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Predator-Prey Interactions. We wish to know: how predators affect prey populations, and vice-versa what stabilizes predator-prey interactions and prevents their collapse how predation can result in complex interactions in natural communities. Consumer-Resource Interactions. Predation

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Predator prey interactions
Predator-Prey Interactions

  • We wish to know:

  • how predators affect prey populations, and vice-versa

  • what stabilizes predator-prey interactions and prevents their collapse

  • how predation can result in complex interactions in natural communities

Consumer resource interactions
Consumer-Resource Interactions

  • Predation

    • Traditional view: carnivory

  • Herbivory

    • Herbivory is non-lethal

  • Parasitism

    • In parasitism, one individual is utilized for the development of more than one parasite

    • May or may not be lethal











The Predator-Prey Cycle: Theory

  • An abundant prey population is a resource for predators; hence they should increase in numbers

  • Once predators are abundant, predation should cause prey to decline

  • In the absence of its food supply, the predator in turn will decline

  • As long as some prey survive, since predators are rare, the prey population should increase again

The predator prey cycle theory
The Predator-Prey Cycle: Theory

  • In theory this cycle neither expands nor contracts, but continues indefinitely in a cycle.

The predator prey cycle evidence
The Predator-Prey Cycle: Evidence

  • Paramecium and its predator will cycle in a test-tube, if prey are provided with a refuge or periodically replenished

  • 100-yr+ record from fur trapping shows a regular cycle between the lynx and hare, with a 10-year period

Predator prey interactions with protozoans
Predator-Prey Interactions with Protozoans

In simple environments, Paramecium either is eliminated by a protozoan predator, or the predator fails to find enough prey and dies out.

In more complex environments, with refugia for the prey or when prey are allowed to “immigrate” into the system, predator-prey cycles persist for some time.

Predator and prey
Predator and Prey

Lynx and Snowshoe Hare

Hudson bay fur trapper captures
Hudson Bay Fur Trapper Captures

Annual fur trapping records demonstrate a 10-year cycle in prey and predator abundance.

Predator prey interactions with mites in a simple environment
Predator-Prey Interactions with Mites in a Simple Environment

Densities per area of orange for the prey mite Eotetranychus and the predator mite Typhlodromus, provided with 20 small areas of food for the prey alternating with 20 foodless positions.

One predator-prey cycle is completed before predators eliminate the prey.

Predator prey interactions with mites in a complex environment
Predator-Prey Interactions with Mites in a Complex Environment

Four cycles are obtained over ~ 60 weeks in a complex laboratory environment consisting of 252 oranges, with 1/20th of each orange exposed, and barriers to dispersal.

Host parasite interactions
Host-Parasite Interactions Environment

Fluctuations in abundance of the azuki bean weavil and its larval parasite (a wasp) in a laboratory setting.

Note the similarity to a predator-prey cycle.

What stabilizes predator prey systems in nature
What Stabilizes Predator-Prey Systems in Nature? Environment

In simple lab systems, predators often extinguish their prey and then starve. Why doesn’t this occur in nature?

  • Spatial heterogeneity or complexity of environment

  • Prey evolve defenses that reduce their vulnerability

  • Other prey species serve as alternate prey when one species becomes rare

Prey defenses
Prey Defenses Environment

Prey defenses1
Prey Defenses Environment

  • Predation provides many examples of adaptation by natural selection

    • plant leaves use chemical compounds to deter herbivores

    • cryptic coloration, chemical and “startle” defenses are widespread in insects

    • bright colors warn of toxicity

  • predators and prey can be locked in an “arms race” -- prey evolves greater defense, predator evolves better attack.

    • E.g., crabs and snails

    • the “red queen” model

Plant Defenses Environment

Cactus- Sharp spines may pierce and cause allergic reactions

Tobacco- Production of toxic secondary compound (nicotine)

Pitcher Plant- Leaves secrete nectar to attract insects to eat

Magnolia Leaves- Tough, waxy covering

Prey switching
Prey-Switching Environment

  • When the currently preferred prey becomes rare, predators may simply switch to an alternate prey. Theoretically, prey-switching could lead to reduced cycling of each prey and comparative constancy of predator abundance.

  • If an alternative prey is sufficiently abundant to maintain high predator densities, some other prey may be forced to very low densities.

  • An example in Newfoundland, involving caribou, lynx, snowshoe and arctic hare illustrates some complexities.

The hare lynx interaction a closer look
The Hare-Lynx Interaction: A Closer Look Environment

  • Does the lynx cause the hare to cycle? Or is it the reverse?

  • Hares cycle on islands where lynx are absent

  • Might hares cycle with their food supply (a hare-plant cycle), and lynx simply “ride” up and down with changes in their food supply?

Hudson bay fur trapper captures1
Hudson Bay Fur Trapper Captures Environment

Logically, the lynx cycle should lag behind the hare cycle, especially if the predator controls the prey. On occasion, the lynx appears to be “ahead” of the hare.

Range of lynx
Range of Lynx Environment

The hare lynx interaction field experiments
The Hare-Lynx Interaction : Field Experiments Environment

  • A large-scale experiment was conducted in 1 km2 plots in the Yukon over 8 years

  • predators were excluded with an electric fence

  • nutrients were added to stimulate plant growth

  • the predator exclusion --> a 2X increase

  • the food (via nutrients) addition --> 3X incr

  • combined treatments --> 10x increase

  • predator and food effects were not additive

Results of hare lynx field experiments
Results of Hare-Lynx Environment Field Experiments

Non native predators cause domino effects
Non-Native Predators Cause Domino Effects Environment

  • Opposum shrimp, introduced to Flathead Lake, Montana, Have strong indirect effects. Kokanee salmon declined, eagles no longer frequent the area, and grizzlies may be affected.

The opossum shrimp in flathead lake montana
The Opossum Shrimp in Flathead Lake, Montana Environment

  • Prior to shrimp introduction, lake trout and kokanee salmon (also introduced) fed on small zooplankton

  • spawning runs of kokanee into rivers provided food for eagles, bears

  • opossum shrimp, introduced as “fish-food”, upset the system

  • shrimp preyed upon and out-competed native zooplankton

  • shrimp migrate to deep waters by day, so inaccessible to kokanee

  • kokanee collapsed, eagles no longer stop over, and bears lack important fall food supply

Summary Environment

  • Predation, a “+/-” interaction, includes predator-prey, herbivore-plant, and parasite-host linkages.

  • These coupled systems are thought to cycle, although in complex systems other factors may play a role.

  • Stability may result from spatial heterogeneity, prey defenses, and availability of alternate prey.

  • Predation can cause complex community interactions, including strong indirect effects (keystone) and cascading effects.