Plant-herbivore interactions

The green earth paradox

• Why don’t predators win?• Why is the earth green?

Plants have antiherbivore defenses.

Plant-herbivore “warfare” is waged primarily through biochemical means.

Full spectrum of plant defenses includes:– low nutritional content of plant tissues– toxic compounds synthesized by the plants– structural defenses:

• spines and hairs• tough seed coats• sticky gums and resins

DigestibilityAnimals typically select plant food according to

its nutrient content:– especially important to young animals,

which have high demands for protein

Some plants deploy compounds that limit the digestibility of their tissues:– tannins produced by oaks and other plants

interfere with the digestion of proteins– some animals can overcome the effect of

tannins through production of digestive dispersal agents

Secondary Compounds

Secondary compounds are produced by plants for purposes (typically defensive) other than metabolism.

Such compounds can be divided into several major classes:– nitrogen compounds (lignin, alkaloids,

nonprotein amino acids, cyanogenic glycosides)

– terpenoids (essential oils, latex, plant resins)– phenolics (simple phenols, tannins)– hormones

Secondary chemical are big business

Popular drugs & spices: Nicotine, caffeine, capsasinsfruit rots, seeds mold, meat spoilsSpices more abundant in tropical

cultures

Addictive drugs morphine, coca, marijuana

Medicinal drugs Taxus bark; aspirin; penicillinChanges in metabolism of predators

The Plant - Herbivore Arms Race

1. Secondary chemical protects --> greater fitness

2. Selection for predator to exploit as food source

3. Specialization on finding & using this resourcegreater efficiency

4. Plant now has protective dependence on chemical which guarantees herbivory

5. Selection for new & supplemental mechanisms

6. Constant genetic change --> chemical races and diversity

Why are insects monophagus?

• Potential reasons?• Are they really?

Classifications of plant defense types

Apparent versus nonapparent

Apparent plants will always be found, Nonapparent plants can hide

Passive vs active protestants

Carbon/nutrient balance

Early vs late succession

Induced and Constitutive Defenses

Constitutive chemical defenses are maintained at high levels in the plant at all times.

Induced chemical defenses increase dramatically following an attack:– suggests that some chemicals are too expensive

to maintain under light grazing pressure– plant responses to herbivory can reduce

subsequent herbivory

Crypsis

Through crypsis, animals blend with their backgrounds; such animals:– are typically palatable or edible– match color, texture of bark, twigs, or

leaves– are not concealed, but mistaken for inedible

objects by would-be predators

Behaviors of cryptic organisms must correspond to their appearances.

Warning Coloration

Unpalatable animals may acquire noxious chemicals from food or manufacture these chemicals themselves:– such animals often warn potential predators

with warning coloration or aposematism:• predators learn to avoid such animals after

unpleasant experiences• certain aposematic colorations occur so

widely that predators may have evolved innate aversions

Batesian Mimicry

Certain palatable species mimic unpalatable species (models), benefiting from learning experiences of predators with the models.

This relationship has been named Batesian mimicry in honor of discoverer Henry Bates.

Experimental studies have demonstrated benefits to the mimic:– predators quickly learn to recognize color

patterns of unpalatable prey– mimics are avoided by such predators

The monarch (model) and viceroy (mimic) butterflies represent a classic case of Batesian mimicry

Müllerian Mimicry

Müllerian mimicry occurs among unpalatable species that come to resemble one another:– many species may be involved– each species is both model and mimic