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Introduction to Predator-Prey Dynamics

  • Long-standing question: Do predators preferentially target the old, young, sick, and weak?

  • Origin: Concept developed in the early 1950s based on wolf behavior (PR campaign to improve public perception).

Preferential Targeting by Predators

Predation Modes

  • Coursing Predators: Often target vulnerable individuals (e.g., wolves).

  • Stalking or Sit-and-Wait Predators: Less likely to preferentially target weaker individuals.

  • Ambush Predators: Do not show preference; prey vulnerability relies on random chance.

Random Chance and Prey Behavior

  • Example of chronic wasting disease in deer:

    • Predators (e.g., mountain lions) showed a preference for diseased deer based on behavioral changes, not physical weakness.

    • Disease alters behavior (e.g., droopy head syndrome), making deer easier to hunt.

Historical Context of Predator Control

  • Viewpoint shifted over centuries; believed that eliminating predators results in increased game species.

  • The 1905 Kaibab Plateau incident: Removal of predators led to a deer population surge then crash due to starvation.

    • Removal of 781 mountain lions, 30 wolves, 5,000 coyotes, etc.

    • Result: Deer increased to 100,000 but decreased to 10,000 due to overgrazing.

Re-evaluation of Evidence

  • 1970s Reevaluation:

    • Research by Graham Cawley suggested habitat quality improvements (e.g., logging increased food for deer) contributed to deer population spikes.

    • Domesticated livestock removal also played a crucial role in enhancing deer numbers.

Isle Royale Moose and Wolves Study

  • Wolves colonized the island in 1949, leading to the longest-running predator-prey study.

  • Findings showed:

    • Wolves dampened population swings of moose.

    • Moose numbers were primarily tied to browse availability, not just predation.

    • Average moose numbers remained stable before and after wolf introduction.

Bottom-Up vs Top-Down Regulation

Bottom-Up Perspective

  • Adequate habitat drives prey abundance.

  • Prey availability then influences predator abundance.

Top-Down Perspective

  • Predators limit prey abundance, impacting habitat through reduced herbivore populations.

  • Ongoing debate: Which dynamic is dominant?

Logical Considerations in Predator-Prey Interactions

Prey Abundance

  • Trophic pyramid explains prey’s higher numbers than predators.

    • Energy transfer is inefficient (90% loss between trophic levels).

Predator Response Dynamics

  • Time lag in predator population increase in response to growing prey populations.

  • Territoriality can restrict predator numbers even when prey is abundant.

Additive vs Compensatory Mortality

  • Additive Mortality: Predation adds to other mortality causes.

  • Compensatory Mortality: Predation replaces existing mortality sources.

    • Full compensatory mortality means no net effect on prey population size.

Situations of Predator Limitation

  • Predators are more likely to limit prey numbers when:

    • Prey populations are not abundant.

    • Prey are highly preferred by predators.

    • Predation mortality is partially additive.

Examples of Research Findings

Bottom-Up Evidence

  • Studies showed short-term increases in prey numbers without long-term changes when predators are removed.

    • Ex: Texas deer study.

    • Birds and hares similarly showed that predation effects were largely compensatory.

Top-Down Evidence

  • Case studies of wolves limiting caribou populations, and mountain lions impacting bighorn sheep abundances despite alternative prey availability.

  • Example: Sea otters regulating sea urchin populations, affecting kelp forests.

Predator Pit Dynamics

  • Temporary shift in predator-prey dynamics where a decline in prey might allow predators to effectively limit numbers due to scarcity.

  • Counterintuitive, suggesting conditions where normally bottom-up dynamics flip.

Conclusion

  • Future discussions on predator-prey dynamics will further explore the implications of bottom-up and top-down effects.

  • Introduction of the predator pit concept to be revisited for deeper understanding.