Predator-prey interactions: lecture content

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 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

Lotka-Volterra Predator-Prey model is the classic model (see “Summary: Lotka-Volterra Predator-Prey Model”, lecture notes on web page) This 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

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 refuge from predator Modifications of predator isocline

Predator carrying capacity Predator interference (e.g., territoriality)

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?

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)

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 (large-insect 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

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

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!)

Review by Andrew Sih (1985): 95% of studies showed some effect of predation; 85% large effect

Introduced predators have disproportionate effect

What is role of predators in causing oscillations of predator, prey?

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

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: 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 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) Introductions of predators often tend to destabilize

predator-prey systems….why?

Conclusions: 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, time-

lags 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.