of whole communities?