Predation, Herbivory, and Prey Defenses

1. Predation and Herbivory

A predator kills and consumes animal prey; an herbivore consumes plants or algae, usually without killing the whole plant. Both are +/− interactions and can be density-dependent regulators. The prey population response includes changes in numbers and nonconsumptive effects: predation risk — fear itself — can reduce foraging and reproduction even without a kill.

2. The Lotka-Volterra Predator-Prey Model

The classic model produces cycling predator and prey populations: prey grow when predators are scarce, predators rise as prey become abundant, prey crash, and predators follow. The predator functional response describes how a predator's per-individual consumption changes with prey density: Type I rises linearly (constant proportion eaten), Type II saturates as handling time limits consumption, and Type III is S-shaped (low at low prey density, e.g., refuges or prey switching). These refinements move the model toward realism.

3. Prey Defenses and the Arms Race

Prey evade detection with cryptic coloration — camouflage matching the background. Others advertise: aposematic coloration pairs bright warning colors with genuine harm (toxins, stings). Batesian mimicry is a harmless species copying a toxic model's signal (a lie); Müllerian mimicry is genuinely toxic species converging on one signal (shared honesty). Prey also use behavioral defenses (fleeing, mobbing, alarm calls), mechanical defenses (spines, shells, thorns), and chemical defenses — including plant secondary metabolites (tannins, alkaloids) that deter herbivores, and induced defenses activated only after attack.

How it works

1. A predator consumes prey, gaining energy while the prey population loses individuals.
2. Prey with cryptic, behavioral, mechanical, or chemical defenses are eaten less and leave more offspring.
3. Predators with better detection or capture eat more and leave more offspring — coevolution tightens both sides.
4. Toxic prey evolve aposematic coloration; Batesian mimics exploit it, Müllerian mimics reinforce it.
5. At the population level, predator and prey numbers can cycle (the Lotka-Volterra pattern), modified by functional responses and refuges.
6. At the community level, predator changes cascade to lower trophic levels — a trophic cascade.

Common confusions

Memory aids

"CAM-BMC" — Camouflage hides, Aposematism warns, Mechanical/Behavioral block, Chemical poisons. For mimics: "Bates is a Bluffer, Müller is the real deal."

Quick review

Topic Recap

• Predation (animal prey) and herbivory (plants) are +/− interactions that regulate populations.
• The Lotka-Volterra predator-prey model produces cycles; functional responses refine it.
• Defenses: cryptic (camouflage), aposematic, behavioral, mechanical, chemical (incl. plant secondary metabolites, induced defenses).
• Batesian (fake) vs. Müllerian (honest) mimicry.
• Coevolution drives the arms race; predation risk and trophic cascades extend effects beyond the kill.
• Recognize the limitations of simplified predator-prey models.

Knowledge Check

1. Name the interaction type of predation and herbivory.
2. A harmless hoverfly resembles a stinging wasp. What is this called?
3. In the Lotka-Volterra model, what does the term aNP represent?
4. Distinguish a Type II from a Type III predator functional response.
5. Give one reason real predator-prey cycles are not the perfect closed cycles the simple model predicts.

Answers and Rationales

1. A +/− interaction — the consumer benefits, the consumed is harmed.
2. Batesian mimicry — a harmless species mimics a toxic/stinging model's warning signal.
3. The prey consumed per unit time — attack rate a × predator density P × prey density N.
4. Type II saturates as handling time limits consumption; Type III is S-shaped with low consumption at low prey density (refuges or switching), which can stabilize cycles.
5. Any of: prey refuges, predator switching, prey density dependence, or environmental variability — all damp or distort the model's neutral cycles.