Predator-prey interactions:
lecture content
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Predator-prey interactions often dramatic, illustrated by
snowshoe hare-lynx population fluctuations
Simple Lotka-Volterra predator-prey model generates
fluctuations of prey, predator
Graphical models identify factors that stabilize, destabilize
predator-prey interaction
Importance of predation in nature attested to by various
lines of evidence
 Diversity,
ubiquity of anti-predator adaptations
 Evidence that predators control prey, under particular
conditions
 Impact of interacting predators and prey in population cycles
Predator-prey interactions are often
dramatic-- “nature red in tooth and
claw”--as illustrated by this lion about to
snag a Hyena
One of the most famous examples of
predator-prey interactions illustrated by
Canada lynx and snowshoe hare, in
Canadian taiga (forest) biome
The Hudson’s Bay Company provided
the best long-term data set, showing the
fluctuations of lynx and hare populations
across Canada
Dramatic fluctuations of hare and lynx
populations
Note regular periodicity, and lag
by lynx population peaks just
after hare peaks
Hare-lynx example
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Charles Elton’s paper (1924), “Periodic fluctuations
In the numbers of animals: their causes and
effects”, British Journal of Experimental Biology,
was first (of MANY) publications to analyze this
data set
Are these cycles regular, i.e., with constant
periodicity?
What causes these cycles?
 Interaction
of predator and prey?
 Hare-resource interaction? (hares feed on fir tree
needles, and other vegetation)
 Sunspot cycles?
 Humans (as hunters) interacting with both predator
and prey?
Modeling is one way ecologists have
studied predator-prey population
dynamics
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Lotka-Volterra Predator-Prey model is the classic
model (see “Summary: Lotka-Volterra Predator-Prey
Model”, lecture notes on web page)
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model generates highly regular oscillations of both
prey and predator population fluctuations, as seen in
hare-lynx data (see next slide)
 However, this model results in “neutral stability”, a very
fragile kind of stability that does not explain the factors
that tend either to stabilize or destabilize population
dynamics of predator-prey interactions
 To appreciate stabilizing, destabilizing influences on
predator-prey systems, we will use graphical analysis
Predator-prey population fluctuations
(neutral stability) in Lotka-Volterra model
Graphical analyses and stability
of predator-prey systems
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Modifications of prey isocline (see lecture, text)
 Humped
prey isocline
 Why
is it often hump-shaped? (Recall slope of logistic
model)
 Allee effect at low prey densities
 Stability depends on relative position of predator isocline
 Prey
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refuge from predator
Modifications of predator isocline
 Predator
carrying capacity
 Predator interference (e.g., territoriality)
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Factors that destabilize predator-prey interactions
 Time
lags, predator efficiency
 Monophagous predator (inability to switch prey)
What evidence that predators are an
important factor in nature?
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Diversity, ubiquity of anti-predator
adaptations in many kinds of prey
Impact of predators on prey
populations
Reviews of literature
Role of predators in oscillating
populations of prey and predators
Some anti-predator adaptations in
insects (and a few vertebrates)
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Warning = aposematic coloration
Batesian mimicry--palatable mimic of unpalatable model
Mullerian mimicry--both model and mimic unpalatable
Camouflage, crypsis--match background, unpalatable object
Catalepsis--frozen posture with appendages retracted
Aggression, counter-attack (bombadier beetle)
Aggression--e.g., stinging, biting such as wasps & bees
Armor--spines, thorns, anti-swallowing devices, large size,
bluffing
Masting--synchronous reproduction (e.g., 13- ,17-year
cicadas)
Escape behaviors--e.g., jumping Homoptera
Aposematic coloration in
poison-arrow frog,
Monteverde, Costa Rica
(photo by T.W. Sherry & T.K.
Werner)
Batesian mimicry of
wasp (unpalatable
model, upper left) by
(1) mantispid
(Neuroptera, palatable
mimic, upper right),
and (2) moth (palatable
mimic, lower); (Ricklefs
2001)
Mullerian mimicry in two pairs of
butterflies (Ricklefs 2001) (Heliconiinae)
Cryptic coloration in
Costa Rican moth
(center of photo)
resting on ground
during day (photo by
T.W. Sherry)
Cryptic (leaf-like)
coloration in
Choeradodis
rhombicolis
mantid, Costa
Rica
Ventral view of
Choeradodis
rhombicolis mantid,
Costa Rica:
Prothoracic flap
(shield-like structure
just behind head)
causes 10-fold
increase in handling
time by Costa Rican
nunbirds (largeinsect predator),
based on experiment
by T. Sherry (Photo by T.
Sherry)
Catalepsis in Costa Rican
katydids: See two insects
along leaf veins (arrows),
with only one pair of legs
protruding out of allignment
with rest of body (photo by
T.W. Sherry)
Bombadier beetle (Bradinus crepitans)
spraying boiling hot acid at predator;
note also aposematic coloration (Photo by
Thomas Eisner, Cornell University)
Active defense--urticating (stinging)
caterpillar in Costa Rica (photo by T.W. Sherry
& T.K. Werner)
Pinned specimens of jumping
Homoptera from Costa Rica
(superfamily Fulgoroidea)--note large
hind-legs (photo by T.W. Sherry)
Some conclusions from examples
of anti-predator adaptations
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Diversity, ubiquity of anti-predator adaptations attests
to intense selection pressure by predators
Some adaptations are subtle, poorly studied to date
(e.g., large body size as a refuge, anti-predator flaps)
Many prey have multiple adaptations, weapons
Tropics (and deep oceans) are arenas for intense
predator-prey co-evolution (long time periods of
stable environments, specialized adaptations in
relatively constant environments, yearlong activity,
diverse predators, prey)
Anti-predator adaptations are one form of evidence
for the impact of predators in ecological systems
Impact of predation on bullfrog tadpole
behavior and growth rate (from Ricklefs 2001)
Impact of birds
as predators
on caterpillars
in the Hubbard
Brook
Experimental
Forest, NH
(Holmes,
Schultz, and
Nothnagle,
1972);
asterisks
indicate
significant
differences
between
treatments
Other examples of prey
control by predator
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Dingo (wild dog) introduced into Australia has huge
impact on several herbivores there: kangaroos,
emus, feral pigs
 Populations
of all these animals significantly reduced
where dingos live (prey eliminated in some areas)
 Feral pigs have different population age-structure
where dingos present versus absent (see text)
Sea otters control abundance of sea urchins, sea
urchins of kelp beds (& orcas of sea otters!)
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Review by Andrew Sih (1985): 95% of studies
showed some effect of predation; 85% large effect
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Introduced predators have disproportionate effect
What is role of predators in causing
oscillations of predator, prey?
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Look at case study, of lynx-hare system
Krebs et al. (1996) study in arctic Canada
Winter food known to be important: Food quality declines when
heavily grazed at high hare density
 Study attempted to get at both factors by reducing predators (using
exclosures) & supplementing food (rabbit chow) during a
population peak and subsequent decline
 Next three slides present some of results of their study
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Abundance of hare populations in
response to treatments and controls,
during population peak, & subsequent
decline (Krebs et al.)
Ratio of
density of
hares in
treatment
versus
controls for
separate and
combined
treatment
effects; note
by far the
greatest
effect of
combined
treatments
(C)
Survival rates of hares also show much
greater impact of combined treatments
Conclusions from Krebs et
al. experiment on lynx-hare
population oscillations:
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It was possible to prolong peak of population
abundance of hares, but difficult!
Both food additions and predator reductions
affected hare populations separately
Effect of both food and predators had greatest
overall effect, indicating an interaction of food and
predators on prolonging hare population at high
level
Some human applications of
predator-prey models
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Humans as super-efficient predator that destabilizes
predator-prey interactions (e.g., fisheries)
 Humped
catch-yield versus fishing effort curve in
some fisheries
 How does increased predator efficiency destabilize?
 Interaction with natural environmental instability (e.g.,
El Niño-La Niña climate fluctuations)
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Introductions of predators often tend to destabilize
predator-prey systems….why?
Conclusions:
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Predator-prey ecological interactions often dramatic,
conspicuous
Models help identify factors that stabilize and
destabilize predator-prey interactions
 Classic
Lotka-Volterra model leads to oscillations, but
neutral stability
 Stabilizing factors--prey self-limitation, prey refuge,
spatial heterogeneity, predator territoriality
 De-stabilizing factors--predator more efficient, timelags
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Importance of predators in nature supported by
experiments on predator-impacts, anti-predator
adaptations, impact of predators on population
oscillations, activities of humans
Acknowledgements:
Most illustrations for this lecture
from R.E. Ricklefs. 2001. The
Economy of Nature, 5th Edition.
W.H. Freeman and Company, New
York.