Transcript Populations

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
Digestibility
Animals 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, capsasins
fruit rots, seeds mold, meat spoils
Spices more abundant in tropical cultures
Addictive drugs
morphine, coca, marijuana
Medicinal drugs
Taxus bark; aspirin; penicillin
Changes 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 resource
greater 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