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

• 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

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

The Predator-Prey Cycle: Theory

• 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

The Predator-Prey Cycle: Evidence

Predator-Prey Interactions with

ProtozoansIn 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

Lynx and Snowshoe Hare

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

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

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

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?

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• 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

• 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

Prey-Switching

• 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

• 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 Captures

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

The Hare-Lynx Interaction : Field Experiments

• 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

Snowshoe Hare Survival

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Results of Hare-Lynx Field Experiments

Non-Native Predators Cause Domino Effects

• Rainbow trout, introduced into New Zealand, exemplifies a trophic cascade. By suppressing invertebrate densities, algae are favored.

• 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

• 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

• 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.