Predator-prey interactions: lecture content

1. 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

2. Predator-prey interactions are often dramatic-- “nature red in tooth and claw”--as illustrated by this lion about to snag a Hyena

3. One of the most famous examples of predator-prey interactions illustrated by Canada lynx and snowshoe hare, in Canadian taiga (forest) biome

4. The Hudson’s Bay Company provided the best long-term data set, showing the fluctuations of lynx and hare populations across Canada

5. Dramatic fluctuations of hare and lynx populations Note regular periodicity, and lag by lynx population peaks just after hare peaks

6. 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?

7. 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

8. Predator-prey population fluctuations (neutral stability) in Lotka-Volterra model

9. 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)

10. 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

11. 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

12. Aposematic coloration in poison-arrow frog, Monteverde, Costa Rica (photo by T.W. Sherry & T.K. Werner)

13. Batesian mimicry of wasp (unpalatable model, upper left) by (1) mantispid (Neuroptera, palatable mimic, upper right), and (2) moth (palatable mimic, lower); (Ricklefs 2001)

14. Mullerian mimicry in two pairs of butterflies (Ricklefs 2001) (Heliconiinae)

15. Cryptic coloration in Costa Rican moth (center of photo) resting on ground during day (photo by T.W. Sherry)

16. Cryptic (leaf-like) coloration in Choeradodis rhombicolis mantid, Costa Rica

17. 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)

18. 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)

19. Bombadier beetle (Bradinus crepitans) spraying boiling hot acid at predator; note also aposematic coloration (Photo by Thomas Eisner, Cornell University)

20. Active defense--urticating (stinging) caterpillar in Costa Rica (photo by T.W. Sherry & T.K. Werner)

21. Pinned specimens of jumping Homoptera from Costa Rica (superfamily Fulgoroidea)--note large hind-legs (photo by T.W. Sherry)

22. 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

23. Impact of predation on bullfrog tadpole behavior and growth rate (from Ricklefs 2001)

24. 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

25. 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)

26. 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

27. 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

28. Abundance of hare populations in response to treatments and controls, during population peak, & subsequent decline (Krebs et al.)

29. 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)

30. Survival rates of hares also show much greater impact of combined treatments

31. 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

32. 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?

33. 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

34. Acknowledgements: Most illustrations for this lecture from R.E. Ricklefs. 2001. The Economy of Nature, 5th Edition. W.H. Freeman and Company, New York.