Key Concepts of Predator-Prey Relationships

Why This Matters

Predator-prey relationships sit at the heart of behavioral ecology and drive some of the most testable concepts on the AP exam. You're being tested on your ability to connect individual behaviors—like fleeing, foraging, or hiding—to population-level outcomes and evolutionary processes. These interactions demonstrate natural selection in action, energy flow through ecosystems, and the mathematical models that predict population dynamics.

Don't just memorize definitions. For each concept below, know what behavioral or ecological principle it illustrates and how it connects to fitness, adaptation, or ecosystem stability. When an FRQ asks about population regulation or adaptive behavior, these predator-prey dynamics are your go-to examples.

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Population Dynamics and Mathematical Models

Predator and prey populations don't exist in isolation—they're locked in feedback loops that create predictable patterns over time.

Predator-Prey Population Dynamics

• Cyclical population fluctuations—prey increases fuel predator growth, which then crashes prey numbers, causing predator decline
• Lotka-Volterra model provides the mathematical foundation, showing how populations oscillate around equilibrium points
• Real-world examples like lynx-hare cycles demonstrate these patterns across decades of data

Lotka-Volterra Equations

• Differential equations model how predator and prey populations change: $\frac{dN}{dt} = rN - aNP$ for prey and $\frac{dP}{dt} = baNP - mP$ for predators
• Key variables include prey growth rate ($r$), predation rate ($a$), conversion efficiency ($b$), and predator mortality ($m$)
• Assumes simplified conditions—no carrying capacity, single predator-prey pair—making it a starting point for more complex models

Functional and Numerical Responses

• Functional response—how an individual predator's kill rate changes as prey density increases (Type I: linear, Type II: saturating, Type III: sigmoidal)
• Numerical response—how predator population size changes with prey availability through reproduction or immigration
• Both responses together determine whether predator populations can regulate prey or allow boom-bust cycles

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Prey Defense Strategies

Natural selection favors prey that survive long enough to reproduce, driving the evolution of diverse anti-predator adaptations.

Predator Avoidance Strategies

• Active defenses include fleeing, mobbing, and alarm calling—behaviors that require energy but directly reduce predation risk
• Passive defenses like freezing or hiding minimize detection by exploiting predator search patterns
• Behavioral plasticity allows prey to adjust strategies based on predator presence, balancing foraging needs against survival

Camouflage and Mimicry

• Crypsis reduces detection by matching background color, pattern, or texture—effective against visually hunting predators
• Batesian mimicry occurs when harmless species imitate dangerous ones; Müllerian mimicry involves multiple toxic species sharing warning signals
• Selection pressure from predators drives refinement of these traits across generations

Chemical Defenses

• Toxins and noxious secretions make prey unpalatable or dangerous to consume—think poison dart frogs or bombardier beetles
• Aposematic coloration advertises these defenses with bright warning colors, reducing attacks from educated predators
• Learned avoidance by predators means chemical defenses benefit the population even when individuals are sacrificed

Predator Satiation

• Synchronized mass reproduction overwhelms predator consumption capacity—cicada emergences are the classic example
• Dilution effect means any individual's predation risk drops when surrounded by many conspecifics
• Timing is critical—this strategy fails if reproduction spreads out, allowing predators to pick off individuals sequentially

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Predator Foraging and Adaptation

Predators face their own selection pressures—they must capture enough prey to survive and reproduce while minimizing costs.

Optimal Foraging Theory

• Energy maximization principle—predators should select prey that provides the highest $\frac{\text{energy gained}}{\text{handling time + search time}}$ ratio
• Diet breadth expands when preferred prey becomes scarce, as lower-value prey becomes worth pursuing
• Risk sensitivity modifies pure energy calculations—predators may avoid dangerous prey even if energetically profitable

Prey Switching

• Frequency-dependent predation—predators focus on the most abundant prey type, forming search images for common prey
• Stabilizing effect on prey communities because rare species experience reduced predation pressure
• Apostatic selection can maintain prey polymorphisms when predators overlook rare color morphs

Evolutionary Arms Race

• Reciprocal selection drives continuous adaptation—faster predators select for faster prey, which selects for even faster predators
• Red Queen hypothesis explains why neither side "wins"—both must keep evolving just to maintain relative fitness
• Escalation can produce extreme traits like cheetah speed or gazelle agility that seem costly but provide survival advantages

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Coevolutionary Dynamics

Predator-prey interactions don't just shape behavior—they drive evolutionary change across generations.

Predator-Prey Coevolution

• Reciprocal adaptation occurs when each species exerts selective pressure on the other, creating evolutionary feedback
• Geographic mosaic theory suggests coevolutionary intensity varies across space, creating hotspots and coldspots
• Drives biodiversity by generating novel traits, specializations, and ecological niches over evolutionary time

Landscape of Fear

• Non-consumptive effects—predators alter prey behavior even without killing, reducing foraging time and changing habitat use
• Risk allocation forces prey to balance starvation risk against predation risk, creating complex behavioral trade-offs
• Ecosystem engineering occurs when fear-driven prey movements affect vegetation, nutrient cycling, and other species

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Ecosystem-Level Effects

Predator-prey relationships ripple outward, shaping entire communities and ecosystems.

Keystone Predators

• Disproportionate impact relative to abundance—removing keystone predators causes cascading ecosystem changes
• Sea otters controlling sea urchins (which would otherwise destroy kelp forests) represent the textbook example
• Top-down regulation maintains prey diversity by preventing competitive exclusion among prey species

Trophic Cascades

• Indirect effects propagate through food webs—wolves reduce elk grazing, allowing willow recovery, which stabilizes riverbanks
• Top-down vs. bottom-up control debates center on whether predators or resource availability primarily structures communities
• Behavioral cascades occur even without population changes when prey alter their activity patterns

Mesopredator Release

• Mid-level predator explosion follows removal of apex predators—coyotes increase when wolves disappear
• Prey populations suffer because mesopredators often have higher consumption rates and less specialized diets
• Conservation implications highlight why protecting top predators matters for entire ecosystem health

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Quick Reference Table

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Self-Check Questions

1. How do functional and numerical responses differ, and why do ecologists need both concepts to predict population dynamics?

2. Which two prey defense strategies represent opposite solutions to predation pressure, and what determines which strategy evolves in a given species?

3. Compare the effects of removing a keystone predator versus removing a non-keystone predator—what ecosystem changes would you predict in each case?

4. An FRQ describes a predator that shifts from eating rabbits to eating voles as rabbit populations decline. Which concepts explain this behavior, and how does it affect prey community structure?

5. Explain how the "landscape of fear" demonstrates that predators can affect ecosystems even when they rarely kill prey. What does this suggest about the limitations of population-focused models?