Predation, Herbivory, and Defense

Core Concepts

Types of Consumer-Resource Interactions

• Predation (carnivory): One animal kills and consumes another. Typically +/−.

• Herbivory: An animal consumes plant tissue. Typically +/−, but plants usually survive partial consumption (unlike prey in carnivory).

• Parasitism: A parasite lives on or in a host, consuming its resources without (usually) immediately killing it. +/−.

• Parasitoidism: An insect lays eggs in or on a host; the larvae consume and eventually kill the host. Common in wasps and flies. +/−.

Predator-Prey Dynamics

Predator and prey populations are dynamically linked. When prey are abundant, predators reproduce and increase. As predators increase, prey mortality rises and prey decline. As prey decline, predators face food shortage and decline. As predators decline, prey recover. This can produce coupled oscillations (cycles). The classic example is the ~10-year cycle of snowshoe hares and lynx in the boreal forest, documented from fur-trapping records. However, hare cycles are also influenced by food availability (hares overbrowse vegetation, which then declines), so the cycle is not purely predator-driven.

Prey Defenses

Prey have evolved a remarkable diversity of defenses:

Camouflage (crypsis): Coloration, pattern, and body shape that match the background, making the organism difficult to detect. Examples: a stick insect resembling a twig, a flounder matching the seafloor, a snowshoe hare turning white in winter.

Warning coloration (aposematism): Bright, conspicuous coloration advertising that the organism is toxic, distasteful, or dangerous. Predators learn to avoid the color pattern after a bad experience. Examples: poison dart frogs (brightly colored and highly toxic), monarch butterflies (toxic from milkweed compounds consumed as larvae), coral snakes (red, yellow, and black bands — venomous).

Mimicry: Resembling another species for protection.

• Batesian mimicry: A harmless species mimics a toxic or dangerous model. The mimic gains protection because predators mistake it for the model. Example: the harmless viceroy butterfly mimics the toxic monarch. (Note: some research suggests viceroys may also be unpalatable, complicating the classic example, but the principle holds.)

• Müllerian mimicry: Multiple toxic or unpalatable species converge on a similar warning pattern. This reinforces the signal for all species — a predator that learns to avoid one species avoids all similar-looking species. Example: several species of poison dart frogs or stinging bees and wasps sharing yellow-and-black striping.

Other animal defenses: Behavioral defenses (fleeing, hiding, mobbing), physical defenses (armor — armadillo, turtle shell; spines — porcupine, hedgehog), chemical defenses (skunk spray, bombardier beetle hot chemical spray), and startle displays (eyespots on moth wings).

Plant Defenses Against Herbivory

Plants cannot flee, but they have evolved extensive defenses:

Physical defenses: Thorns (modified stems — roses), spines (modified leaves — cacti), prickles (epidermal outgrowths), tough or fibrous leaves (sclerenchyma, silica inclusions in grasses), and sticky resins or latex that trap insects.

Chemical defenses: Secondary metabolites — compounds not involved in primary metabolism that deter or poison herbivores. Examples: alkaloids (caffeine, nicotine, morphine — neurotoxins), tannins (bind proteins, reducing digestibility), cyanogenic glycosides (release cyanide when tissues are damaged), and terpenoids (mint oils, latex). Many of these compounds are the basis for human medicines, spices, and recreational drugs.

Indirect defenses: Plants can release volatile organic compounds that attract predators and parasitoids of the herbivores feeding on them. Example: corn plants attacked by caterpillar larvae release volatiles that attract parasitoid wasps, which lay eggs in the caterpillars.

Coevolution

Predators and prey, herbivores and plants, are locked in coevolutionary arms races. As prey evolve better defenses, predators evolve counter-adaptations. As plants evolve toxins, herbivores evolve detoxification enzymes. Monarch caterpillars evolved the ability to sequester milkweed toxins; milkweeds evolved higher toxin concentrations and sticky latex. This back-and-forth, over evolutionary time, drives diversification.

ELI-10

Predators and prey are locked in an endless arms race. Predators get faster, prey get faster. Predators get better senses, prey get better camouflage. Every improvement on one side drives counter-improvements on the other.

Prey have three main defensive paint jobs:

Camouflage: “You cannot see me.” Blending into the background — like a stick insect that looks exactly like a twig.

Warning coloration: “You do not want to eat me.” Bright, obvious colors that advertise poison or danger. A poison dart frog is basically wearing a neon sign that says TOXIC. Predators learn to avoid those colors after one bad experience.

Mimicry: “I look like someone you do not want to mess with.” A harmless fly that looks like a bee (Batesian mimicry) gets left alone because predators confuse it with the real thing. Multiple toxic species that all look similar (Müllerian mimicry) share the advertising cost — a predator that learns to avoid one avoids them all.

Plants cannot run away, so they fight back with thorns, tough leaves, and an arsenal of chemical weapons — caffeine, nicotine, cyanide precursors, and thousands of other compounds. Some plants even call for help: when a caterpillar starts chewing, the plant releases chemical signals that attract wasps — which lay their eggs inside the caterpillar. The plant cannot move, but it can hire mercenaries.

ELI Example

The predator-prey arms race is like a never-ending game of hide-and-seek where both sides keep inventing better equipment. The prey gets camouflage (invisibility cloak), the predator gets thermal vision. The prey gets toxic skin (poison armor), the predator evolves resistance. The harmless prey starts dressing like the toxic one (costume party — Batesian mimicry). Every upgrade triggers a counter-upgrade. No one ever wins permanently — the race just keeps going.

Do Not Confuse

• Batesian vs. Müllerian Mimicry: Batesian = harmless mimic + toxic model (parasitic on the model’s signal). Müllerian = multiple toxic species converge (mutualistic reinforcement).

• Camouflage vs. Warning Coloration: Camouflage = hide (crypsis). Warning = advertise (aposematism). Opposite strategies.

• Herbivory vs. Predation: Herbivory = eating plants (prey usually not killed). Predation = killing and eating animals. Both are +/− consumer-resource interactions.

High-Yield Memory Anchors

• Predation = +/− (predator benefits, prey harmed). Drives coevolutionary arms races.

• Defenses: camouflage (hide), warning coloration (advertise toxicity), mimicry (look like someone else).

• Batesian mimicry = harmless mimics toxic. Müllerian = toxic species converge on same warning.

• Plant defenses: physical (thorns, toughness) + chemical (toxins, digestibility reducers) + indirect (attract predators of herbivores).

Quick Check

Q1: A harmless hoverfly has yellow and black stripes resembling a stinging wasp. This is an example of:

A) Müllerian mimicry

B) Batesian mimicry

C) Camouflage

D) Warning coloration

Q2: A new, very toxic butterfly species evolves bright orange coloration. Over time, several other toxic butterfly species in the same habitat evolve similar orange patterns. Explain why this convergence occurs and what type of mimicry it represents.

Q3: Predator and prey populations in the boreal forest show coupled ~10-year cycles. Explain why the predator population peaks AFTER the prey population, rather than simultaneously.

Quick Check Answers

A1: B. Batesian mimicry. The harmless hoverfly (mimic) gains protection by resembling the stinging wasp (toxic model). This is Batesian mimicry — a harmless species parasitizing the warning signal of a defended species.

A2: This is Müllerian mimicry — multiple toxic species converging on a shared warning pattern. The convergence occurs because: (1) predators learn to avoid the color pattern after a bad experience with ANY of the species; (2) by sharing the same pattern, each species contributes to predator education, and all benefit from reduced attack rates; (3) the shared pattern reduces the “training cost” for each species — fewer individuals of each species are killed during predator learning. Natural selection favors individuals that resemble the locally common warning pattern because predators avoid them more reliably.

A3: The predator population peaks after the prey population because of a time lag. When prey are abundant, predators have abundant food and their reproduction increases — but this takes time (gestation, juvenile development). The predator population grows while prey are still abundant, but by the time the predator population peaks, it has reduced the prey population through high predation pressure. The prey population then declines due to overpredation, followed by the predator population declining due to food shortage. This lag — predator reproduction not being instantaneous — is a key driver of the coupled oscillations.

Chapter Summary

Predation and herbivory are +/− consumer-resource interactions that transfer energy through food webs and drive the evolution of diverse defenses. Prey defenses include camouflage, warning coloration, and mimicry (Batesian and Müllerian). Plants defend against herbivores through physical and chemical defenses. Predator-prey dynamics often exhibit coupled cycles driven by time-lagged responses. Coevolution between consumers and their targets is a major force generating biological diversity.

Common Mistakes

• “Warning coloration attracts predators.” Warning coloration is conspicuous, but its purpose is to be noticed AND recognized as a danger signal, not to attract attacks. Predators learn to avoid it. A naive predator may attack once, but the negative experience teaches avoidance.

• “Mimicry always involves a harmless species copying a dangerous one.” Batesian mimicry does. Müllerian mimicry involves multiple dangerous species resembling each other — all are toxic, and they share a common warning pattern.