Transcript No Slide Title

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
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
• In theory this
cycle neither
expands nor
contracts, but
continues
indefinitely in
a cycle.
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
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
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
Results of Hare-Lynx
Field Experiments
annual survival rate
Snowshoe Hare Survival
0.25
0.2
0.15
0.1
0.05
0
controls
food
predator
exclosure
predator
exclosure +
food
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 predatorprey, 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.